branched polymer via free radical polymerization of chain transfer monomer: a theoretical and...

Branched Polymer Via Free Radical Polymerization ofChain Transfer Monomer: A Theoretical andExperimental Investigation

JIANHUA LIU,1 YUSONG WANG,2 QI FU,1 XIANGYANG ZHU,1 WENFANG SHI1

1Department of Polymer Science and Engineering, Joint Laboratory of Polymer Thin Films and Solution,University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

2Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China,Hefei, Anhui 230026, People’s Republic of China

Received 19 July 2007; accepted 17 October 2007DOI: 10.1002/pola.22484Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A simple mathematic model for the free radical polymerization of chaintransfer monomers containing both polymerizable vinyl groups and telogen groupswas proposed. The molecular architecture of the obtained polymer can be prognosti-cated according to the developed model, which was validated experimentally byhomopolymerization of 4-vinyl benzyl thiol (VBT) and its copolymerization with sty-rene. The chain transfer constant (CT) of telogen group in a chain transfer monomeris considered to play an important role to determine the architecture of obtained poly-mer according to the proposed model, either in homopolymerization or copolymeriza-tion. A highly branched polymer will be formed when the CT value is around unity,while a linear polymer with a certain extent of side chains will be obtained when theCT value is much bigger or smaller than unity. The CT of VBT was determined to bearound 15 by using the developed model and 1H NMR monitored experiments. Theobtained poly(VBT) and its copolymers were substantiated to be mainly consisted oflinear main chain with side branching chains, which is in agreement with the antici-pation from the developed model. The glass transition temperature, number averagemolecular weight, and its distribution of those obtained polymer were primarilyinvestigated. This model is hopefully to be used as a strategy to select appropriatechain transfer monomers for preparing hyperbranched polymers. VVC 2008 Wiley Periodi-

cals, Inc. J Polym Sci Part A: Polym Chem 46: 1449–1459, 2008

Keywords: chain transfer; highly branched; hyperbranched; modeling; polystyrene;radical polymerization

INTRODUCTION

Telomers are usually achieved through the telo-merization by using transfer agents to controlboth the molecular weights and the chain endfunctionality. Many compounds can be used as

chain transfer agents in free radical polymeriza-tion. The most commonly used transfer agentsare halogens and mercaptans.1–3 Normally, atransfer agent, that is, telogen, used in telomeri-zation bears a reactive functional group such ashydroxyl, carboxylic or amino group, and so on.Besides their capacity to react as chain transferagents, these molecules also supply well-definedend groups as mentioned above to the final poly-mers.4 For a long time, telomerization remained

Correspondence to: W. Shi (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 1449–1459 (2008)VVC 2008 Wiley Periodicals, Inc.

1449

primary technique allowing for the synthesis oftelechelic oligomers before the introduction ofliving radical polymerization technique.3,5 Todate, few studies on telomerization have beenreported, mostly by Boutevin and coworkers,whose work focused on the synthesis of macro-monomers.6,7

Recently, one-pot synthesis of highly branchedpolymers has received considerable attentiondue to their ease of preparation when comparedwith the step by step synthesis of dendrimers.8,9

Many synthesis approaches using living radicaltechnique have been investigated for preparinghighly branched polymers based on the strategyof self-condensing vinyl polymerization.10–15

Moreover, Sherrington and coworkers16–20 havedeveloped a facile and generic strategy for syn-thesizing highly branched vinyl polymers viaconventional free radical polymerization thatuses only low cost and readily available startingmaterials. The strategy involves the conven-tional free radical polymerization of a vinylmonomer in the presence of a multifunctionalvinyl comonomer using mercaptans as chain-transfer agents to prevent crosslinking.

