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Full Paper MacromolecularChemistry and Physics

wileyonlinelibrary.com DOI:10.1002/ macp. 201100610

Aqueous Emulsion Polymerization of Substituted Acetylenes: Effects of Organic

Solvent and Analysis of Blue Shifts andEmulsion Polymerization Mechanism

Bo Chen, Xuan Liu, Chuqi Xu, Ci Song, Xiaofeng Luo, Wantai Yang, Jianping Deng*

A series of optically active nanoparticles composed of helical substituted polyacetylene is pre-pared via emulsion polymerization. A specic organic solvent is required for solid substituted

acetylene monomers to undergo emulsion polymerization.The effects of this organic solvent are investigated. Its amountis found to be decisive for obtaining stable polymer emulsions.The helical conformations of the substituted polyacetylenesin nanoparticle state are identied with the help of CD andUV-vis spectra. A blue shift occurs in CD and UV-vis spectra of some of the polyacetylene emulsions. The underlying drivingforce for the blue shift is discussed, and an emulsion poly-merization mechanism is proposed.

B. Chen, X. Liu, C. Song, X. Luo, Prof. W. Yang, Prof. J. DengState Key Laboratory of Chemical Resource Engineering,College of Materials Science and Engineering, BeijingUniversity of Chemical Technology, Beijing 100029, ChinaE-mail: [email protected]. XuXiamen Sunrui Ship Coatings Co. Ltd., China Shipbuildingindustry Co., Xiamen 361101, China

We have performed a series of emulsion polymeriza-tions of acetylene derivatives, providing a variety of NPscomposed of substituted polyacetylenes. [ 7 ] Such emulsionpolymerizations were performed to circumvent to somedegree the limitations frequently observed in substitutedpolyacetylenes, that is, low solubility, low processability,and low stability against heat and oxygen, as mentionedearlier. [ 7c ] We further revealed that the formation of NPslittle affected the polymers secondary structures. In moredetail, the substituted polyacetylenes constituting NPscan adopt helical conformations, just like in the state of polymer solution. More interestingly, emulsion polymeri-

zation is favorable for the substituted polyacetylenes toadopt helical structures. Based on these investigations, wefurther prepared core/shell structured and hollow two-layered hybrid NPs composed of helical substituted poly-acetylenes. [ 8 ] The structural distinction and the poten-tial applications of such NPs encourage us to deepen theinvestigations.

Some key factors affecting the emulsion polymeriza-tions mentioned above were investigated in detail, bywhich the formula for polymerizations was optimized. [ 7 ] Nevertheless, for solid substituted acetylene mono-mers, how to effectively charge them into the aqueous

1. Introduction

Conjugated polymers have attracted a great deal of interestbecause of the unique structures and the remarkablypotential applications, in particular, as novel optical andelectrical materials. The recent progress in polymer chem-istry afforded the easy synthesis of (composite) nano-particles (NPs) constituted by conjugated polymers, forinstance, polyaniline, [ 1 ] polypyrrole, [ 2 ] polythiophene, [ 3 ] and so forth. However, for polyacetylene and the deriva-tives, even though large achievements have been made inthis intriguing research eld, [ 4 ] only few studies have been

found focusing on the preparation of NPs derived fromthem, [ 5 ] to which Mecking and co-workers [ 6 ] have maderemarkable contribution.

Macromol. Chem. Phys. 2012 , 213 , 603609 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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with deionized water before measurement. Number-averagemolecular weight ( M n ) and molecular weight distributions( M w / M n ) of the polymers were determined by gel permeationchromatography (GPC, Waters 515-2410 system) calibrated byusing polystyrenes as standard and tetrahydrofuran (THF) aseluent. Transmission electron microscopy (TEM) images weretaken on a Hitachi H-800 electron microscope. Before measuringTEM and CD spectra, the original polymer emulsions were dilutedwith water to a suitable concentration. Specic rotations weremeasured on a Jasco P-1020 digital polarimeter with a sodiumlamp as the light source at room temperature.

