Anionic homo- and copolymerization of styrenic triple-tailed polybutadiene macromonomers

Anionic Homo- and Copolymerization of StyrenicTriple-Tailed Polybutadiene Macromonomers

ANASTASIA NIKOPOULOU,1 HERMIS IATROU,1 DAVID J. LOHSE,2 NIKOS HADJICHRISTIDIS1

1Department of Chemistry, University of Athens, Panepistimiopolis, Zografou, 15771 Athens, Greece

2Corporate Strategic Research Labs, ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801

Received 30 January 2007; accepted 15 March 2007DOI: 10.1002/pola.22098Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A number of well-defined complex macromolecular architectures have beensynthesized using an efficient macromonomer technique. Styrenic triple-tailed polybu-tadiene (PBd) macromonomers (sTMMB), synthesized by selective reaction (titration) ofliving PBd tails with the SiCl groups of 2-(dichloromethylsilyl)ethylchloromethylsilyl-4-styrene (TCDSS), were polymerized in situ without isolation of sTMMB with s-BuLi,using high vacuum techniques. Taking advantage of the living character of the anionicpolymerization of sTMMB, the following complex macromolecular architectures wereprepared: PBd-g-(PBd)3 (triple-combs), PS-g-(PBd)3 (triple-grafts), [PBd-g-(PBd)3]3 (3-arm triple-comb stars), and triple molecular brushes (tmbPBd) or triple polymacromono-mers (tPMMB). Characterization carried out by size exclusion chromatography (SEC),equipped with refractive index and light scattering detectors, indicated that the synthe-sized novel architectures have a high degree of molecular and compositionalhomogeneity. VVC 2007Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3513–3523, 2007

Keywords: graft copolymers; light scattering; living polymerization; star polymers;triple molecular brushes; triple-tailed macromonomers

INTRODUCTION

In recent years, a great deal of effort has beendirected to the synthesis of well-defined novelmacromolecular architectures, since new topolo-gies can result in unprecedented properties andfunctions for polymeric materials. Among them,the precise architectural control of comb-like top-ologies has been mostly exploited, since the varia-tion of the different structural parameters (e.g.,number, molecular weight and nature of thebranches) has a great impact on material behav-ior. For the synthesis of comb-like polymers the‘‘macromonomer’’ approach1 is being more widely

used as compared to the ‘‘grafting from/onto’’methods. This is due to the lack of the need for afractionation step in the macromonomerapproach, to separate the branched materialsfrom the linear precursors.2,3

Single-tailed macromonomers, that is macro-monomers bearing only one chain, have been syn-thesized and used successfully for the synthesis ofa variety of nonlinear macromolecular architec-tures.3 Double-tailed macromonomers of the[(PS)2SiCl2]

4 type along with tetra-tailed macro-monomers of the [(PS)2ClSiCH2CH2Si(PS)2Cl]

5

type have been synthesized and used to preparedouble- and quarter-combs/grafts by step poly-merization with a,x-difunctional living chains.The polymeric materials produced exhibited highpolydispersity indices, and fractionation wasrequired to eliminate the linear precursors andcyclic side products. However, this approach allows

Correspondence to: N. Hadjichristidis (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 3513–3523 (2007)VVC 2007Wiley Periodicals, Inc.

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for the precise control of branch point spacing.Double-combs were also prepared by hydrosilyla-tion of the vinyl side groups of PBd-1,4 withdichloromethylsilane and reaction of the producedSiCl2 groups, along the PBd backbone, with livingchains.6,7 Finally, dense double PS combs were pre-pared by coupling living PS chains with the bromo-benzyl side groups (two per monomeric unit) of aPS backbone.8 The polymers prepared by the lasttwo methodologies suffer from the incomplete reac-tion of the living polymers with the reactive sidegroups, as well as the need of an extra fractionationstep to eliminate excess living polymer. Recently,by using the macromonomer strategy, double-combpolymers with narrow polydispersity have beensynthesized and used for the synthesis of complexmacromolecular architectures.9

In this article we report, for the first time, thesynthesis and characterization of triple-tailedstyrenic-tipped macromonomers of polybutadiene(sTMMB), as well as their in situ anionic homopo-lymerization and copolymerization with butadi-

ene (B) or styrene (S) (without the need for iso-lation of the sTMMB). The novel complex macro-molecular architectures synthesized in this workare presented in Scheme 1.