Chain transfer monomers process a polymeriz-able vinyl group and a telogen group. In its freeradical polymerization, low molecular weightoligomers having a terminal double bond, whichcan be used as macromonomers in the followingpolymerization, will be generated through the tel-omerization of telogen groups. As the process goeson iteratively, branched polymer will be obtainedvia one-pot free radical polymerization of a chaintransfer monomer. The polymerization process isanalogous to the use of an AB2 monomer in con-densation polymerization. No gelation occurs theo-retically, which was discussed by Flory.21

To the best of our knowledge, the work on thepolymerization of chain transfer monomers wasreported seldom. Very few early literaturedescribed the formation of long chain branchesin the free radical polymerization of styrene.22

The branching density and theoretical molecularweight of obtained polystyrene were computedin theory using polymerization kineticsmethod23,24 and Monto Carlo simulation.25 How-ever, the abstruse mathematic methods wereused and the formation of short branchingchains from the chain transfer monomers hasnot been investigated. Recently, a US patent hasreported the preparation of a series of highlybranched polymers via the telomerization ofvinyl monomers containing haloid groups.26 At

the same time, Gaynor has reported the vinylchloride as a chain transfer monomer in olefinpolymerization to prepare highly branched poly-olefins.27 However, the correlation of the mono-mers with the structure of finally obtained poly-mers needs to be further discussed. Therefore, itis necessary to investigate the polymerization ofchain transfer monomers as well as the corre-sponding polymer structures.

In this study, the free radical polymerizationof chain transfer monomers was investigatedboth theoretically and experimentally. A simplefundamental equation was proposed to simulatethe change of average chain length stemmingthiyl radical during the polymerization. Accord-ing to the analytical solution of the equation, thechain transfer constant (CT) of a chain transfermonomer plays an important role in the determi-nation of molecular architecture of the obtainedpolymer. A highly branched polymer will beformed when the value of CT is around unity. Amethod for the determination of CT was devel-oped according to the proposed mathematicalmodel combined with NMR monitored measure-ment. 4-Vinyl benzyl thiol (VBT) was synthesizedas a chain transfer monomer and its chain trans-fer constant was estimated. The homopolymeriza-tion of VBT and its copolymerization with styreneat different feed ratios were investigated. Themolecular architectures of obtained polymers arebasically in agreement with the prediction fromthe developed mathematical model.

EXPERIMENTAL

Materials

4-Chloromethyl styrene was used as receivedfrom Aldrich. Azobis(isobutyronitrile) (AIBN)was recrystallized twice from ethanol and driedunder vacuum at room temperature and storedat �4 8C. Styrene was washed with 10% alka-line solution and then distilled water, followedby dried with calcium chloride and distilled atreduced pressure. The middle fraction was col-lected and kept under dark at 0 8C. Thiourea, p-methoxylphenol, anhydrous sodium sulfate andall solvents were supplied by Shanghai First Re-agent and used as received.

Characterization

The 1H and 13C NMR measurements were con-ducted on a Bruker 300-MHz NMR spectrometer

1450 LIU ET AL.

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

with CDCl3 or benzene-d6 as solvents. The FTIRspectra were recorded on a Nicolet MAGNA-IR750 spectrometer. The molecular weights andtheir distributions were determined by a Watersgel permeation chromatography (GPC) systemequipped with a refractive index detector, usingchloroform as an eluent, and calibrated usingpolystyrene standards. The differential scanningcalorimetry (DSC) measurement was carried outwith Shimazhu DSC-60 equipment. The sampleswere first heated at 10 8C/min from �60 to150 8C under nitrogen, then cooled to �60 8C at40 8C/min, and immediately heated at 10 8C/minfrom �60 to 150 8C again. The X-ray diffraction(XRD) measurement was performed using aRigaku D/Max-rA rotating anode X-ray diffrac-tometer equipped with a CuKa tube and Ni filter(k ¼ 0.1542 nm).

To perform the NMR monitored experiments,in a typical procedure, all reagents includingVBT (116 lL, 0.8 mmol), styrene (228 lL, 2mmol), AIBN (4.5 mg) were dissolved in 0.6 mLbenzene-d6, and then added directly to the NMRtube attached to a Schlenk line, where the solu-tion was subjected to be degassed for threefreeze-pump-thaw cycles. After the final cycle,the NMR tube was sealed, followed by NMRmeasurement at 70 8C for 2 h. Every 5 min, aspectrum was recorded. The signals of all aro-matic protons between d7.0–8.0 ppm were inte-grated as the internal calibration.