2.3. Monomer Synthesis and Emulsion Polymerization

Monomer 1 (M1 in Scheme 1 ) was synthesized following a previ-ously reported method. [ 10 ] The emulsion polymerization processwas introduced in detail earlier. [ 7 ] Taking H 2 O/DMF = 9/12 v/v asan example (Table 1 ), a typical procedure is stated as the following:

a 50 mL glass reactor equipped with a reux condenser, a stirrer,an N 2 inlet, and a dropping funnel was rst deaerated by purgingN2 for 30 min. A predetermined amount of Triton X-100 (1.52 g,2.4 mmol) and a given amount of deionized water ( 9 mL) wereadded in the reactor. The water used for polymerization was alsodeaerated for 30 min in advance. The aqueous solution was stirredunder a stirring rate of 350 rpm at 30 C for 15 min to allow theTriton X-100 to dissolve completely. During this process and thesubsequent polymerization, N 2 bubbling was maintained allthe time. A certain amount of M 1 (0.09 g, 0.33 mmol) was dissolvedin DMF ( 6 mL) in advance. The solution was purged with N 2 toexclude the dissolved oxygen. Afterward, this solution was addeddropwise into the reactor containing the emulsier solution. Thesolution was stirred at 350 rpm under nitrogen gas at 30 C for

10 min. By the end of this stage, homogeneous and clear emul-sion was obtained. It indicates that the hydrophobic monomermolecules diffused inside the micelles formed by the emulsier.Next, a predetermined amount of catalyst (0.0038 g, 7.4 mol)was dissolved in DMF (6 mL) beforehand and the solution wasbubbled with nitrogen. This solution was added dropwise in theabove reactor like the monomer/DMF solution. The dropping

polymerization system is still an open question. For thispurpose, we utilized a specic solvent which can bothsolubilize the hydrophobic monomer and dissolve inwater. We found that the content of this specic solventis one of the key factors determining the formation of

stable polymer emulsions. In the present study, we thusinvestigated in detail the effects of this special solvent onemulsion polymerization. In addition, we also found thatsome polymer emulsions show a blue shift in both cir-cular dichroism (CD) and UV-vis spectra (compared withthe corresponding polymer solution), whereas others didnot show this phenomenon. Therefore, the present articlealso discusses the occurrence of blue shift. Finally, basedon the present investigation and the earlier ones, [ 7 , 8 ] wepropose an aqueous catalytic emulsion polymerizationmechanism of substituted acetylene monomers. We areconvinced that the present article will be helpful for our

understanding more deeply the unique type of emulsionpolymerization and will be useful for further designingand preparing advanced conjugated materials.

2. Experimental Section

2.1. Materials

The solvents were distilled under reduced pressure in advance.1S -(+ )-10-Camphorsulfonyl chloride, phenylacetylene, propar-gylamine, pyridine, poly(ethylene glycol) tert -octylphenyl ether(Triton X-100), and sodium dodecylsulfate (SDS) were purchasedfrom Aldrich and used as received without further purication.

(nbd)Rh+ B

(C6 H5 )4 was prepared as reported.

[ 9 ]

2.2. Measurements

Circular dichroism and UV-vis absorption spectra were recordedon a Jasco 810 spectropolarimeter equipped with a JULABOheating immersion circulator. For emulsions, they were diluted

Table 1. Emulsion polymerization results of M 1 with varied H 2 O/DMF ratio (Monomer: 0.33 mmol, catalyst: 7.4 mol, monomer/TritonX-100= 1:7.3 (mol mol 1 ), temperature: 30 C, polymerization time: 1.5 h).