EXPERIMENTAL

Reagents

The purification of the monomer (Bd, 99%Aldrich), the solvents (benzene, 99.8% and tetra-hydrofuran (THF), 99.9%; both from Aldrich),the terminating (methanol, 99.9% Aldrich) andthe linking agent (Cl3(CH3)Si, 99% Aldrich), tothe standards required for anionic polymeriza-tion, was performed using well-established high-vacuum procedures.10 sec-Butyllithium (s-BuLi),prepared from sec-butylchloride (99.9% Aldrich)and lithium dispersion (99% high sodium,Aldrich), was the initiator for all homo- andcopolymerizations. Magnesium turnings (Aldrich)were washed with HCl 0.1N, diethylether (99.9%

Scheme 1. Novel complex macromolecular structures based on triple-comb- or triple-brush like structures. [Color figure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]

3514 NIKOPOULOU ET AL.

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

Aldrich) and acetone (99.9% Aldrich), and then leftto dry in vacuum oven overnight. p-chlorostyrene(97% Aldrich) was distilled in the vacuum-line,

from calcium hydride, to ampules equipped withbreak-seals. Potassium 2,3-dimethyl-pentyloxide-3(ROK) was prepared from 2,3-dimethyl-3-penta-

Scheme 2. General reaction scheme for the synthesis of the linking agent TCDSS.

Figure 1. 1H NMR spectrum of the linking agent TCDSS.

STYRENIC TRIPLE-TAILED PBd MACROMONOMERS 3515


nol (99% Aldrich) and potassium, according to aprocedure similar to the one used for the synthe-sis of s-BuLi. n-BuLi in hexane (1.6N, Aldrich)was used as received. 1,2-Bis(dichloromethylsi-lyl)ethane was purified by fractional sublimationunder high vacuum, diluted with hexane (50 mL),and stored at �20 8C.

Synthesis of 2-(dichloromethylsilyl)ethylchloromethylsilyl-4-styrene (TCDSS)

The general reactions for the synthesis of 2-(dichloromethylsilyl)ethylchloromethylsilyl-4-sty-rene (TCDSS) are given in Scheme 2.

The apparatus is similar to the one describedin a previous article for the synthesis of4-(dichloromethylsilyl)styrene.9 Initially, the re-actor was rinsed with Cl(CH3)3Si to remove tracesof humidity from the glass walls. Magnesiumturnings (8.226 g, 0.338 mol) were introduced intothe reactor, followed by addition of few dropsof 1,2-dibromoethane solution diluted in THF.The produced ethane was eliminated through thevacuum line and the appropriate amount of p-chlorostyrene (13.4 g, 0.097 mol) was introduceddrop-wise into the reactor. At this point, reflux ofthe THF was started (an exothermic reaction).During the last 30 min of the p-chlorostyrene

Scheme 3. Synthesis of the triple-combs PBd (tcPBd) and triple-grafts PS (tgSB).[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]



addition the reflux was supported by slightlyheating (� 30 8C) the reactor. The heating wasmaintained after all of the addition steps (� 2 h)for another hour. Then, the formed Grignard rea-gent was added drop-wise to an excess of 1,2-bis(dichloromethylsilyl)ethane THF solution (� 50%)at 0 8C. The produced TCDSS was transferred,under vacuum, into a specially designed appara-tus and was filtered twice to partially eliminatethe produced MgCl2. THF was distilled off the ap-paratus in the vacuum line, and was replaced by

hexane. Hexane is a good solvent for TCDSS and1,2-bis(dichloromethylsilyl)ethane, but not forMgCl2, which precipitates and is removed quanti-tavely. Hexane was removed by distillation, andthen TCDSS was fractionally distilled three times,to remove quantitavely the excess of 1,2-bis(di-chloromethylsilyl)ethane and any unreacted p-chlorostyrene. The final product was 9.4 g (� 30%yield). Finally the purified TCDSS was dilutedwith benzene, and stored at �20 8C in ampoulesequipped with break-seals. The NMR spectrum,

Scheme 4. Synthesis of the 3-arm triple-comb PBd star (3-tcPBd). [Color figure canbe viewed in the online issue, which is available at www.interscience.wiley.com.]