Synthesis

Synthesis of 4-Vinyl benzyl thiol

VBT was synthesized from 4-chloromethyl sty-rene via thiourea method.28 About 31.1 g of 4-chloromethyl styrene (0.2 mol), 21.1 g thiourea(0.27 mol), 250 mL of methanol, and 0.5 g p-methoxylphenol (as an inhibitor) were addedinto a three-necked flask equipped with a refluxcondenser, and stirred at 80 8C for 12 h undernitrogen. 200 mL of 20% solution of sodium hy-droxide was added to the above reactant mix-ture after cooled to room temperature, then im-mediately heated to 80 8C for 6 h. Finally, theresulting solution was cooled to room tempera-ture and 200 mL of chloroform was added. Theorganic phase was separated and washed withdistilled water until neutral, then dried by an-hydrous sodium sulfate. The crude product as ayellowish liquid was obtained by removing thedesiccant and organic solvent. After distilled at

reduced pressure, VBT was obtained as a color-less liquid with a yield of 78%. The freshlydistilled VBT should be stored in a brown glassbottle in a refrigerator. Caution: VBT has an ob-noxious odour. IR (cm�1, KBr): 3085, 3025, 2924,2850 2560, 1628, 1601, 1509, 1492, 1452, 1404,1028, 989, 908, 842, 757, 699; 1H NMR (ppm,CDCl3): 7.37(d, 2H), 7.27 (d, 2H), 6.73 (q, 1H),5.74(d, 1H), 5.25(d, 1H), 3.75 (d, 2H), 1.76(t,1H); 13C NMR (ppm, CDCl3): 140.7, 136.5,136.4, 128.2, 126.5, 113.8, 28.7.

Polymerization procedure

A typical polymerization was carried out as fol-lows. VBT (1.5 g, 10 mmol), styrene (2.6 g, 25mmol), and AIBN (1 mol % relative to doublebond) were added sequentially into a 10-mLSchlenk flask, and then degassed for threefreeze-pump-thaw cycles, flushed with nitrogen,and finally sealed off. After the polymerizationreaction proceeded at 70 8C for 2 h, the productwas dissolved in chloroform and precipitatedinto methanol three times, and finally dried at40 8C in vacuum for 24 h to give a white solid.The yield was determined by gravimetry.

RESULTS AND DISCUSSION

Theoretical Consideration

As well known, the mechanism of telomerizationis different from that of conventional radical po-lymerization.29 Ideally, the chain growth startswith the addition of a telogen radical to the firstmonomer rather than the radical stemmingfrom an initiator. The propagation proceeds bythe addition of a certain number of monomers,while the chain termination takes place typi-cally by the chain transfer reaction to telogengroups.

Chain transfer monomers contain a polymer-izable vinyl group and a telogen group, such asthiol group, and mainly undergo the polymeriza-tion routes as shown in Scheme 1. Firstly, athiyl radical (A) transferred from the radical viainitiator decomposition is formed, and thenreacts with the double bond of another chaintransfer monomer or added comonomer, forminga dimer carbon radical (B). The formed radicalB will further undergo two different reactionsdepending on the rate of chain transfer reaction(Rtr) and the rate of chain propagation (Rp). If

POLYMERIZATION OF CHAIN TRANSFER MONOMER 1451


Rtr > Rp, the chain transfer takes place, result-ing into the formation of a dormant dimer (C)with a polymerizable terminal double bond anda new thiyl radical. If Rp > Rtr, the chain propa-gation takes place, which results into the forma-tion of an oligomer containing a carbon radical(D). The further reaction of species D dependsupon the instantaneous rates of chain growthand chain transfer. As the polymerization pro-ceeds, the thiyl and carbon radicals will repeatthe reactions as described above. Therefore, thepolymerization reaction undergoes a complexmechanism that combines step growth withchain growth. As a result, highly branched poly-mer might be obtained.