H2 O/DMF v/v Molecular weight of polymer Particle diameter b) [nm]

c) [ ]

Polymerization d)

M n a) M w / M n a)

18/3 e) precipitation15/6 e) precipitation12/9 9900 1.31 89.7 847 emulsion9/12 10 180 1.27 89.1 851 emulsion6/15 e) solutionMonomer 2 f) 25 700 2.80 90 solution

a) Determined by GPC (polystyrenes as standard, THF as eluent); b) Obtained according to TEM; c) Obtained from the obtained emulsions;d) The appearance of the product after polymerization; e) Stable emulsion could not be obtained; f) Monomer 2 is liquid; concentration5 10 2 M , catalyst concentration 10 3 M , monomer/SDS = 1/5 mol/mol, temperature 30 C, polymerization time 3 h.


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Accordingly, we investigated the inuence of DMF con-tent on the emulsion polymerization. The relevant poly-merization results are listed in Table 1 .

When the concentration of DMF was lower (H 2 O/DMF= 18/3 and 15/6 v/v in Table 1 ), M1 could indeed undergo

polymerization; however, the polymer emulsion wasunstable, since observable precipitation occurred in thesetwo cases. This indicates that DMF was not enough inamount to transfer all the monomer molecules into theemulsier micelles. It further means that DMF diffusestoo quickly in water before the monomer molecules (and/or Rh-catalyst) were all transferred inside the emulsiermicelles and some polymerization took place outside themicelles. Thus, the polymerization systems are not stableenough.

Table 1 shows that when the amount of DMF isincreased (H 2 O/DMF = 12/9 and 9/12 v/v), the formed

polymer emulsions became quite stable, without notice-able precipitation occurring. The emulsions can be storedfor at least half a year, without noticeable change. Theemulsions were characterized by TEM, CD, and UV-visspectroscopy techniques. These measurements will bediscussed in detail later on. The pure polymer from mon-omer 1 (with H 2 O/DMF = 12/9 v/v) was subjected to FTIRand 1 H NMR measurements, and the obtained spectra areshown in Figures S1 and S2 (see Supporting Information).

When DMF was further increased to H 2 O/DMF = 6/15 v/v, M 1 underwent solution polymerization ratherthan emulsion polymerization. This assumption wasmade according to the TEM images (to be discussedlater) in which no particles can be found. In addition, thepolymerization product seems more transparent thanthe others, as illustrated in Figure 1 . The photographs of polymerization product obtained with varied H 2 O/DMFratio are all presented in Figure 1 for a clear visual com-parison. Among the ve polymer samples, the rst two(A and B) contained precipitation at the bottom of thebottle, whereas C and D are similar to the earlier oneswith SDS as emulsier for the monomers emulsionpolymerization. [ 7 ] For the last one (E), it seems the mosttransparent, indicating to some degree its solution nature

speed was about 40 drops per minute. After the complete catalystsolution had been added, the stirring speed was slowed to about300 rpm to make the emulsion particles grow, and the polymeri-zation proceeded under N 2 at 30 C for 1.5 h.

At the beginning of polymerization, the solution was colorlessand transparent; with the occurrence of polymerization, thecolor of the solution changed slowly to yellow-green. Afterpolymerization, rotary evaporator was used to remove the waterand DMF in the obtained emulsion, providing a mixture of emulsier and polymer. Afterward, the product was placed inwater and stirred at room temperature for at least 5 min to makethe emulsier dissolve completely. Eventually, CHCl 3 solution wasltered, collected, and concentrated to provide the pure polymer.

3. Results and Discussion

3.1. Effects of DMF on Emulsion Polymerization

In the present study, M 1 and M 2 (shown in Scheme 1 ) areutilized to perform emulsion polymerization, providingpolymer 1 emulsion and polymer 2 emulsion from M 1 and M 2 , respectively. M 1 and M 2 are used because both of them are hydrophobic, and M 1 is a chiral solid monomer,whereas M 2 is an achiral liquid monomer. The emulsionof polymer 1 was found to demonstrate optical activity,because of the formation of helical structures of pre-dominant screw sense in polymer 1 . In addition, polymer 1 emulsion and polymer 2 emulsion differ largely withrespect to blue shifts in CD and UV-vis spectra, as to be dis-cussed later.

