Macromolecular Rapid Communications
Communication
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization Huajun Huang, Chunni Chen, Dongyue Zhang, Jianping Deng,* Youping Wu*
This article reports the first fluorescent microparticles (MPs, approximately 600 nm in diameter) constructed using helical substituted polyacetylene and prepared via a precipitation polymerization approach. The MPs judiciously combine this interesting helical conjugated acetylene, fluorescent material and polymeric particles in one entity. The monomer containing a dansyl group undergoes precipitation polymerization in butanone/n-heptane mixed solvent, with (nbd)Rh+B−(C6H5)4 as a catalyst. MPs with a regular morphology are formed in a high yield (>80 wt%). UV-vis spectroscopy demonstrates that the polymer chains making up the MPs adopt helical structures. The MPs show considerable fluorescence emission (λmax, 500 nm; excited at 340 nm). Based on SEM and fluorescence images, the formation mechanism of the MPs is proposed. This methodology opens up new ways to prepare functional microstructured materials derived from substituted polyacetylenes, and may also result in opportunities for new practical applications of polyacetylene and its derivatives.
1. Introduction Fluorescent particles (at both the nano- and microscale) have gathered ever-increasing attention due to their unique potential applications in biolabelling, bioimaging, detection, sensing, diagnosis, etc.[1] Quantum dots[2] and small-molecular organic dyes[3] are the fluorophores
H. Huang, C. Chen, D. Zhang, J. P. Deng State Key Laboratory of Chemical Resource Engineering; College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029, China E-mail: [email protected] Y. P. Wu State Key Laboratory of Organic-Inorganic Composites; College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029, China E-mail: [email protected]
908
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
primarily used for preparing fluorescent particles. However, despite the high fluorescence emission, easy leakage from the host materials frequently occurs with these commonly used fluorophores which is disadvantageous in practical applications. One of the alternative strategies to circumvent the problem is to combine the fluorophores with host materials using chemical bonds[4] or to construct core/shell structures in which the cores are composed of fluorescent materials while the shells are made from protective materials.[5] Unfortunately, such practices may weaken the fluorescence intensity due to the presence of an outer shell. More recently, conjugated polymers have attracted much attention and have provided new opportunities for developing novel fluorescent microparticles.[6] To date, a variety of conjugated polymers[7] have been explored in depth for this purpose. However, as a typical class of conjugated polymers, substituted polyacetylenes have rarely been exploited for constructing fluorescent particles, even though a variety of fluorescent polymers have been prepared from them.[8] The underlying issue is how
wileyonlinelibrary.com
DOI: 10.1002/marc.201400046
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
the growing process of the MPs could be followed. Furto efficiently prepare particles from acetylenic monomers. thermore, based on the present study and our earlier We have made efforts in this area and have successfully investigations,[19b] we propose here a formation mechaprepared a new class of fluorescent polymeric microparticles (MPs) derived from helical substituted polyacetylene. nism for the MPs. This article reports such fluorescent MPs, which combine the respective advantages of helical conjugated polyacetylenes, fluorescent materials, and polymeric particles in one 2. Experimental Section entity. Also notably, the presence of the fluorescent groups enabled us to investigate in depth the formation mecha2.1. Materials nism of the MPs reported in this article. (nbd)Rh+B−(C6H5)4 was prepared according to a procedure Artificial helical polymers have resulted in a unique reported previously.[20] Solvents were purified as presented research area in polymer science.[9] Stimulated by the in the Supporting Information. Dansyl chloride and proparfascinating helical structures in naturally occurring macgylamine were purchased from Aldrich and used without further romolecules, such as proteins and DNA, synthetic helical purification. polymers have attracted much interest in the last few decades. Some research groups have made large contri2.2. Measurements butions to progress in the field of synthetic helical polymers.[10] However, so far, only limited success has been FT-IR spectra were recorded using a Nicolet NEXUS 870 infrared achieved in constructing microscale particles using helical spectrometer. 1H and 13C NMR spectra were recorded on a Bruker polymers, despite the great number of analogous microAV 400 spectrometer. The morphology of the microparticles was observed with a HITACHI S-4700 scanning electron microparticles created from vinyl polymers[11] and even from scope (SEM). Circular dichroism (CD) and UV-vis absorption other conjugated polymers[12] like poly(thiophene),[13] spectra were obtained using a Jasco-810 spectropolarimeter. The poly(phenylene ethynylene),[14] and poly(fluorene).[15] For molecular weights and molecular weight polydispersities were substituted polyacetylenes, although treated as typical determined by GPC (Waters 515–2410 system) calibrated with synthetic helical polymers, they have advanced much polystyrenes, with THF as the eluent. Fluorescence spectra were more slowly in forming microscale particles, with only a measured on a Varian Cary Eclipse spectrophotometer (Varian, few studies reported in the literature.[16] Moreover, some USA). For all measurements of fluorescence spectra, the excitastudies did not explore whether or not the polymer chains tion wavelength was kept at 340 nm with a slit width of 4 nm. forming the particles adopted helical structures. MeckFluorescence images were observed with a fluorescence microing's group has performed extensive investigations on scope (Olympus IX81). Elemental analysis was performed on an preparing nanoparticles from polyacetylenes,[17] which Elementar vario EL cube element analyzer. Thermogravimetric have shown interesting potential in inkjet printing. In earlier studies,[18,19] we successfully prepared both nanoand microparticles consisting of optically active helical substituted polyacetylenes. Such nano- and microarchitectures demonstrated remarkable optical activity and significant potential applications ranging from asymmetric catalysis, chiral recognition/resolution, and enantiomer-selective crystallization to enantio-selective release. Based on previous investigations, in the present study we have synthesized fluorescent MPs derived from substituted acetylene, following a simple but efficient strategy presented in Scheme 1A. Interestingly, the polymer chains constructing the MPs were found to form helical conformations. The specific dansyl groups in the polymer side Scheme 1. (A) Schematic illustration of the strategy for preparing helical polyacetylenechains facilitated the MPs to demonbased fluorescent microparticles. (B) Schematic illustration of the microparticle formastrate fluorescent properties, by which tion process.
www.MaterialsViews.com
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
909
Macromolecular Rapid Communications
H. Huang et al.
www.mrc-journal.de
analysis (TGA) was carried out using a Q50 TGA at a scanning rate of 10 °C min–1 under N2. Differential scanning calorimetry (DSC) was performed at a scanning rate of 10 °C min–1 under N2 on a DSC1 (Mettler-Toledo).
2.3. Fabrication of Fluorescent Microparticles (MPs) The monomer, as presented in Scheme 1A, was synthesized and structurally identified according to our earlier method.[18,19] The detailed preparation process is explained in the Supporting Information. Referring to our earlier successful preparation of chiral helical polymer particles,[19b] a typical process for the preparation of fluorescent MPs is briefly presented as follows. All the solvents were obtained from Aldrich and purified by distillation. Pre-determined amounts of monomer ([M] = 0.1 mol L−1) and catalyst (nbd)Rh+B−(C6H5)4 ([Rh] = 1 ×10−3 mol L−1) were separately charged in two tubes, and 0.5 mL of butanone were added dropwise into each tube after a vacuum was applied to the tubes and they were charged with N2 three times. After complete dissolution, the Rh catalyst solution was immediately transfered into the monomer tube. A certain amount of n-heptane was slowly added to the reaction vessel. The polymerization lasted for 3 h at 30 °C. After filtration, washing with n-hexane three times, and drying in a vacuum, the yellow precipitate was collected for further measurements.