By assuming that all double bonds have equalreactivity and the chain termination takes placeby the chain transfer manner typically, thereare four species including the thiol and vinylgroups as well as the thiyl and carbon radicalsin a quasi equilibrium during the polymerizationreported by Bowman and coworkers.30,31 Thegoverning equations are given in eqs 1–4:

d½SH�dt

¼ �ktr½SH�½C�� ð1Þ

d½C ¼ C�dt

¼ �ki½C ¼ C�½S�� kp½C ¼ C�½C�� ð2Þ

d½S��dt

¼ ktr½SH�½C�� ki½C ¼ C�½S�� ð3Þ

d½C��dt

¼ �ktr½SH�½C�� þ ki½C ¼ C�½S�� ð4Þ

where [SH], [C ¼ C], [C�], and [S�] denote theconcentrations of thiol and vinyl groups and car-bon and thiyl radicals, respectively. The ktr, ki,and kp are the reaction constants of chain trans-fer, thiyl radical initiation, and carbon radicalpropagation, respectively. Equation 1 accountsfor the consumption of thiol functional group bychain transfer to a carbon radical, where theconsumption by chain transfer to radicalsformed by the decomposition of initiator is negli-gible. Equation 2 accounts for the consumptionof vinyl group via the initiation for thiyl radialsand the propagation for carbon radicals. Equa-tions 3 and 4 describe the concentrations of thiyland carbon radicals, containing the generationand consumption by thiyl radical initiation andcarbon radical transfer. A pseudo steady-stateconcentration of the radials is assumed duringthe polymerization. Therefore, eqs 1–4 can besimplified into eqs 5 and 6.

ktr½SH�½C�� ¼ ki½C ¼ C�½S�� ð5Þ

d½SH�d½C ¼ C� ¼

1

1þ kp½C¼C�ktr½SH�

ð6Þ

Equation 5 has the same form as that for therelationship of two different monomer radicalsin a copolymerization system, which can begiven by statistical derivation without invokingthe steady-state assumption.32 Equation 6 givesa ratio of the consumption rates of vinyl to thiolfunctional group.

In traditional free radical telomerization, Mayoequation (as shown in eq 7) gives the degree of poly-merization as a function of the transfer constant CT,whereXn is number average degree of polymeriza-tion, 1=Xn0 is the value of 1=Xn in the absence ofchain transfer agent.33 When a large amount ofchain transfer agent is added, 1=Xn0 is negligibleand eq 7 can be simplified into eq 8.

1

Xn¼ 1

Xn0þ CT

½S�½M� ; CT ¼ ktr

kpð7Þ

CT½S�½M� >>

1

Xn0;

1

Xn� CT

½T�½M� ð8Þ

According to the proposed assumptions, allthe propagation chains start from the thiyl radi-

Scheme 1. Schematic illustration for the ideal poly-merization route of a chain transfer monomer.

1452 LIU ET AL.


cals. It is important to define the number aver-age chain length from every thiyl radical (Sn),which means the possible branched chain lengthformed during the polymerization. The relation-ship between Sn and Xn can be formulated as eq9 according to eq 6, which is easy to be under-stood that the carbon propagating radical is gen-erated after the addition of a thiyl radical to avinyl group.

Sn ¼ d½C ¼ C�d½SH� ¼ Xn þ 1 ð9Þ

It is assumed that the ratio of added comono-mer, which has the same reactivity for all unsat-urated groups, to the chain transfer monomer isa, and the x denotes the conversion of thiolgroup. Therefore, Sn is a function of x, whichwill be changed as the polymerization proceeds,regarding as Sn(x). From eqs 8 and 9, the funda-mental eq 10 can be proposed. Sn(0) ¼ 1 þ (a þ1)/CT, 0 � x � 1 and Sn(x) � 1 are considered tobe the initial values.