For performing emulsion polymerization in aqueousmedia, solid monomer 1 (M1 ) should be dissolved rst ina suitable organic solvent, which was then added in emul-sier (Triton X-100)/H 2 O aqueous system. This organicsolvent should be soluble in water and simultaneouslycan solubilize M 1 . DMF was proved to be a good choicefor this specic requirement. In the course of our experi-ments, we found that the content of DMF played a largerole in the emulsion polymerization under investigation.

Scheme 1. The structures of monomers 1and 2 .

Figure 1. Photographs of polymer 1emulsions prepared using var- ying H2 O/DMF ratios: (A) 18/3, (B) 15/6, (C) 12/9, (D) 9/12, (E) 6/15 v/v(for detailed polymerization conditions, see Table 1 ).


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subsequent investigations seem not to approve this view.In more detail, we ever thought that DMF should play arole of co-emulsier for the substituted acetylene mono-mers to undergo microemulsion polymerization. But asfar as a liquid monomer is concerned, such as M 2 (phe-nylacetylene in Scheme 1 ) to be involved in detail later,it can undergo emulsion polymerization without usingDMF as a co-emulsier or carrier. (The relevant data arepresented in Table 1 . The FTIR and 1 H NMR spectra of thepure polymer derived from monomer 2 are illustrated inFigures S3 and S4 in Supporting Information.) In addition,we ever tried both hydrophobic [(nbd)Rh + B (C6 H5 )4 ] andhydrophilic [Rh(cod) 2 BF4 ] catalyst for performing emulsionpolymerization, but only the former offered stable substi-tuted polyacetylene emulsions. [ 7 ] Therefore, the presentstudy together with the previous ones enables us to pro-pose a mechanism on this novel type of catalytic emulsionpolymerization in aqueous media. The proposed emulsion

polymerization mechanism is outlined in Scheme 3 .Even though a variety of emulsion polymerizations have

been accomplished with substituted acetylene monomersin our laboratory, the involved polymerization mechanismhas not yet been specically considered. On the basis of thepresent study and our earlier relevant studies, the under-lying mechanism will be analyzed herein. Still taking M 1 as representative, our consideration is schematically pre-sented in Scheme 3 . Emulsier (Triton X-100 or SDS) formsmicelles in water rst, when the monomer solution (e.g.,M1 in DMF) or liquid monomer (e.g., M 2 ) is added in this

rather than emulsion. The differences among the vesamples are considered as being originated from thevaried amount of DMF, as discussed earlier.

The TEM images in Figure 2 also provide further sup-port for our consideration above. In the TEM images of A and B, no regular spherical particles can be observed,demonstrating the formation of precipitation in these twosystems. For C and D, regular NPs are clearly observed.The NPs are of 80 nm in diameter. For the last samplewith H 2 O/DMF ratio being 6/15 v/v, nothing can beobserved in the TEM image (the image thus not shown).These observations veried our conclusions made on theeffects of DMF on emulsion polymerization. The aboveconsideration is illustratively presented in Scheme 2 .Briey, DMF is required to transfer solid M 1 and Rh-basedcatalyst inside the micelles in aqueous media. However,only DMF in an appropriate amount is favorable for themonomer to undergo emulsion polymerization, providing

stable polymer emulsion. This nding is helpful for us tocontinue our research dealing with emulsion polymeriza-tion of substituted acetylene monomers.

3.2. Emulsion Polymerization Mechanism

Initially, we thought that the catalytic emulsion poly-merization stated above should be considered asmicroemulsion polymerization. [ 7 ] Nevertheless, the

Figure 2. TEM images of polymer 1 emulsions prepared using different H 2 O/DMF ratios: (A) 18/3; (B) 15/6; (C) 12/9; (D) 9/12 v/v (for poly-merization conditions, see Table 1 ).