3. Results and Discussion
as-obtained MPs can be dissolved in appropriate solvents, since no crosslinking agent was used in preparing them. As a consequence, the MPs can be further characterized by GPC to analyze the molecular weight and molecular weight distribution of the polymer chains constructing the particles. The relevant data are presented in Table S1. When the butanone/n-heptane ratio was higher than 1/12 (mL/mL), no particles were formed (data not included here). When the solvent ratio of butanone/n-heptane was adjusted to 1/12 and 1/13 (mL/mL), particles were observed in the SEM images (Figure 1A and 1B); however, the MPs were not regular enough in morphology (CV, coefficient of variance, was ca. 5, Table S1). At a ratio of butanone/n-heptane of 1/14 (mL/mL), the MPs seemed to have a more regular morphology (Figure 1C; CV, 4.87, Table S1). A further increase in n-heptane again led to irregular particles (Figure 1D). Accordingly, the SEM images clearly demonstrate that the solvent mixture of butanone/n-heptane = 1/14 (mL/mL) can provide MPs that are regular in size. Therefore, in the subsequent polymerizations, the solvent mixture was kept at butanone/n-heptane = 1/14 (mL/mL). The resulting particles could be obtained in rather high yields (84−88 wt%) in the above cases. In addition, GPC measurements showed that the polymer chains forming the particles had a molecular weight ( Mn ) in the range 4100−4600 (Table S1). This is in good agreement with our earlier studies[19b] and the low Mn can be readily understood in terms of the features of precipitation polymerizations, as described later. The polydispersity of the molecular weights ( Mw /Mn ) was
In the study, the newly synthesized monomer (structurally presented in Scheme 1A) was used to prepare the designed fluorescent polymer MPs. The dansyl group makes the monomer an ideal candidate for preparing fluorescent polymer materials. The monomer was synthesized using a well-investigated approach, as described in detail in the Supporting Information. The monomer was obtained in a 50 wt% yield and then characterized by FT-IR and NMR spectroscopies and elemental analysis, as presented in Figure S1 and S2 in the Supporting Information. The detailed analysis of the spectra, together with the data from elemental analysis, are also presented in the Supporting Information. The characterizations clearly identified the structure of the monomer, according to our earlier series studies on substituted acetylenes.[18,19] The monomer underwent precipitation polymerization in a solvent mixture consisting of butanone and n-heptane. Nonetheless, only a suitable ratio of butanone/n-heptane led to regular MPs, Figure 1. SEM images of the microparticles prepared in different solvent mixtures of as illustrated in Table S1 in the Supbutanone/n-heptane: (A) 1/12; (B) 1/13; (C) 1/14; (D) 1/15 (v/v). For more details, refer to porting Information and Figure 1. The Table S1 in the Supporting Information.
910
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
In Figure 2A, the MPs were dissolved separately in 1.66−1.70 (Table S1). The MPs were also characterized by 1H NMR and 13C NMR, as shown in Figure S3 in the SupCHCl3 and DMF for measuring the UV-vis absorption spectra. Both spectra show a maximum UV-vis absorpporting Information. tion at around 340 nm. This absorption indicates the forIt is well known that for substituted polyacetylenes, mation of helical conformations in the polymer chains, if the pendant groups are appropriately bulky and when by referring to our earlier intensive studies dealing intramolecular hydrogen bonds are formed between the with helical substituted polyacetylenes and the partineighboring pendant groups, the polymer main chains cles thereof.[18,19,22] This assumption is further supported can form helical conformations under suitable condiby the investigations discussed below. A comparison tions.[21] Furthermore, the helical conformations can be between the two spectra in Figure 2A shows that in the confirmed by circular dichroism (CD) and UV-vis absorppolar solvent DMF, the UV-vis absorption was lower than tion spectroscopy measurements. Our previous studies[22] that measured in the relatively apolar solvent CHCl3. This also demonstrated that achiral substituted acetylenes observation can be explained by the stability of the helcould form helical polymers, but the polymers failed to ical conformations formed in the polymer backbones. We show optical activity due to the absence of chiral factors, know that, for helical substituted polyacetylenes, one of i.e., the right- and left-handed helices were identical in the major driving forces for them to form stable helical content. Fortunately, helix-sense-selective polymerizations conformations is the intramolecular hydrogen bonding performed in chiral micelles may lead to optically active occurring in the neighboring pendant amide moieties particles because of the polymer chains adopting predomialong the polymer chains.[22] The polar solvent DMF may nantly one-handed helicity.[18b,18d] In the present study, we destroy the intramolecular hydrogen bonds formed in the took a precipitation polymerization approach to prepare polymer chains. DMF is therefore not as favorable as CHCl3 the helical polymer-based fluorescent particles, since this to maintain helical conformations in the polymer chains. process can be carried out in the absence of stabilizers and emulsifiers, which are otherwise required in dispersion polymerizations and emulsion polymerizations for preparing polymeric particles. This feature makes precipitation polymerizations advantageous over their counterparts in preparing pure polymeric particles. The obtained MPs were characterized by CD and UV-vis absorption spectroscopies. As expected, they demonstrated no CD effects (300–500 nm) due to the lack of chiral factors (the CD spectra are thus omitted here), as previously observed for achirally helical polymerbased nanoparticles.[18,19,21,22] Figure 2 presents the UV-vis absorption spectra of the MPs (in Figure 2A, the MPs were dissolved in CHCl3 and DMF; in Figure 2B, the MPs were dissolved in DMF and the spectra were measured at different temperatures), together with the UV-vis absorption spectra of the polymeric MPs pressed after swelling with a little ethanol (Figure 2C) for a clear comparison. The polymeric MPs investigated hereafter are those prepared with butanone/n-heptane = 1/14 (mL/mL) Figure 2. UV-vis spectra of microparticles dissolved in solvent (A) and microparticles dis(Table S1 in the Supporting Informasolved in DMF, measured at different temperatures (B) (c = 0.1 g dL–1). (C) UV-vis spectra of tion). The subsequent characterizamicroparticles pressed after swelling with ethanol. (D) Fluorescence emission spectra of tions, unless otherwise noted, were also monomer and microparticles; the microparticles were directly measured in the powder accomplished using MPs prepared state. All the microparticles were prepared with butanone/n-heptane = 1/14 (mL/mL). See Table S1 in the Supporting Information. under the same conditions.
www.MaterialsViews.com
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
911
Macromolecular Rapid Communications
H. Huang et al.
www.mrc-journal.de
Despite this disadvantage, DMF was still used to investigate the effects of temperature on the helical conformations due to its relatively high boiling point. Figure 2B presents the UV-vis spectra of MPs dissolved in DMF. The spectra were measured at varied temperatures (30−90 °C). They show an enhancement of temperature from 30 to 90 °C leading to a gradual decrease in UV-vis absorption. This observation agrees well with earlier studies,[22] demonstrating the instability of the helical conformations at higher temperatures. Furthermore, when the temperature was decreased from 90 to 30 °C again, the UV-vis absorption increased gradually (Figure S4 in the Supporting Information). This demonstrates that the polymer chains underwent a reversible transition between helical conformations and disordered conformations, as reported previously.[22] This phenomenon also offers further evidence for the aforementioned conclusion that the polymer chains in the particles adopted helical conformations under the investigated conditions. In Figure 2C, both the monomer solution and the pressed MPs after swelling with ethanol (ethanol was used to swell the MPs for pressing them and to keep the particulate morphology) were investigated using UV-vis spectroscopy. The monomer showed a large UV-vis absorption at around 340 nm due to the dansyl groups. However, this UV-vis absorption did not change as temperature varied from 20−50 °C (in CHCl3), unlike the UV-vis absorption of the MPs in solution (Figure 2B). For the MPs, UV-vis spectra were obtained using pressed samples, as presented in Figure 2C. The spectrum showed a noticeable UV-vis absorption with λmax also at 340 nm, just like Figure 2A and 2B. We also previously investigated helical polyacetylene-derived microspheres[19] and swelled (hydro)gels[23] by UV-vis and CD spectroscopies. We found that the helical polyacetylene-based materials always showed remarkable UV-vis absorption at wavelengths ranging from 330−500 nm; if the helical polymer chains possessed a predominant helicity, CD effects could be observed at the corresponding wavelength range. The observation in this study is similar to earlier observations (Figure 2C), in other words, the noticeable UV-vis absorption (around 340 nm) demonstrates the helical conformations of the polyacetylene chains constructing the MPs. After successful preparation of the MPs, we began to consider another significant question, namely the growth of the MPs, since there has not yet been a detailed discussion on the formation mechanism of MPs derived from substituted acetylenes and prepared by a precipitation polymerization approach. Therefore, we observed the growing process of the MPs by SEM microscopy, as displayed in Figure 3A, 3C, 3E, and 3G. In the SEM images, the formation of MPs and their growth can be observed clearly and visually. They demonstrate that, in the initial stage of the precipitation polymerization (Figure 3A),