SnðxÞ ¼aþ 1� R x

0 SnðxÞdxð1� xÞ:CT

þ 1 ð10Þ

Therefore the analytical solution of eq 10 canbe given as eq 11:

SnðxÞ ¼ CT

CT � 1þ aþ 1

CTþ 1

1� CT

� �ð1� xÞ 1

CT�1

ð11Þ

From eq 11, the relationship between doublebond conversion, regarding as y, and thiol groupconversion x can be formulated as eq 12.

y ¼R x0 SnðxÞdx1þ a

¼ 1� CT

1þ að1� xÞ½SnðxÞ � 1� ð12Þ

The denominator in eq 11 is zero when CT

equals one. Therefore, there is no solution of eq10 at that point. However, in practical, the valueof 0.99 or 1.01 can be used to approach to thepoint of unity. Almost the same curve will beobtained according to eq 11 when CT is 0.99 and1.01. Actually, it is a homogeneously polymeriza-tion where the chain transfer monomer can beregarded as an ideal AB2 monomer.23

When the feed ratio (a) is set as zero, that is,the homopolymerization of chain transfer mono-

mer takes place, the curves of Sn versus the con-version of thiol group x are plotted as shown inFigure 1 according to eq 11.

From Figure 1, the conversion of thiol groupreaches to a maximal value as the Sn decreasesto unity. The maximal conversion of thiol groupis determined by the CT value of a chain trans-fer monomer. The smaller the CT value is, thesmaller the maximal conversion of thiol groupcan be achieved, and the more sharply the valueof Sn decreases as the polymerization proceeds.When the CT value is far smaller than unity, thepolymerization behavior approaches the conven-tional radical polymerization. The obtained poly-mer has a linear main chain with pendant thiolfunctional groups. When the CT value is far big-ger than unity, the value of Sn is nearly unity atany stage of polymerization. Therefore, a linearpolymer will be formed via step growth polymer-ization mechanism. When the CT value is aroundunity, Sn maintains a lower value of around twoduring polymerization. Consequently, a highlybranched polymer will be preferred to form.Summarizing the above discussion, Scheme 2shows the ideal molecular architectures ofpolymers obtained by homopolymerization ofthe chain transfer monomers with different CT

values.When the feed ratio (a) is set as a value

beyond zero, the copolymerization of a chaintransfer monomer with another comonomer hav-ing the equal vinyl reactivity takes place. Ifa comonomer with unequal vinyl reactivity

Figure 1. Calculated number average chain lengthstemming from the thiyl radical of chain transfermonomers with different chain transfer constants(a ¼ 0, CT ¼ 0.1, 0.5, 2, and 10).



compared with VBT is used, the reactivity ratiosof two monomers should be considered in thecopolymerization as well as the different CT val-ues of the thiol group for two unequal reactivityvinyl groups, which makes the copolymerizationprocess more complex. Figure 2 shows the Sn

value versus the conversion of thiol group x aswell as the conversion of double bond y at differ-ent feed ratios of monomers with different chaintransfer constants.

The similar conclusions for the copolymeriza-tion as the homopolymerization of a chain trans-fer monomer can be drawn when CT value isaround unity. The copolymerization process isanalogous to that of AB2 and AB monomerscopolymerization in traditional condensationsystems. However, an important difference fromthe homopolymerization is that the thiol groupwill undergo a full conversion, while the doublebond will achieve a maximal conversion as cal-

culated from eq 11 when CT > 1. The bigger thefeed ratio is, the lower the maximal conversionof double bond is achieved. In fact, the higherconversion of double bond can be obtainedexperimentally than calculated one. This can beexplained that the polymerization directly initi-ated by the radical decomposed from initiatorrather than the thiyl radical, that is, the copoly-merization of unreacted monomer with theformed macromonomer, which has a polymeriz-able terminal double bond, will take placepredominately after most thiol groups wereconsumed.

From Figure 2, it can be seen that when thehigher feed ratio of comonomer to chain transfermonomer is added into the system, the Sn valuechanges more steeply during the polymerization,which indicates that the more irregular linearchains and the lower degree of branching will beobtained. Therefore, the feed ratio of comonomerto transfer monomer is preferred to be between0 and 5 for copolymerization to prepare a highlybranched polymer.