Scheme 2. Schematic illustration for polymerization of M 1 withvariying H2 O/DMF ratios.

Scheme 3. Proposed emulsion polymerization mechanism to formhelical substituted polyacetylene NPs.


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be assigned to the phenyl group in theemulsier Triton X-100. This strongabsorption shields the absorption of thepolymer in the emulsions.

To explore the underlying reason for

the blue shifts occurring in Figure 3 , wefurther investigated M 2 in a similar wayfor conducting emulsion polymerizationof M1 . M2 smoothly underwent emul-sion polymerization, as evidenced bythe TEM images (A) and photograph (B)in Figure 4 . In this case, the NPs are reg-ular, with 90 nm in diameter. Figure 4 Bshows the yellowish color in polymer 2 emulsion, much different from the ones

in Figure 1 . We think that the difference in emulsion coloris originated in the different effective conjugation length

of the polymer main chains. This will be involved againbelow. The M n and M w / M n of polymer 2 thus obtainedwere 25 700 and 2.80, respectively.

Figure 5 presents the UV-vis absorption spectra of polymer 2 in the forms of both emulsion and solution (inCHCl3 ). The insert is an enlarged spectrum of polymer 2 in CHCl3 solution. From the spectra in Figure 5 and refer-ring to the reports, [ 7 , 8 , 11 , 13 , 14 ] we conclude that in the stateof NPs, polymer 2 also adopted helical conformations, justlike polymer 1 discussed above. Nevertheless, polymer 2 in solution did not show CD effect, because there is nochiral factor in this polymer, and the helices formed in itare composed of both right- and left-handed screw sensein equal amount. For the same reason, polymer 2 emul-sion failed to show CD signal in CD spectra either.

We can nd an interesting difference between thespectra in Figure 3 and Figure 5 . In more detail, both theCD and UV-vis spectra of polymer 1 emulsion demon-strated a remarkable blue shift when compared with thecorresponding spectra of polymer 1 in solution (men-tioned above). However, such a blue shift is not observed inpolymer 2 emulsion (vs. polymer 2 solution). The blue shiftwas also found in emulsication of helical substituted poly-acetylenes, which was specically investigated earlier. [ 15 ]

The above-mentioned blue shift can be explained by

Scheme 4 A. We considered that whether the blue shift

aqueous system, the hydrophobic monomer moleculesdisperse into the micelles. When the hydrophobic Rh

catalyst is added in, its molecules also disperse inside themicelles, in which Rh catalyst initiates the polymerizationof monomer. Finally, substituted polyacetylene NPs emul-sions are generated. More interestingly, the polymers con-stituting the NPs can adopt helical structures just like thecorresponding polymer chains in solution. Such polymeremulsions exhibited considerable optical activity, as willbe observed in polymer 1 emulsion.

3.3. Blue Shifts in CD and UV-vis Spectra

With stable emulsions in hand, we further characterizedthe polymer emulsions with CD and UV-vis spectroscopy,aiming at elucidating the secondary structures of the sub-stituted polyacetylene forming the NPs. Herein, it shouldbe stressed that CD and UV-vis spectroscopies have beenproved highly efcient for exploring the helical conforma-tions of substituted polyacetylenes in solution, [ 7 ] NPs,[ 8 , 11 ] and (hydro)gel state. [ 12 ] For polymer 1 emulsions, the rele-vant spectra are presented in Figure 3 A (CD) and 3B (UV-visabsorption spectra).