912
when the polymer chains propagated to a certain length (approximately 4300, Table S2 in the Supporting Information), they were precipitated from the polymerization media because of their low solubility. Therefore, at this stage, only irregular agglomerations were observed in the SEM images. Subsequently, some aggregates changed to nucleations which grew larger by adsorbing more polymer chains until 60 min (Figure 3C and 3E). Finally, all the particles grew into regular ones in terms of both morphology and size (Figure 3G). In this process, no further pronounced propagation occurred to the polymer chains ( Mn just increased from 4300 to ca. 4600, Table S2). The nucleation and growing process of the MPs seem to be the same as that observed in our earlier study,[19] in which chiral monomers were utilized for forming chiral particles constituted by optically active helical substituted polyacetylenes. The pendant dansyl groups along the polymer main chains in theory enable the MPs to show fluorescence emission properties. Accordingly, we next observed the MPs by fluorescence microscopy, with the aim of further verifying our aforementioned analyses of the growing process of the MPs. To perform the measurements, the pure MPs obtained were dispersed in ethanol. A certain amount of the dispersion was placed dropwise on a piece of coverslip and then dried at room temperature. The sample was then observed by fluorescence microscopy. In such a process, the morphology of the MPs can be maintained because of the insolubility of the MPs in ethanol. The observed fluorescent images are presented in Figure 3B, 3D, 3F, and 3H. Figure 3B shows intensively fluorescent bulks; however, fluorescence is also noticeable in the whole view, owing to the fluorescent polymer chains disorderedly distributed in the specimen. In comparison, Figure 3D demonstrates fluorescent bulks decreased in both number and size, and the tendency of granulation became pronounced. In Figure 3F and 3H, the fluorescence in the whole view background weakened in intensity and even disappeared, while the particles became more distinguishable. The corresponding SEM and fluorescent images are consistent with each other, reflecting the formation process of the fluorescent MPs. Herein we tentatively put forward a growing process for the MPs based on both the above investigations and earlier ones.[19b] The proposed process is schematically presented in Scheme 1B. Scheme 1B shows, after the beginning of polymerization, that monomer units were initiated to undergo polymerization, resulting in polymer chains. With increasing polymerization time, the reactive polymer chains propagated gradually until a certain length (Mn, approximately 4500 in the present case) was reached, and then the polymer chains precipitated out due to poor solubility in the mixed solvent. Then nucleation occurred in the precipitate, and the nucleated sites
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
It further indicates that, for both alkyne and alkene monomers undergoing precipitation polymerizations, they follow the same mechanism for constructing MPs, even though they follow different polymerization mechanisms, namely, vinyl monomers undergo free radical polymerizations, while alkyne monomers undergo catalytic polymerizations. To acquire more insights into the polymeric microparticles, we subsequently explored some factors potentially influencing the formation of MPs, including the monomer and Rh-catalyst concentrations. The related SEM images for the MPs are presented in Figure S5 and S6 in the Supporting Information, and other data are listed in Table S3 and S4 in the Supporting Information. Based on these investigations, the optimal conditions for preparing MPs were determined to be a solvent mixture of butanone/nheptane at 1/14 (v/v), and monomer and catalyst concentrations of 0.1 M and 10 × 10–3 M, respectively. Under these conditions, MPs with rather regular size and morphology can be prepared in high yields. These findings are useful for designing and preparing other analogous MPs and even optically active MPs.[19] The MPs were subsequently investigated using fluorescence spectroscopy measurements. The relevant spectra are presented in Figure 2D. The monomer exhibited an intense fluorescence emission at λmax = 492 nm (in THF solution). For the MPs, we felt it was better to keep the spherical shape when characterizing their fluorescence emission, so they were directly characterized by fluorescence spectroscopy. Like the monomer, the MPs also showed a fluorescence Figure 3. SEM images (A, C, E, G) and fluorescent images (B, D, F, H) showing the growth of the microparticles as a function of polymerization time: (A, B) 10 min; (C, D) 25 min; emission peak at 492 nm (Figure 2D). (E, F) 45 min; (G, H) 60 min. For polymerization conditions, refer to Table S1 in the Sup- This indicates that the fluorescence porting Information. emission performance of the MPs originated from the dansyl moieties. Other dansyl-containing polymers in the literature also show a grew larger gradually by adsorbing other polymer chains. fluorescence emission band at wavelengths ranging from Finally, MPs with regular size and morphology were 450 to 600 nm.[25] Finally, the obtained MPs were subformed, as observed in the SEM images in Figure 3G. The formation mechanism illustratively shown in jected to TGA and DSC analyses, as presented in Figure S7 Scheme 1B seems to be similar to the corresponding mechand S8 in the Supporting Information. These showed that anism for forming vinyl polymer-derived microspheres.[24] the polymer constructing the MPs was thermally stable
www.MaterialsViews.com
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
913
Macromolecular Rapid Communications
H. Huang et al.
www.mrc-journal.de
at temperatures up to 200 °C, indicating that they could be practically applied under normal conditions. Additionally, the polymer chains underwent a glass transition at around 110 °C.
4. Conclusion We synthesized a substituted acetylene monomer containing a dansyl group. The monomer underwent catalytic precipitation polymerization in the presence of Rh-catalyst in butanone/n-heptane (1/14, v/v), resulting in microparticles in high yields. The polymer chains constructing the microparticles adopted helical conformations, as did the corresponding polymer solution. The dansyl groups afforded the microparticles with interesting fluorescence emission properties. Based on the present investigations and earlier ones, we tentatively proposed a mechanism for forming microparticles using substituted acetylene as a monomer. The unique fluorescent particles, after optimization of composition and structure, e.g., fluorescent chiral particles, may hopefully be used as chiral catalysts,[9a,26] through which asymmetric reactions could be followed to investigate the reaction mechanisms involved. Investigations are currently ongoing along these interesting directions in our laboratory.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This work was supported by the National Natural Science Foundation of China (21274008, 21174010), the Funds for Creative Research Groups of China (51221002), and the “Specialized Research Fund for the Doctoral Program of Higher Education” (SRFDP 20120010130002).
Received: January 22, 2014; Revised: January 28, 2014; Published online: February 18, 2014; DOI: 10.1002/marc.201400046 Keywords: fluorescence; helical polymers; particle nucleation; polyacetylenes; precipitation polymerization [1] Recent typical articles: a) K. Li, B. Liu, J. Mater. Chem. 2012, 22, 1257; b) C. Paquet, S. Ryan, S. Zou, A. Kell, J. Tanha, J. Hulse, L-L. Yay, B. Simard, Chem. Commun. 2012, 48, 561; c) S. Bonacchi, D. Genovese, R. Juris, M. Montalti, L. Prodi, E. Rampazzo, N. Zaccheroni, Angew. Chem. Int. Ed. 2011, 50, 4056; d) A. Palma, L. A. Alvarez, D. Scholz, D. O. Frimannsson, M. Grossi, S. J. Quinn, D. F. O’Shea, J. Am. Chem. Soc. 2011, 133, 19618; e) L. Baù, P. Tecilla, F. Mancin, Nanoscale 2011, 3, 121; f) J. Gallo, I. García, N. Genicio, D. Padro, S. Penadés, Biomaterials 2011, 32, 9818. [2] Y. Zhu, Z. Li, M. Chen, H. M. Cooper, G. Q. Lu, Z. P. Xu, Chem. Mater. 2012, 24, 421.