Scheme 5. Synthesis of triple molecular PBd brushes polymacromonomers (tmbPBd).[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]



given in Figure 1, confirms the successful syn-thesis of TCDSS: d 7.42–7.59 (C6H4); triplet6.75–6.82 (CH2¼¼CH); two doublets: 5.33 (trans1,4) and 5.9 (cis-1,4) (CH2¼¼CH); 0.65–0.80(CH3SiCl2CH2 CH2SiCl2CH3); and 1.1–1.3(��CH2CH2��). Integration of the peaks gave theexpected ratios for TCDSS.

Synthesis of triple-comb polybutadienes (tcPBd) andtriple-graft PS-g-(PBd)3 (tgSB)

The reactions used for the synthesis of the tri-ple-tailed PBd combs are shown in Scheme 3.Polymerizations were carried out in evacuated, n-BuLi-washed, and solvent-rinsed glass reactors.

Reagents were introduced via break-seals andtubes of aliquots for characterization wereremoved by heat-sealing of the correspondingconstrictions. Full details of the high vacuum

Figure 2. SEC chromatograms of (A) the polybuta-diene living tail, (B) the triple-tailed macromonomerand (C) the final triple-comb (sample 2, Table 1).

Figure 3. SEC chromatograms of (A) the polybuta-diene living tail, (B) the triple-tailed macromonomerand (C) the final triple-graft (sample 1, Table 2).



techniques are given elsewhere.10 The livingPBd�Li+ tails were prepared in a secondary reac-tor and were added drop-wise to the solution ofTCDSS. The reaction was monitored by removingsmall aliquots and analyzing them by SEC. After� 3 equivalents of PBd�Li+, relative to TCDSS,were added to the reactor, the titration wasstopped. The procedure of the titration lastedabout 5 h. The linking reaction of the first tailwith TCDSS took place in about 1 h, and the addi-tion of the second one also took about an hour.However, the reaction of the third tail requiredabout 3 h because of excluded volume effect. Bu-tadiene was then added to the main reactor,followed by s-BuLi and the randomizer ROK(mols-BuLi/molROK ¼ 1/30). The polymerizationwas monitored by removing small samples andanalyzing them by SEC. The reaction was consid-ered complete only when the SEC peak of the

macromonomer disappeared. After the comple-tion of the copolymerization (9–10 days), the tri-ple-comb PBd was terminated with degassedmethanol, precipitated into an excess of metha-nol, and dried under vacuum, until the weightbecame constant.

A similar procedure was used for the synthesisof triple-graft PS/PBd (tgSB). The difference wasthat after the synthesis of the macromonomer,styrene added to the main reactor instead of buta-diene, followed by the addition of the initiator(Scheme 3). The polymerization was monitored,as before, by removing small samples and analyz-ing them by SEC. The reaction was consideredcomplete only when the SEC peak of the macro-monomer disappeared. After the completion ofthe copolymerization (9–10 days), the tgSB wasterminated with degassed methanol, precipitatedinto an excess of methanol, and dried under vac-

Figure 4. 1H-NMR spectrum of the triple-graft (tgSB3) (Table 2).

Table 1. Molecular Characteristics of Living Tails, Triple-Tailed Styrenic Macromonomers (sTMMB)and Corresponding Triple-Comb Polybutadienes (tcPBd)

Sample

Average Numberof Branch Points

PBdTail sTMMB tcPBd

p-valueMw

a

(kg/mol)Mw/Mn

bMw

b

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

b

tcPBd1 1.2 9.9 1.03 29.5 1.05 60 1.05tcPBd2 7.5 6.4 1.01 19 1.03 112 1.02tcPBd3 9.3 4.8 1.02 19.6 1.08 235 1.07

a Size exclusion chromatography (SEC) in THF at 25 8C, using PBd standards.b SEC-TALLS in THF at 35 8C.



uum, until constant weight. No randomizer wasused in this case because both monomer and mac-romonomer were styrenic.

Synthesis of 3-arm triple-comb polybutadienes star(3-tcPBd)

The reactions used for the synthesis of the 3-armtriple-comb PBd star are shown in Scheme 4. Tothe living triple-tailed PBd combs, an appropri-ate amount of trichloromethyl silane was added(molSiCl/moltcPBd ¼ 1/1.2). After the completion ofthe linking reaction (� 1 month) the excess lin-ear triple- comb precursor was eliminated by thewell-known solvent/nonsolvent fractionationmethodology using toluene (solvent) and metha-nol (nonsolvent).