Experimental Determination of Chain TransferConstant

From above discussion, the molecular architec-ture of finally obtained polymer is determinedby the CT value of a chain transfer monomer.There have been several methods offering todetermine CT values. The most widely usedapproach was described as a general equationby Mayo and coworkers.33 However, thismethod is only applied for the systems withknown average polymerization degrees of thepolymers, which are obtained at lower conver-sions and constant concentrations of monomersand initiators. O’Brien and Gornick proposedan approximate method for the determinationof CT in which the concentration of thiol groupis related to the reaction progress, as given aseq 13.34

ln½T�0½T� ¼ CTln

½M�0½M� ð13Þ

In eq 13, the subscript 0 denotes the initialconcentrations of double bond and thiol group.Thus, plotting ln([T]0/[T]) versus ln([M]0/[M])permits to calculate the CT value from the slopeof the obtained straight line.

Equation 13 can be derived from eq 6. When[C ¼ C] < CT � [SH], eq 6 can be approximately

Scheme 2. Schematic illustration for ideal architec-tures of polymers obtained by homopolymerization ofchain transfer monomers with different CT values.

1454 LIU ET AL.


expressed as eq 14. Integrating the eq 14, thatis eq 13.

d½C ¼ C�d½SH� � 1

CT

½C ¼ C�½SH� ;

that is;d½C ¼ C�½C ¼ C� ¼ d½SH�

CT½SH�

ð14Þ

In this study, real time 1H NMR spectroscopywas used to monitor the polymerization process.The relationship between the conversions ofdouble bond and thiol group can be establishedas the polymerization proceeds. However, toobtain the reliable data from the 1H NMR spec-tra, the concentrations of double bond and thiolgroup are preferred to maintain at a same mag-

nitude. Therefore, the precondition of [C ¼ C] �CT � [SH] is always not tenable, especially whenthe CT value is much bigger than unity. The CT

value calculated by O’Brien method will be inac-curate in some extent. However, the relationshipof the conversions of two functional groups fol-lows eq 12. The CT value can be evaluated bycomparing the conversion curves obtained fromthe 1H NMR spectra with simulated curves cal-culated from eq 12 at different reaction condi-tions.

VBT was synthesized and investigated as atypical chain transfer monomer. Styrene, whichcan be regarded to have an equal reactivity ofdouble bond with VBT approximately, was usedas a comonomer. So far, no literature hasreported the CT value of VBT, while the CT

Figure 2. Calculated number average chain length of copolymerization of a chaintransfer monomer with an equal reactivity comonomer at different chain transferconstants [CT ¼ 10 (a), 2 (b), 0.1 (c), and 0.5 (d)] and different feed ratios [a ¼ 0 ( ),1 (u), 3 (*), 5 (~), 10 (!)].



value of benzyl thiol for the polymerization ofstyrene was reported to be 25.35 To determinethe CT value of VBT, the 1H NMR monitoredexperiments were carried out at two differentfeed ratios of 1:2.4 and 1:9.0 for VBT to styrene,respectively. Figure 3 shows the 1H NMR moni-tored spectra with the feed ratio of 1:2.4 at dif-ferent reaction times. The signals of doublebond and thiol group are assigned. From the 1HNMR monitored spectra, the consumption of cor-responding functional groups during the poly-merization was shown in Figure 4. According toeq 13, the value of CT can be determined by

O’Brien method as seen in Figure 5. For thefeed ratio of 1:2.4, the value of CT was calculatedto be 4.5, while 9.5 for the feed ratio of 1:9.0,which are not accordant with each other. Thereason is mainly due to that the precondition ofO’Brien method is not available as discussedabove.