914
[3] W. Liu, X. Huang, H. Wei, X. Tang, L. Zhu, Chem. Commun. 2011, 47, 11447. [4] B. Yu, X. Jiang, J. Yin, Soft Matter 2011, 7, 6853. [5] S. Wang, W. Zhao, J. Song, S. Cheng, L. Fan, Macromol. Rapid Commun. 2013, 34, 102. [6] a) J. Bai, C. Liu, T. Yang, F. Wang, Z. Li, Chem. Commun. 2013, 49, 3887; b) A. Patra, J.-M. Koenen, U. Scherf, Chem. Commun. 2011, 47, 9612; c) S. W. ThomasIII, G. D. Joly, T. M. Swager, Chem. Rev. 2007, 107, 1339. [7] Typical fluorescent conjugated polymers: a) L. Wang, Y. Liu, J. Yang, X. Tao, Z. Liu, Chem. Commun. 2012, 48, 5742; b) F. Schutze, B. Stempfle, C. Jungst, D. Woll, A. Zumbusch, S. Mecking, Chem. Commun. 2012, 48, 2104; c) T. L. Kelly, M. O. Wolf, Chem. Soc. Rev. 2010, 39, 1526; d) M. C. Baier, J. Huber, S. Mecking, J. Am. Chem. Soc. 2009, 131,14267. [8] a) X. Lou, Q. Zeng, Y. Zhang, Z. Wan, J. Qin, Z. Li, J. Mater. Chem. 2012, 22, 5581; b) C. Zhou, Y. Gao, D. Chen, J. Phys. Chem. B 2012, 116, 11552; c) R. Sivkova, J. Vohlídal, M. Bláha, J. Svoboda, J. Sedlácˇek, J. Zednik, Macromol. Chem. Phys. 2012, 213, 411; d) B. A. S. Jose, S. Matsushita, Y. Moroishi, K. Akagi, Macromolecules 2011, 44, 6288; e) W. Z. Yuan, H. Zhao, X. Y. Shen, F. Mahtab, J. W. Y. Lam, J. Z. Sun, B. Z. Tang, Macromolecules 2009, 42, 9400; f) G. Kwak, W.-E. Lee, H. Jeong, T. Sakaguchi, M. Fujiki, Macromolecules 2009, 42, 20; g) K. Tamura, T. Fujii, M. Shiotsuki, F. Sanda, T. Masuda, Polymer 2008, 49, 4494. [9] Recent review articles: a) V. Jain, K.-S. Cheon, K. Tang, S. Jha, M. M. Green, Isr. J. Chem. 2011, 51, 1067; b) R. P. Megens, G. Roelfes, Chem. Eur. J. 2011, 17, 8514; c) L. Liu, J. W. Y. Lam, B. Z. Tang, Chem. Rev. 2009, 109, 5799; d) E. Yashima, K. Maeda, H. Iika, Y. Furusho, K. Nagai, Chem. Rev. 2009, 109, 6102; e) J. G. Rudick, V. Percec, Acc. Chem. Res. 2008, 41, 1641; f) M. Fujiki, Top. Curr. Chem. 2008, 284, 119; g) K. Akagi, T. Mori, Chem. Rev. 2008, 8, 395; h) T. Masuda, J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 165; i) T. Nakano, Y. Okamoto, Chem. Rev. 2001, 101, 4013; j) M. M. Green, K.-S. Cheon, S.-Y. Yang, J.-W. Park, S. Swansburg, W. H. Liu, Acc. Chem. Res. 2001, 34, 672. [10] Typical articles: a) J. X. Cui, J. Zhang, X. H. Wan, Chem. Commun. 2012, 48, 4341; b) J. Budhathoki-Uprety, B. M. Novak, Macromolecules 2011, 44, 5947; c) X. A. Zhang, M. R. Chen, H. Zhao, Y. Gao, Q. Wei, S. Zhang, A. J. Qin, J. Z. Sun, B. Z. Tang, Macromolecules 2011, 44, 6724; d) K. Maeda, H. Mochizuki, K. Osato, E. Yashima, Macromolecules 2011, 44, 3217; e) M. Shiotsuki, F. Sanda, T. Masuda, Polym. Chem. 2011, 2, 1044; f) Y. Nakano, M. Fujiki, Macromolecules 2011, 44, 7511; g) F. Helmich, C. C. Lee, A. P. H. J. Schenning, E. W. J. Meijer, J. Am. Chem. Soc. 2010, 132, 16753; h) H. Jia, M. Teraguchi, T. Aoki, Y. Abe, T. Kaneko, S. Hadano, T. Namikoshi, T. Ohishi, Macromolecules 2010, 43, 8353; i) T. Sakamoto, Y. Fukuda, S. Sato, T. Nakano, Angew. Chem. Int. Ed. 2009, 48, 9308. [11] a) M. T. Gokmen, F. E. Du Prez, Prog. Polym. Sci. 2012, 37, 365; and the references cited therein. [12] a) T. L. Kelly, M. O. Wolf, Chem. Soc. Rev. 2010, 39, 1526; b) Z. Tian, J. Yu, C. Wu, C. Szymanski, J. McNeill, Nanoscale 2010, 2, 1999; c) A. Salinas-Castillo, M. Camprubí-Robles, R. Mallavia, Chem. Commun. 2010, 46,1263. [13] T. L. Kelly, S. P. Y. Che, Y. Yamada, K. Yano, M. O. Wolf, Langmuir 2008, 24, 9809. [14] P. Zhang, J. Guo, C. Wang, J. Mater. Chem. 2012, 22, 21426. [15] A. Álvarez-Diaz, A. Salinas-Castillo, M. Camprubí-Robles, J. M. Costa-Fernández, R. Pereiro, R. Mallavia, A. Sanz-Medel, Anal. Chem. 2011, 83, 2712.
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
[16] a) C. Song, X. Liu, D. Liu, C. Ren, W. Yang, J. P. Deng, Macromol. Rapid Commun. 2013, 34, 1426; b) M. Rassetti, I. Fratoddi, L. Lilla, C. Pasquini, M. V. Russo, O. Ursini, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 5097; c) F. Freire, J. M. Seco, E. Quiñoá, R. Riguera, Angew. Chem. Int. Ed. 2011, 50, 11692. [17] a) J. Huber, S. Mecking, Macromolecules 2010, 43, 8718; b) J. Pecher, S. Mecking, Chem. Rev. 2010, 110, 6260. [18] a) X. F. Luo, J. P. Deng, W. T. Yang, Angew. Chem. Int. Ed. 2011, 50, 4909; b) B. Chen, J. P. Deng, W. T. Yang, Adv. Funct. Mater. 2011, 21, 2345; c) L. Ding, Y. Y. Huang, Y. Y. Zhang, J. P. Deng, W. T. Yang, Macromolecules 2011, 44, 736; d) X. F. Luo, L. Li, J. P. Deng, T. T. Guo, W. T. Yang, Chem. Commun. 2010, 46, 2745; e) B. Chen, J. P. Deng, X. Q. Liu, W. T. Yang, Macromolecules 2010, 43, 3177. [19] a) C. Song, C. Zhang, F. Wang, W. Yang, J. P. Deng, Polym. Chem. 2013, 4, 645; b) D. Y. Zhang, C. Song, J. P. Deng, W. T. Yang, Macromolecules 2012, 45, 7329; c) D. Liu, L. Zhang, M. Li, W. Yang, J. P. Deng, Macromol. Rapid Commun. 2012, 33, 672. [20] R. R. Schrock, J. A. Osborn, Inorg. Chem. 1970, 9, 2339. [21] a) R. Nomura, Y. Fukushima, H. Nakako, T. Masuda, J. Am. Chem. Soc. 2000, 122, 8830; b) J. P. Deng, J. Tabei,
www.MaterialsViews.com
[22]
[23]
[24]
[25]
[26]
M. Shiotsuki, F. Sanda, T. Masuda, Macromolecules 2004, 37, 9715. a) J. P. Deng, J. Tabei, M. Shiotsuki, F. Sanda, T. Masuda, Macromolecules 2004, 37, 1891; b) J. P. Deng, J. Tabei, M. Shiotsuki, F. Sanda, T. Masuda, Macromolecules 2004, 37, 7156. a) X. Du, J. Liu, J.P. Deng, W. Yang, Polym. Chem. 2010, 1, 1030; b) F Yao, D. Zhang, C. Zhang, W. Yang, J. P. Deng, Bioresource Technol. 2013, 129, 58; c) D. Liu, H Chen, J.P. Deng, W. Yang, J. Mater. Chem. C 2013, 1, 8066. a) Y. Chen, L. Tong, D. Zhang, W. Yang, J. P. Deng, Ind. Eng. Chem. Res. 2012, 51, 15610; b) J. S. Downey, G. McIsaac, R. S. Frank, H. D. H. Stöver, Macromolecules 2001, 34, 4534; c) H. Macková, V. Proks, D. Horák, J. Kucˇka, M. Trchová, J. Polym. Sci., Part A: Polym. Chem. 2001, 49, 4820. a) N. Haridharan, R. Bhandary, K. Ponnusamy, R. Dhamodharan, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1491; b) J. Wang, D.-T. Pham, T. W. Kee, S. N. Clafton, X. Guo, P. Clements, S. F. Lincoln, R. K. Prud'homme, C. J. Easton, Macromolecules 2011, 44, 9782. D. Zhang, C. Ren, W. Yang, J. P. Deng, Macromol. Rapid Commun. 2012, 33, 652.