Synthesis of triple molecular brush of PBd (tmbPBd)or triple polymacromonomer (tPMMB)

The preparation of the tmbPBd (triple-polymacro-monomer) followed a similar procedure (Scheme5). A deep, orange-red color appeared immediatelyafter the addition of the initiator. The homopoly-merization of the macromonomer was completedin 1 month.

Molecular Characterization

SEC analysis was performed using a WatersHPLC system equipped with a Waters 501 highpressure liquid chromatographic pump, fourWaters Styragel columns having a porosity rangeof 102–106, a Waters 410 differential refractome-ter (RI) and a UV detector. THF was used as theeluent at a rate of 1 mL/min at 30 8C. The instru-ment was used to determine the polydispersity

factor (I ¼Mw/Mn) of all samples and the number-average molecular weight (Mn) of the PBd triple-tailed macromonomers. Calibration was performedwith seven standard PBd samples covering themolecular weight range from 2.0 to 350 kg mol�1.Multidetector GPC analysis (GPC-RI and GPC-TALLS) was performed using a Waters systemequipped with a Waters 1525 high pressure liquidchromatographic pump, Waters Ultrastyragel col-umns (HR-2, HR-4, HR-5E, and HR-6E) withTHF eluent at a rate of 1 mL/min at 30 8C. AWaters 2410 differential refractometer detectorand a Precision PD 2020 two angles (158, 908)laser light scattering (TALLS) detector at 35 8Cwere used. The instrument was used to determinethe weight–average molecular weight (Mw) of theintermediate and final products. The NMR spec-tra of the TCDSS and PBd macromonomers wereobtained with a Varian Unity Plus 300/54 instru-ment in CDCl3 at room temperature. The analysisshowed that the final comb has practically thesame microstructure (1,4 �90%; 1,2 �10%) asPBd polymerized without potassium alkoxide.

RESULTS AND DISCUSSION

The synthesis of the TCDSS was performed underhigh vacuum conditions to protect the moisturesensitive SiCl group. The raw product contains,besides the TCDSS, small amounts of p-chloros-tyrene and an excess of 1,2-bis(dichloromethylsi-lyl)ethane along with the produced MgCl2. Theremoval of MgCl2 from the raw products beforethe purification of the TCDSS is critical, since itspresence increases the boiling point of TCDSSsignificantly and so introduces the danger of

Table 2. Molecular Characteristics of Living Tails, Styrenic Triple-Tailed Macromonomers (sTMMB) andCorresponding Triple-Graft of PS and PBd (tgSB)

Sample

AverageNumber of

Branch Points Tail sTMMB tgSB%PS(w/w)

(Stoichiometry)

%PS(w/w)(NMR)p-value

Mwa

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

b

tgSB1 3.5 15 1.02 43 1.03 220 1.10 61.6 58.2tgSB2 6.9 6 1.03 20 1.05 211 1.08 86.5 82.7tgSB3 2.6 2.3 1.04 9 1.05 54 1.09 36.2 34.5

a Size exclusion chromatography (SEC) in THF at 25 8C, using PS standards.b SEC-TALLS in THF at 35 8C.



decomposition. After the removal of the MgCl2three fractional distillations are required to sepa-rate quantitavely the TCDSS.

The key factor for the synthesis of the macro-monomers is the faster replacement of the threechlorines by the PBd chains rather than the reac-tion with the double bond of TCDSS.11,12 In thecase of triple-tailed macromonomers, this wasachieved by using 1,2-bis(dichloromethylsilyl)-ethane rather than SiCl4. It was found that thelinking reaction of PBdLi is fast only for the firsttwo chlorine groups attached on the same Si atomwhile the third chlorine reacts much moreslowly.13 TCDSS has one or two Si��Cl groups perSi atom, rendering the linking reaction fasterthan the reaction of PBdLi with the double bondof the styrenic group. Another critical step forthe synthesis of triple-combs or triple-grafts isthe copolymerization of the macromonomer withBd or S in the same reactor used for the prepara-tion of the macromonomer without isolation(in situ). The isolation introduces impuritiesbecause it requires precipitation of the macro-monomer in a nonsolvent.14 The macromono-mers are solid materials and therefore theirpurification to the standards required for anionicpolymerization is difficult, and this is the reasonthat previous attempts to polymerize macromono-mers failed.15