According to eq 12, the conversion of doublebond (y) is a function of the conversion of thiolgroup (x). The CT value is related to the conver-sions of both groups. By comparing the experi-mental curve with simulated curve with differ-ent CT values, it can be found that the curvesare greatly overlapped when the value of CT wasset as 15, as shown in Figures 6 and 7. Conse-

Figure 3. 1H NMR monitored spectra for copolymer-ization of VBT with styrene with a feed ratio of 1:2.4at different reaction times. The feed ratio was deter-mined from the integrations of corresponding signalsin the NMR spectrum at 0 min.

Figure 4. 1H NMR monitored curves for consump-tions of functional groups versus reaction time withdifferent feed ratios. [Feed ratio of VBT and styrene:1:2.4 (a); 1:9.0 (b)].

Figure 5. Evaluation of CT value according toO’Brien method by copolymerization of VBT with sty-rene at different feed ratios. (!: feed ratio of 1:9.0;~: feed ratio of 1:2.4).

Figure 6. Computer simulated and experimentallymeasured conversions of double bond versus conver-sion of thiol group at the feed ratio of 1:2.4.

1456 LIU ET AL.


quently, the CT value of VBT was determined tobe around 15.

Therefore, a method for determining the CT

values of chain transfer monomers was devel-oped according to the proposed model and 1HNMR monitored experiments. Furthermore, it isalso supposed to be suitable for determining theCT values of ordinary transfer agents after asmall modification of replacing 1 þ a with a ineq 12. This method can be easily used and leadsto accurate results especially in determining theCT value of a transfer agent having a higherchain transfer constant.

Homopolymerization of VBT andCopolymerization with Styrene

From the above discussion for Figures 6 and 7,the CT value of VBT was determined to be 15.

According to the developed mathematical model,the homopolymerization of VBT will result inthe formation of quasi-linear polymer via a stepgrowth mechanism as shown in Scheme 3. Afterthe polymerization was carried at 70 8C for 2 h,a white powder was obtained by precipitationand purification. The 1H NMR spectra (Fig. 8)confirmed the formation of linear benzyl ethyl-ene sulfur ether main chain containing fewbranching chains. The number average moleculeweight and its distribution determined by GPCwere 2100 and 2.15, respectively. The lower mo-lecular weight is mainly because of the earlyprecipitation of the polymer from the solution ata low conversion during the polymerization. It ispresumed that the strong intermolecular inter-action of benzyl ethylene sulfur ether mainchains makes the polymer easily aggregated.The XRD analysis and DSC measurement con-firmed the obtained polymer as semicrystalline,which has a Tg of 16.7 8C and a Tm of 89.7 8C.

For the copolymerization with styrene,according to the developed mathematical model,the conversion of double bond and the numberaverage degree of polymerization versus the con-version of thiol group are simulated, as shownin Figure 9 when the feed ratio is set as 1:2.5.The number average degree of polymerizationfor every thiyl radical maintains a very lowvalue (<2), and increases steeply only aftermore than 95% thiol groups were consumed,whereas the conversion of double bond is lessthan 50%. Therefore, the polymerization proce-dure can be divided into two steps approxi-mately as shown in Scheme 4. The first step is

Figure 7. Computer simulated and experimentallymeasured conversions of double bond versus conver-sion of thiol group at the feed ratio of 1:9.0.

Scheme 3. Schematic illustration for homopolymeri-zation of VBT.

Figure 8. 1H NMR spectra of the homopolymer ofVBT and its copolymer with styrene with a feed ratioof 1:2.5.



regarded as the telomerization of VBT with sty-rene; the formed oligomers are mainly dimersand trimers with terminal double bond, whichcan be demonstrated by 1H NMR measurements(Figs. 3 and 8). After most thiol groups wereconsumed, the second step is regarded as thecopolymerization of unreacted styrene with theformed dimers and trimers. The final obtainedpolymer is supposed to have a comb-like archi-tecture with short pendant chains bearing atthe polystyrene-type main chain.