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
915
Communication
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization Huajun Huang, Chunni Chen, Dongyue Zhang, Jianping Deng,* Youping Wu*
This article reports the first fluorescent microparticles (MPs, approximately 600 nm in diameter) constructed using helical substituted polyacetylene and prepared via a precipitation polymerization approach. The MPs judiciously combine this interesting helical conjugated acetylene, fluorescent material and polymeric particles in one entity. The monomer containing a dansyl group undergoes precipitation polymerization in butanone/n-heptane mixed solvent, with (nbd)Rh+B−(C6H5)4 as a catalyst. MPs with a regular morphology are formed in a high yield (>80 wt%). UV-vis spectroscopy demonstrates that the polymer chains making up the MPs adopt helical structures. The MPs show considerable fluorescence emission (λmax, 500 nm; excited at 340 nm). Based on SEM and fluorescence images, the formation mechanism of the MPs is proposed. This methodology opens up new ways to prepare functional microstructured materials derived from substituted polyacetylenes, and may also result in opportunities for new practical applications of polyacetylene and its derivatives.
1. Introduction Fluorescent particles (at both the nano- and microscale) have gathered ever-increasing attention due to their unique potential applications in biolabelling, bioimaging, detection, sensing, diagnosis, etc.[1] Quantum dots[2] and small-molecular organic dyes[3] are the fluorophores
H. Huang, C. Chen, D. Zhang, J. P. Deng State Key Laboratory of Chemical Resource Engineering; College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029, China E-mail: [email protected] Y. P. Wu State Key Laboratory of Organic-Inorganic Composites; College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029, China E-mail: [email protected]
908
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
primarily used for preparing fluorescent particles. However, despite the high fluorescence emission, easy leakage from the host materials frequently occurs with these commonly used fluorophores which is disadvantageous in practical applications. One of the alternative strategies to circumvent the problem is to combine the fluorophores with host materials using chemical bonds[4] or to construct core/shell structures in which the cores are composed of fluorescent materials while the shells are made from protective materials.[5] Unfortunately, such practices may weaken the fluorescence intensity due to the presence of an outer shell. More recently, conjugated polymers have attracted much attention and have provided new opportunities for developing novel fluorescent microparticles.[6] To date, a variety of conjugated polymers[7] have been explored in depth for this purpose. However, as a typical class of conjugated polymers, substituted polyacetylenes have rarely been exploited for constructing fluorescent particles, even though a variety of fluorescent polymers have been prepared from them.[8] The underlying issue is how
wileyonlinelibrary.com
DOI: 10.1002/marc.201400046
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
the growing process of the MPs could be followed. Furto efficiently prepare particles from acetylenic monomers. thermore, based on the present study and our earlier We have made efforts in this area and have successfully investigations,[19b] we propose here a formation mechaprepared a new class of fluorescent polymeric microparticles (MPs) derived from helical substituted polyacetylene. nism for the MPs. This article reports such fluorescent MPs, which combine the respective advantages of helical conjugated polyacetylenes, fluorescent materials, and polymeric particles in one 2. Experimental Section entity. Also notably, the presence of the fluorescent groups enabled us to investigate in depth the formation mecha2.1. Materials nism of the MPs reported in this article. (nbd)Rh+B−(C6H5)4 was prepared according to a procedure Artificial helical polymers have resulted in a unique reported previously.[20] Solvents were purified as presented research area in polymer science.[9] Stimulated by the in the Supporting Information. Dansyl chloride and proparfascinating helical structures in naturally occurring macgylamine were purchased from Aldrich and used without further romolecules, such as proteins and DNA, synthetic helical purification. polymers have attracted much interest in the last few decades. Some research groups have made large contri2.2. Measurements butions to progress in the field of synthetic helical polymers.[10] However, so far, only limited success has been FT-IR spectra were recorded using a Nicolet NEXUS 870 infrared achieved in constructing microscale particles using helical spectrometer. 1H and 13C NMR spectra were recorded on a Bruker polymers, despite the great number of analogous microAV 400 spectrometer. The morphology of the microparticles was observed with a HITACHI S-4700 scanning electron microparticles created from vinyl polymers[11] and even from scope (SEM). Circular dichroism (CD) and UV-vis absorption other conjugated polymers[12] like poly(thiophene),[13] spectra were obtained using a Jasco-810 spectropolarimeter. The poly(phenylene ethynylene),[14] and poly(fluorene).[15] For molecular weights and molecular weight polydispersities were substituted polyacetylenes, although treated as typical determined by GPC (Waters 515–2410 system) calibrated with synthetic helical polymers, they have advanced much polystyrenes, with THF as the eluent. Fluorescence spectra were more slowly in forming microscale particles, with only a measured on a Varian Cary Eclipse spectrophotometer (Varian, few studies reported in the literature.[16] Moreover, some USA). For all measurements of fluorescence spectra, the excitastudies did not explore whether or not the polymer chains tion wavelength was kept at 340 nm with a slit width of 4 nm. forming the particles adopted helical structures. MeckFluorescence images were observed with a fluorescence microing's group has performed extensive investigations on scope (Olympus IX81). Elemental analysis was performed on an preparing nanoparticles from polyacetylenes,[17] which Elementar vario EL cube element analyzer. Thermogravimetric have shown interesting potential in inkjet printing. In earlier studies,[18,19] we successfully prepared both nanoand microparticles consisting of optically active helical substituted polyacetylenes. Such nano- and microarchitectures demonstrated remarkable optical activity and significant potential applications ranging from asymmetric catalysis, chiral recognition/resolution, and enantiomer-selective crystallization to enantio-selective release. Based on previous investigations, in the present study we have synthesized fluorescent MPs derived from substituted acetylene, following a simple but efficient strategy presented in Scheme 1A. Interestingly, the polymer chains constructing the MPs were found to form helical conformations. The specific dansyl groups in the polymer side Scheme 1. (A) Schematic illustration of the strategy for preparing helical polyacetylenechains facilitated the MPs to demonbased fluorescent microparticles. (B) Schematic illustration of the microparticle formastrate fluorescent properties, by which tion process.
www.MaterialsViews.com
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
909
Macromolecular Rapid Communications
H. Huang et al.
www.mrc-journal.de
analysis (TGA) was carried out using a Q50 TGA at a scanning rate of 10 °C min–1 under N2. Differential scanning calorimetry (DSC) was performed at a scanning rate of 10 °C min–1 under N2 on a DSC1 (Mettler-Toledo).
2.3. Fabrication of Fluorescent Microparticles (MPs) The monomer, as presented in Scheme 1A, was synthesized and structurally identified according to our earlier method.[18,19] The detailed preparation process is explained in the Supporting Information. Referring to our earlier successful preparation of chiral helical polymer particles,[19b] a typical process for the preparation of fluorescent MPs is briefly presented as follows. All the solvents were obtained from Aldrich and purified by distillation. Pre-determined amounts of monomer ([M] = 0.1 mol L−1) and catalyst (nbd)Rh+B−(C6H5)4 ([Rh] = 1 ×10−3 mol L−1) were separately charged in two tubes, and 0.5 mL of butanone were added dropwise into each tube after a vacuum was applied to the tubes and they were charged with N2 three times. After complete dissolution, the Rh catalyst solution was immediately transfered into the monomer tube. A certain amount of n-heptane was slowly added to the reaction vessel. The polymerization lasted for 3 h at 30 °C. After filtration, washing with n-hexane three times, and drying in a vacuum, the yellow precipitate was collected for further measurements.