For the two polymer emulsions, both of them showedintense CD signal around 300 nm (Figure 3 A). Accordingto our proceeding investigations and the specic opticalrotations in Table 1 , the CD spectra in Figure 3 A demon-

strate the helical structures formed in the substitutedpolyacetylenes constituting the NPs. However, the CDspectra showed a considerable blue shift when comparedwith the corresponding CD spectra of the same polymersolution reported in our earlier article. [ 10 ] (In solution, theCD spectra appeared around 410 nm. [ 10 ] ) It is important topoint out that some of the helical substituted polyacety-lenes did not show such a blue shift, for example, polymer2 , which will be discussed in detail in next section. Thedriving force for the blue shifts will also be consideredbelow, taking polymers 1 and 2 as two representatives. InFigure 3 B, the strong absorption around 275 nm should

Figure 3. (A) CD and (B) UV-vis spectra of polymer1 emulsions with varied H 2 O/DMFratio (for detailed polymerization conditions, see Table 1 ). For measuring CD and UV-visspectra, the original emulsions were diluted 1:20 with water ( 0.8 10 3 M by monomerunit). The spectra were recorded at 25 C.

Figure 4 . (A) TEM image and (B) photograph of polymer 2 emul-sion (for polymerization conditions, see Table 1 ).


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occurs or not predominantly depends on the substituentgroups in the side chains of the polyacetylenes. For thepolymers with relatively less bulky pendent groups, thepolymer backbones can be compacted to some degree inthe course of forming NPs. This leads to the shortened

Scheme 4 . Illustration of blue shift occurring in helical substitutedpolyacetylene emulsion. (A) The substituted pendant groups arenot bulky enough and in the formation of NPs, compacting of thepolymer chains occurs, resulting in blue shifts in CD and UV-visspectra. (B) The bulky pendant groups prevent the polymer chainsfrom such a considerable compact.

effective conjugation length of the polymer main chains,as reected by blue shifts in the CD and UV-vis spectrain Figure 3 . This type of substituted polyacetylene is wellexemplied by polymer 1 in the present study.

On the other hand, for the polymers with relatively

bulky or rigid pendent groups, when the polymers formNPs, the bulky pendent groups will prevent the polymermain chains from compacting so tightly as in the poly-mers with not-so-bulky pendent groups. Consequently,the effective conjugation length of the polymer back-bones will change little in NPs state when compared withthe same polymer in solution. This idea is schematicallyillustrated in Scheme 4 B. Polymer 2 in the current study issuch a substituted polyacetylene.

4. Conclusion

Monomers 1 and 2 smoothly underwent catalytic emulsionpolymerization in aqueous media. A specic organic sol-vent (DMF in the current study) is required for introducingsolid monomer 1 into the emusier micelles. The amountof this solvent played a crucial role for forming the stablepolymer emulsion. The as-prepared emulsions exhibitedconsiderable optical activity because of the substitutedpolyacetylene chains adopting helical structures with pref-erential screw sense. Blue shifts were observed in polymer1 emulsion. For polymer 2 emulsion, no such a blue shifttook place. The underlying driving force for the occurrenceof blue shifts was put forward. According to the present

study and the earlier related investigations, a polymeriza-tion mechanism was provided for the catalytic emulsionpolymerizations of substituted acetylene monomers.

Supporting Information

Supporting Information is available from the Wiley Online Libraryor from the author.

Acknowledgements : This work is supported by the NationalNatural Science Foundation of China (21174010, 20974007,20574004), the Fundamental Research Funds for the CentralUniversities (ZZ1117), and the Project of Polymer Chemistryand Physics, Beijing Municipal Commission of Education.

Received: November 3, 2011 ; Revised: December 14, 2011;Published online: March 2, 2012; DOI: 10.1002/macp.201100610

Keywords: blue shifts; chiral; emulsion polymerization; helicalpolymers; substituted polyacetylenes

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Figure 5. UV-vis spectra of polymer 2 emulsion and solution(insert). (For the conditions for emulsion polymerization, seeFigure 4). The concentration for measuring UV-vis absorbance:emulsion after dilution with water by 250 times ( 0.2 10 3 M );solution, 0.1 10 3 M . Temperature, 25 C.


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