When the feed ratio of VBT to styrene varies,the similar analysis method can be adoptedapproximately. If more styrene is added, the av-erage chain length of the oligomer generated inthe first telomerization step increases slightly.However, the component ratio of unreacted sty-

rene to formed macromonomer for the secondcopolymerization step increases sharply, result-ing in more styrene segment copolymerized inthe linear main chain.

The molecular weights and their distributionsof the obtained copolymers determined by GPCare listed in Table 1. No precipitation occursduring the copolymerization. The molecularweights of obtained copolymers are far biggerthan that of homopolymer. These copolymershave broad distributions due to the nature offree radical polymerization when a chain trans-fer agent is added.7,16

The glass transition temperatures of obtainedcopolymers determined by DSC are listed inTable 1. It can be found that the Tg of thecopolymers are quite lower than that of amor-phous linear polystyrene, which was reported tobe 85 8C.35 The reason for the lower Tg values ispresumed to be the irregular short thioethertype side chains existing in the copolymers, andthe broad distributions of molecular weights.

CONCLUSIONS

A strategy for using chain transfer monomers toproduce polymers by radical polymerization wasinvestigated theoretically in this study. A math-ematical model was developed to describe thepolymerization process. For the homopolymeri-zation of a chain transfer monomer, the conver-sion of double bond is higher than that of thiolgroup. An ideal hyper-branched polymer is sup-posed to be obtained when the CT value of atransfer monomer is around unity. If the CT

Figure 9. Computer simulated curves for numberaverage degree of polymerization and conversion ofdouble bond versus conversion of thiol group (CT ¼15, feed ratio ¼ 2.5).

Scheme 4. Schematic illustration of copolymeriza-tion of VBT with styrene.

Table 1. Data for Homopolymer of TMSt and itsCopolymers with Styrene at Various Feed Ratios

Feed Ratio(mol) [St]:[TMSt] Mn

a PDI Tg

Yield(%)

0:1 2100b 2.15 16.7/89.7c 431:1 45,000 5.25 27.7 571.75:1 59,600 6.10 30.6 612.5:1 73,000 4.85 40.4 645:1 82,500 4.55 43.8 6610:1 84,400 4.30 47.5 68

a Measured by GPC with chloroform as an eluent andcalibrated using polystyrene standards.

b The homopolymer of TMSt has a different main struc-ture from polystyrene, the GPC value is only for reference.

c Tm of the homopolymer of TMSt.

1458 LIU ET AL.


value is much smaller than unity, a linear poly-mer will be obtained similarly with traditionalradical polymerization. If the CT value is muchbigger than unity, a linear polymer via a stepgrowth mechanism will be obtained similarlywith traditional condensation process. For thecopolymerization of a chain transfer monomerwith a comonomer having equal reactivity ofdouble bond, the similar conclusion can bedrawn as its homopolymerization system.

VBT was synthesized and used as a chaintransfer monomer. Its CT value was determinedto be 15 by the developed mathematical modeltogether with 1H NMR monitored experiments.Therefore, the homopolymerization of VBT givesa linear poly((4-ethanyl)benzyl thioether) via astep-growth polymerization, which is in accord-ance with the theoretical analysis, while thecopolymerization of VBT with styrene was inves-tigated to give a comb-like polystyrene. Thecopolymerization can be regard as two stepsapproximately, that is, the telomerization ofVBT and copolymerization of unreacted mono-mer with the oligomer formed in the first step.

The developed model was demonstrated to beavailable by the determination of CT value andprediction of homopolymer and copolymer archi-tectures of VBT. Therefore, it is feasible to syn-thesize highly branched polymers via free radi-cal polymerization of chain transfer monomersfrom theoretical consideration. The further workis focused on finding appropriate chain transfermonomers with the CT value of around unity toprepare hyper-branched polymers.

The authors gratefully acknowledge the financial sup-port of National Natural Science Foundation of China(No. 50633010). And also many thanks to Mr. C.Zhang at department of chemistry in USTC for NMRmonitored experiment, and Ms. S. S. Chong and Mr.S. L. Zhang for valuable assistance.

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branched polymer via free radical polymerization of chain transfer monomer: a theoretical and...

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