3. Results and Discussion
as-obtained MPs can be dissolved in appropriate solvents, since no crosslinking agent was used in preparing them. As a consequence, the MPs can be further characterized by GPC to analyze the molecular weight and molecular weight distribution of the polymer chains constructing the particles. The relevant data are presented in Table S1. When the butanone/n-heptane ratio was higher than 1/12 (mL/mL), no particles were formed (data not included here). When the solvent ratio of butanone/n-heptane was adjusted to 1/12 and 1/13 (mL/mL), particles were observed in the SEM images (Figure 1A and 1B); however, the MPs were not regular enough in morphology (CV, coefficient of variance, was ca. 5, Table S1). At a ratio of butanone/n-heptane of 1/14 (mL/mL), the MPs seemed to have a more regular morphology (Figure 1C; CV, 4.87, Table S1). A further increase in n-heptane again led to irregular particles (Figure 1D). Accordingly, the SEM images clearly demonstrate that the solvent mixture of butanone/n-heptane = 1/14 (mL/mL) can provide MPs that are regular in size. Therefore, in the subsequent polymerizations, the solvent mixture was kept at butanone/n-heptane = 1/14 (mL/mL). The resulting particles could be obtained in rather high yields (84−88 wt%) in the above cases. In addition, GPC measurements showed that the polymer chains forming the particles had a molecular weight ( Mn ) in the range 4100−4600 (Table S1). This is in good agreement with our earlier studies[19b] and the low Mn can be readily understood in terms of the features of precipitation polymerizations, as described later. The polydispersity of the molecular weights ( Mw /Mn ) was
In the study, the newly synthesized monomer (structurally presented in Scheme 1A) was used to prepare the designed fluorescent polymer MPs. The dansyl group makes the monomer an ideal candidate for preparing fluorescent polymer materials. The monomer was synthesized using a well-investigated approach, as described in detail in the Supporting Information. The monomer was obtained in a 50 wt% yield and then characterized by FT-IR and NMR spectroscopies and elemental analysis, as presented in Figure S1 and S2 in the Supporting Information. The detailed analysis of the spectra, together with the data from elemental analysis, are also presented in the Supporting Information. The characterizations clearly identified the structure of the monomer, according to our earlier series studies on substituted acetylenes.[18,19] The monomer underwent precipitation polymerization in a solvent mixture consisting of butanone and n-heptane. Nonetheless, only a suitable ratio of butanone/n-heptane led to regular MPs, Figure 1. SEM images of the microparticles prepared in different solvent mixtures of as illustrated in Table S1 in the Supbutanone/n-heptane: (A) 1/12; (B) 1/13; (C) 1/14; (D) 1/15 (v/v). For more details, refer to porting Information and Figure 1. The Table S1 in the Supporting Information.
910
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
In Figure 2A, the MPs were dissolved separately in 1.66−1.70 (Table S1). The MPs were also characterized by 1H NMR and 13C NMR, as shown in Figure S3 in the SupCHCl3 and DMF for measuring the UV-vis absorption spectra. Both spectra show a maximum UV-vis absorpporting Information. tion at around 340 nm. This absorption indicates the forIt is well known that for substituted polyacetylenes, mation of helical conformations in the polymer chains, if the pendant groups are appropriately bulky and when by referring to our earlier intensive studies dealing intramolecular hydrogen bonds are formed between the with helical substituted polyacetylenes and the partineighboring pendant groups, the polymer main chains cles thereof.[18,19,22] This assumption is further supported can form helical conformations under suitable condiby the investigations discussed below. A comparison tions.[21] Furthermore, the helical conformations can be between the two spectra in Figure 2A shows that in the confirmed by circular dichroism (CD) and UV-vis absorppolar solvent DMF, the UV-vis absorption was lower than tion spectroscopy measurements. Our previous studies[22] that measured in the relatively apolar solvent CHCl3. This also demonstrated that achiral substituted acetylenes observation can be explained by the stability of the helcould form helical polymers, but the polymers failed to ical conformations formed in the polymer backbones. We show optical activity due to the absence of chiral factors, know that, for helical substituted polyacetylenes, one of i.e., the right- and left-handed helices were identical in the major driving forces for them to form stable helical content. Fortunately, helix-sense-selective polymerizations conformations is the intramolecular hydrogen bonding performed in chiral micelles may lead to optically active occurring in the neighboring pendant amide moieties particles because of the polymer chains adopting predomialong the polymer chains.[22] The polar solvent DMF may nantly one-handed helicity.[18b,18d] In the present study, we destroy the intramolecular hydrogen bonds formed in the took a precipitation polymerization approach to prepare polymer chains. DMF is therefore not as favorable as CHCl3 the helical polymer-based fluorescent particles, since this to maintain helical conformations in the polymer chains. process can be carried out in the absence of stabilizers and emulsifiers, which are otherwise required in dispersion polymerizations and emulsion polymerizations for preparing polymeric particles. This feature makes precipitation polymerizations advantageous over their counterparts in preparing pure polymeric particles. The obtained MPs were characterized by CD and UV-vis absorption spectroscopies. As expected, they demonstrated no CD effects (300–500 nm) due to the lack of chiral factors (the CD spectra are thus omitted here), as previously observed for achirally helical polymerbased nanoparticles.[18,19,21,22] Figure 2 presents the UV-vis absorption spectra of the MPs (in Figure 2A, the MPs were dissolved in CHCl3 and DMF; in Figure 2B, the MPs were dissolved in DMF and the spectra were measured at different temperatures), together with the UV-vis absorption spectra of the polymeric MPs pressed after swelling with a little ethanol (Figure 2C) for a clear comparison. The polymeric MPs investigated hereafter are those prepared with butanone/n-heptane = 1/14 (mL/mL) Figure 2. UV-vis spectra of microparticles dissolved in solvent (A) and microparticles dis(Table S1 in the Supporting Informasolved in DMF, measured at different temperatures (B) (c = 0.1 g dL–1). (C) UV-vis spectra of tion). The subsequent characterizamicroparticles pressed after swelling with ethanol. (D) Fluorescence emission spectra of tions, unless otherwise noted, were also monomer and microparticles; the microparticles were directly measured in the powder accomplished using MPs prepared state. All the microparticles were prepared with butanone/n-heptane = 1/14 (mL/mL). See Table S1 in the Supporting Information. under the same conditions.
www.MaterialsViews.com
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
911
Macromolecular Rapid Communications
H. Huang et al.
www.mrc-journal.de
Despite this disadvantage, DMF was still used to investigate the effects of temperature on the helical conformations due to its relatively high boiling point. Figure 2B presents the UV-vis spectra of MPs dissolved in DMF. The spectra were measured at varied temperatures (30−90 °C). They show an enhancement of temperature from 30 to 90 °C leading to a gradual decrease in UV-vis absorption. This observation agrees well with earlier studies,[22] demonstrating the instability of the helical conformations at higher temperatures. Furthermore, when the temperature was decreased from 90 to 30 °C again, the UV-vis absorption increased gradually (Figure S4 in the Supporting Information). This demonstrates that the polymer chains underwent a reversible transition between helical conformations and disordered conformations, as reported previously.[22] This phenomenon also offers further evidence for the aforementioned conclusion that the polymer chains in the particles adopted helical conformations under the investigated conditions. In Figure 2C, both the monomer solution and the pressed MPs after swelling with ethanol (ethanol was used to swell the MPs for pressing them and to keep the particulate morphology) were investigated using UV-vis spectroscopy. The monomer showed a large UV-vis absorption at around 340 nm due to the dansyl groups. However, this UV-vis absorption did not change as temperature varied from 20−50 °C (in CHCl3), unlike the UV-vis absorption of the MPs in solution (Figure 2B). For the MPs, UV-vis spectra were obtained using pressed samples, as presented in Figure 2C. The spectrum showed a noticeable UV-vis absorption with λmax also at 340 nm, just like Figure 2A and 2B. We also previously investigated helical polyacetylene-derived microspheres[19] and swelled (hydro)gels[23] by UV-vis and CD spectroscopies. We found that the helical polyacetylene-based materials always showed remarkable UV-vis absorption at wavelengths ranging from 330−500 nm; if the helical polymer chains possessed a predominant helicity, CD effects could be observed at the corresponding wavelength range. The observation in this study is similar to earlier observations (Figure 2C), in other words, the noticeable UV-vis absorption (around 340 nm) demonstrates the helical conformations of the polyacetylene chains constructing the MPs. After successful preparation of the MPs, we began to consider another significant question, namely the growth of the MPs, since there has not yet been a detailed discussion on the formation mechanism of MPs derived from substituted acetylenes and prepared by a precipitation polymerization approach. Therefore, we observed the growing process of the MPs by SEM microscopy, as displayed in Figure 3A, 3C, 3E, and 3G. In the SEM images, the formation of MPs and their growth can be observed clearly and visually. They demonstrate that, in the initial stage of the precipitation polymerization (Figure 3A),
912
when the polymer chains propagated to a certain length (approximately 4300, Table S2 in the Supporting Information), they were precipitated from the polymerization media because of their low solubility. Therefore, at this stage, only irregular agglomerations were observed in the SEM images. Subsequently, some aggregates changed to nucleations which grew larger by adsorbing more polymer chains until 60 min (Figure 3C and 3E). Finally, all the particles grew into regular ones in terms of both morphology and size (Figure 3G). In this process, no further pronounced propagation occurred to the polymer chains ( Mn just increased from 4300 to ca. 4600, Table S2). The nucleation and growing process of the MPs seem to be the same as that observed in our earlier study,[19] in which chiral monomers were utilized for forming chiral particles constituted by optically active helical substituted polyacetylenes. The pendant dansyl groups along the polymer main chains in theory enable the MPs to show fluorescence emission properties. Accordingly, we next observed the MPs by fluorescence microscopy, with the aim of further verifying our aforementioned analyses of the growing process of the MPs. To perform the measurements, the pure MPs obtained were dispersed in ethanol. A certain amount of the dispersion was placed dropwise on a piece of coverslip and then dried at room temperature. The sample was then observed by fluorescence microscopy. In such a process, the morphology of the MPs can be maintained because of the insolubility of the MPs in ethanol. The observed fluorescent images are presented in Figure 3B, 3D, 3F, and 3H. Figure 3B shows intensively fluorescent bulks; however, fluorescence is also noticeable in the whole view, owing to the fluorescent polymer chains disorderedly distributed in the specimen. In comparison, Figure 3D demonstrates fluorescent bulks decreased in both number and size, and the tendency of granulation became pronounced. In Figure 3F and 3H, the fluorescence in the whole view background weakened in intensity and even disappeared, while the particles became more distinguishable. The corresponding SEM and fluorescent images are consistent with each other, reflecting the formation process of the fluorescent MPs. Herein we tentatively put forward a growing process for the MPs based on both the above investigations and earlier ones.[19b] The proposed process is schematically presented in Scheme 1B. Scheme 1B shows, after the beginning of polymerization, that monomer units were initiated to undergo polymerization, resulting in polymer chains. With increasing polymerization time, the reactive polymer chains propagated gradually until a certain length (Mn, approximately 4500 in the present case) was reached, and then the polymer chains precipitated out due to poor solubility in the mixed solvent. Then nucleation occurred in the precipitate, and the nucleated sites
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
It further indicates that, for both alkyne and alkene monomers undergoing precipitation polymerizations, they follow the same mechanism for constructing MPs, even though they follow different polymerization mechanisms, namely, vinyl monomers undergo free radical polymerizations, while alkyne monomers undergo catalytic polymerizations. To acquire more insights into the polymeric microparticles, we subsequently explored some factors potentially influencing the formation of MPs, including the monomer and Rh-catalyst concentrations. The related SEM images for the MPs are presented in Figure S5 and S6 in the Supporting Information, and other data are listed in Table S3 and S4 in the Supporting Information. Based on these investigations, the optimal conditions for preparing MPs were determined to be a solvent mixture of butanone/nheptane at 1/14 (v/v), and monomer and catalyst concentrations of 0.1 M and 10 × 10–3 M, respectively. Under these conditions, MPs with rather regular size and morphology can be prepared in high yields. These findings are useful for designing and preparing other analogous MPs and even optically active MPs.[19] The MPs were subsequently investigated using fluorescence spectroscopy measurements. The relevant spectra are presented in Figure 2D. The monomer exhibited an intense fluorescence emission at λmax = 492 nm (in THF solution). For the MPs, we felt it was better to keep the spherical shape when characterizing their fluorescence emission, so they were directly characterized by fluorescence spectroscopy. Like the monomer, the MPs also showed a fluorescence Figure 3. SEM images (A, C, E, G) and fluorescent images (B, D, F, H) showing the growth of the microparticles as a function of polymerization time: (A, B) 10 min; (C, D) 25 min; emission peak at 492 nm (Figure 2D). (E, F) 45 min; (G, H) 60 min. For polymerization conditions, refer to Table S1 in the Sup- This indicates that the fluorescence porting Information. emission performance of the MPs originated from the dansyl moieties. Other dansyl-containing polymers in the literature also show a grew larger gradually by adsorbing other polymer chains. fluorescence emission band at wavelengths ranging from Finally, MPs with regular size and morphology were 450 to 600 nm.[25] Finally, the obtained MPs were subformed, as observed in the SEM images in Figure 3G. The formation mechanism illustratively shown in jected to TGA and DSC analyses, as presented in Figure S7 Scheme 1B seems to be similar to the corresponding mechand S8 in the Supporting Information. These showed that anism for forming vinyl polymer-derived microspheres.[24] the polymer constructing the MPs was thermally stable
www.MaterialsViews.com
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
913
Macromolecular Rapid Communications
H. Huang et al.
www.mrc-journal.de
at temperatures up to 200 °C, indicating that they could be practically applied under normal conditions. Additionally, the polymer chains underwent a glass transition at around 110 °C.
4. Conclusion We synthesized a substituted acetylene monomer containing a dansyl group. The monomer underwent catalytic precipitation polymerization in the presence of Rh-catalyst in butanone/n-heptane (1/14, v/v), resulting in microparticles in high yields. The polymer chains constructing the microparticles adopted helical conformations, as did the corresponding polymer solution. The dansyl groups afforded the microparticles with interesting fluorescence emission properties. Based on the present investigations and earlier ones, we tentatively proposed a mechanism for forming microparticles using substituted acetylene as a monomer. The unique fluorescent particles, after optimization of composition and structure, e.g., fluorescent chiral particles, may hopefully be used as chiral catalysts,[9a,26] through which asymmetric reactions could be followed to investigate the reaction mechanisms involved. Investigations are currently ongoing along these interesting directions in our laboratory.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This work was supported by the National Natural Science Foundation of China (21274008, 21174010), the Funds for Creative Research Groups of China (51221002), and the “Specialized Research Fund for the Doctoral Program of Higher Education” (SRFDP 20120010130002).
Received: January 22, 2014; Revised: January 28, 2014; Published online: February 18, 2014; DOI: 10.1002/marc.201400046 Keywords: fluorescence; helical polymers; particle nucleation; polyacetylenes; precipitation polymerization [1] Recent typical articles: a) K. Li, B. Liu, J. Mater. Chem. 2012, 22, 1257; b) C. Paquet, S. Ryan, S. Zou, A. Kell, J. Tanha, J. Hulse, L-L. Yay, B. Simard, Chem. Commun. 2012, 48, 561; c) S. Bonacchi, D. Genovese, R. Juris, M. Montalti, L. Prodi, E. Rampazzo, N. Zaccheroni, Angew. Chem. Int. Ed. 2011, 50, 4056; d) A. Palma, L. A. Alvarez, D. Scholz, D. O. Frimannsson, M. Grossi, S. J. Quinn, D. F. O’Shea, J. Am. Chem. Soc. 2011, 133, 19618; e) L. Baù, P. Tecilla, F. Mancin, Nanoscale 2011, 3, 121; f) J. Gallo, I. García, N. Genicio, D. Padro, S. Penadés, Biomaterials 2011, 32, 9818. [2] Y. Zhu, Z. Li, M. Chen, H. M. Cooper, G. Q. Lu, Z. P. Xu, Chem. Mater. 2012, 24, 421.
914
[3] W. Liu, X. Huang, H. Wei, X. Tang, L. Zhu, Chem. Commun. 2011, 47, 11447. [4] B. Yu, X. Jiang, J. Yin, Soft Matter 2011, 7, 6853. [5] S. Wang, W. Zhao, J. Song, S. Cheng, L. Fan, Macromol. Rapid Commun. 2013, 34, 102. [6] a) J. Bai, C. Liu, T. Yang, F. Wang, Z. Li, Chem. Commun. 2013, 49, 3887; b) A. Patra, J.-M. Koenen, U. Scherf, Chem. Commun. 2011, 47, 9612; c) S. W. ThomasIII, G. D. Joly, T. M. Swager, Chem. Rev. 2007, 107, 1339. [7] Typical fluorescent conjugated polymers: a) L. Wang, Y. Liu, J. Yang, X. Tao, Z. Liu, Chem. Commun. 2012, 48, 5742; b) F. Schutze, B. Stempfle, C. Jungst, D. Woll, A. Zumbusch, S. Mecking, Chem. Commun. 2012, 48, 2104; c) T. L. Kelly, M. O. Wolf, Chem. Soc. Rev. 2010, 39, 1526; d) M. C. Baier, J. Huber, S. Mecking, J. Am. Chem. Soc. 2009, 131,14267. [8] a) X. Lou, Q. Zeng, Y. Zhang, Z. Wan, J. Qin, Z. Li, J. Mater. Chem. 2012, 22, 5581; b) C. Zhou, Y. Gao, D. Chen, J. Phys. Chem. B 2012, 116, 11552; c) R. Sivkova, J. Vohlídal, M. Bláha, J. Svoboda, J. Sedlácˇek, J. Zednik, Macromol. Chem. Phys. 2012, 213, 411; d) B. A. S. Jose, S. Matsushita, Y. Moroishi, K. Akagi, Macromolecules 2011, 44, 6288; e) W. Z. Yuan, H. Zhao, X. Y. Shen, F. Mahtab, J. W. Y. Lam, J. Z. Sun, B. Z. Tang, Macromolecules 2009, 42, 9400; f) G. Kwak, W.-E. Lee, H. Jeong, T. Sakaguchi, M. Fujiki, Macromolecules 2009, 42, 20; g) K. Tamura, T. Fujii, M. Shiotsuki, F. Sanda, T. Masuda, Polymer 2008, 49, 4494. [9] Recent review articles: a) V. Jain, K.-S. Cheon, K. Tang, S. Jha, M. M. Green, Isr. J. Chem. 2011, 51, 1067; b) R. P. Megens, G. Roelfes, Chem. Eur. J. 2011, 17, 8514; c) L. Liu, J. W. Y. Lam, B. Z. Tang, Chem. Rev. 2009, 109, 5799; d) E. Yashima, K. Maeda, H. Iika, Y. Furusho, K. Nagai, Chem. Rev. 2009, 109, 6102; e) J. G. Rudick, V. Percec, Acc. Chem. Res. 2008, 41, 1641; f) M. Fujiki, Top. Curr. Chem. 2008, 284, 119; g) K. Akagi, T. Mori, Chem. Rev. 2008, 8, 395; h) T. Masuda, J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 165; i) T. Nakano, Y. Okamoto, Chem. Rev. 2001, 101, 4013; j) M. M. Green, K.-S. Cheon, S.-Y. Yang, J.-W. Park, S. Swansburg, W. H. Liu, Acc. Chem. Res. 2001, 34, 672. [10] Typical articles: a) J. X. Cui, J. Zhang, X. H. Wan, Chem. Commun. 2012, 48, 4341; b) J. Budhathoki-Uprety, B. M. Novak, Macromolecules 2011, 44, 5947; c) X. A. Zhang, M. R. Chen, H. Zhao, Y. Gao, Q. Wei, S. Zhang, A. J. Qin, J. Z. Sun, B. Z. Tang, Macromolecules 2011, 44, 6724; d) K. Maeda, H. Mochizuki, K. Osato, E. Yashima, Macromolecules 2011, 44, 3217; e) M. Shiotsuki, F. Sanda, T. Masuda, Polym. Chem. 2011, 2, 1044; f) Y. Nakano, M. Fujiki, Macromolecules 2011, 44, 7511; g) F. Helmich, C. C. Lee, A. P. H. J. Schenning, E. W. J. Meijer, J. Am. Chem. Soc. 2010, 132, 16753; h) H. Jia, M. Teraguchi, T. Aoki, Y. Abe, T. Kaneko, S. Hadano, T. Namikoshi, T. Ohishi, Macromolecules 2010, 43, 8353; i) T. Sakamoto, Y. Fukuda, S. Sato, T. Nakano, Angew. Chem. Int. Ed. 2009, 48, 9308. [11] a) M. T. Gokmen, F. E. Du Prez, Prog. Polym. Sci. 2012, 37, 365; and the references cited therein. [12] a) T. L. Kelly, M. O. Wolf, Chem. Soc. Rev. 2010, 39, 1526; b) Z. Tian, J. Yu, C. Wu, C. Szymanski, J. McNeill, Nanoscale 2010, 2, 1999; c) A. Salinas-Castillo, M. Camprubí-Robles, R. Mallavia, Chem. Commun. 2010, 46,1263. [13] T. L. Kelly, S. P. Y. Che, Y. Yamada, K. Yano, M. O. Wolf, Langmuir 2008, 24, 9809. [14] P. Zhang, J. Guo, C. Wang, J. Mater. Chem. 2012, 22, 21426. [15] A. Álvarez-Diaz, A. Salinas-Castillo, M. Camprubí-Robles, J. M. Costa-Fernández, R. Pereiro, R. Mallavia, A. Sanz-Medel, Anal. Chem. 2011, 83, 2712.
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.MaterialsViews.com
Helical Substituted Polyacetylene-Derived Fluorescent Microparticles Prepared by Precipitation Polymerization
Macromolecular Rapid Communications www.mrc-journal.de
[16] a) C. Song, X. Liu, D. Liu, C. Ren, W. Yang, J. P. Deng, Macromol. Rapid Commun. 2013, 34, 1426; b) M. Rassetti, I. Fratoddi, L. Lilla, C. Pasquini, M. V. Russo, O. Ursini, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 5097; c) F. Freire, J. M. Seco, E. Quiñoá, R. Riguera, Angew. Chem. Int. Ed. 2011, 50, 11692. [17] a) J. Huber, S. Mecking, Macromolecules 2010, 43, 8718; b) J. Pecher, S. Mecking, Chem. Rev. 2010, 110, 6260. [18] a) X. F. Luo, J. P. Deng, W. T. Yang, Angew. Chem. Int. Ed. 2011, 50, 4909; b) B. Chen, J. P. Deng, W. T. Yang, Adv. Funct. Mater. 2011, 21, 2345; c) L. Ding, Y. Y. Huang, Y. Y. Zhang, J. P. Deng, W. T. Yang, Macromolecules 2011, 44, 736; d) X. F. Luo, L. Li, J. P. Deng, T. T. Guo, W. T. Yang, Chem. Commun. 2010, 46, 2745; e) B. Chen, J. P. Deng, X. Q. Liu, W. T. Yang, Macromolecules 2010, 43, 3177. [19] a) C. Song, C. Zhang, F. Wang, W. Yang, J. P. Deng, Polym. Chem. 2013, 4, 645; b) D. Y. Zhang, C. Song, J. P. Deng, W. T. Yang, Macromolecules 2012, 45, 7329; c) D. Liu, L. Zhang, M. Li, W. Yang, J. P. Deng, Macromol. Rapid Commun. 2012, 33, 672. [20] R. R. Schrock, J. A. Osborn, Inorg. Chem. 1970, 9, 2339. [21] a) R. Nomura, Y. Fukushima, H. Nakako, T. Masuda, J. Am. Chem. Soc. 2000, 122, 8830; b) J. P. Deng, J. Tabei,
www.MaterialsViews.com
[22]
[23]
[24]
[25]
[26]
M. Shiotsuki, F. Sanda, T. Masuda, Macromolecules 2004, 37, 9715. a) J. P. Deng, J. Tabei, M. Shiotsuki, F. Sanda, T. Masuda, Macromolecules 2004, 37, 1891; b) J. P. Deng, J. Tabei, M. Shiotsuki, F. Sanda, T. Masuda, Macromolecules 2004, 37, 7156. a) X. Du, J. Liu, J.P. Deng, W. Yang, Polym. Chem. 2010, 1, 1030; b) F Yao, D. Zhang, C. Zhang, W. Yang, J. P. Deng, Bioresource Technol. 2013, 129, 58; c) D. Liu, H Chen, J.P. Deng, W. Yang, J. Mater. Chem. C 2013, 1, 8066. a) Y. Chen, L. Tong, D. Zhang, W. Yang, J. P. Deng, Ind. Eng. Chem. Res. 2012, 51, 15610; b) J. S. Downey, G. McIsaac, R. S. Frank, H. D. H. Stöver, Macromolecules 2001, 34, 4534; c) H. Macková, V. Proks, D. Horák, J. Kucˇka, M. Trchová, J. Polym. Sci., Part A: Polym. Chem. 2001, 49, 4820. a) N. Haridharan, R. Bhandary, K. Ponnusamy, R. Dhamodharan, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1491; b) J. Wang, D.-T. Pham, T. W. Kee, S. N. Clafton, X. Guo, P. Clements, S. F. Lincoln, R. K. Prud'homme, C. J. Easton, Macromolecules 2011, 44, 9782. D. Zhang, C. Ren, W. Yang, J. P. Deng, Macromol. Rapid Commun. 2012, 33, 652.
Macromol. Rapid Commun. 2014, 35, 908−915 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
915