A randomizer (potassium 2,3-dimethyl-pentyl-oxide-3, ROK) is required for the synthesis of thetriple-combs, to equilibrate the reactivity ratios ofthe styrenic macromonomer and the butadienemonomer, because otherwise a tapered copolymerwould be produced.16 ROK is used in a molar ratioof s-BuLi to ROK equal to 1/30. It is not requiredto use this polar additive when the monomer isstyrene since the styrenic macromonomer andstyrene have similar reactivity. During thecopolymerization of the PBd triple-tailed macro-monomers with the Bd monomer, the solution hadthe characteristic color of PBdLi. Nine to ten daysafter the addition of monomers the macromono-mer peak disappeared and a new peak, at smallerelution volume, appeared, indicating the successof the polymerization. As an example, Figure 2gives the chromatograms of the PBd living tail,the macromonomer and the final triple-comb(tcPBd1).

During the copolymerization of the PBd triple-tailed macromonomers with styrene, the solutionhad a bright orange color characteristic of thestyryllithium. Nine to ten days after the additionof the monomers the macromonomer peak disap-

Figure 5. SEC chromatograms of (A) the polybuta-diene living tail, (B) the triple-tailed macromonomer,(C) the triple-comb, (D) the unfractionated and (D) thefractionated star (sample 1, Table 3).



peared and a new peak, at smaller elution volume,appeared, indicating the success of the polymer-ization. As an example, Figure 3 gives the chro-matograms of the PBd living tail, the macromono-mer and the final triple-graft (tgSB1).

The average number of branch points (p) oftcPBd can be calculated using eq 1, given thefact that the macromonomer and the Bd aretransformed practically completely into copoly-mers:

p ¼ molsTMMB

moltcPBd¼ WsTMMB 3 Mn;tcPBd

ðWBd þWsTMMBÞ 3 Mn;sTMMBð1Þ

where WsTMMB and WBd are the weights of thetwo monomers (macromonomer and Bd) andMn,tcPBd and Mn,sTMMB are the number-averagemolecular weights of the triple-comb and themacromonomer, respectively. The molecular char-acteristics are summarized in Table 1. The compo-sition of the tgSB graft copolymers was obtained byNMR spectroscopy (Fig. 4, Table 2). The averagenumber of brunch points (p) of triple-tailed graftscan be calculated by eq 2,

p ¼ Mn;tgSB � a 3 Mn;tgSB

3 3 Mn;branchð2Þ

where Mn,tgSB, Mn,branch and a are the number–average molecular weights of the triple-tailedgraft and the branch and the weight percent ofthe PS blocks, respectively. The results were ingood agreement with those from stoichiometryused during the synthesis, indicating that thecopolymers exhibited a high degree of molecularand compositional homogeneity.

Table 3. Molecular Characteristics of Living Tails, Styrenic Triple-Tailed Macromonomer (sTMMB), Triple-CombPolybutadienes (tcPBd) Arms and Corresponding Symmetric Stars (3-tcPBd)

Sample

AverageNumber of

Branch Points Tail Tcpbd Arm 3-tcPBd

p-valueMw

a

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

b

tcPBd 1 5.55 20 1.04 185 1.07 535 1.05tcPBd 2 7.45 6.35 1.03 112 1.04 310 1.05tcPBd 3 3.8 3 1.02 23 1.03 64 1.03


Figure 6. SEC chromatograms of (A) the polybuta-diene living branch, (B) the triple-tailed macromono-mer and (C) the final triple polymacromonomers(sample 1, Table 4).



Because of the enhanced excluded volumeeffect of the tcPBd, the linking reaction for theformation of the 3-arm tcPBd stars (3-tcPBd)required quite a long time for completion. Themolecular characteristics of the synthesized3-tcPBd stars are summarized in Table 4. Typicalchromatograms of the living tail, the macromono-mer, the triple-comb, the unfractionated and thefractionated star (3-tcPBd 1) are given in Figure 5,Table 3.

Finally a typical example of the synthesis oftriple-molecular brush tmbPBd1, as monitored bySEC, are given in Figure 6, and the molecularcharacteristics are summarized in Table 4.Although the target degree of polymerization was20, the obtained degree was 2–4. This is attrib-uted to the increased steric hindrance of the dou-ble bond, which practically lies in the center of athree-arms PBd star.

CONCLUSIONS

The successful synthesis of a styrenic heterofunc-tional linking agent bearing three differentSi��Cl groups on two silicon atoms separated by��CH2CH2�� led to the synthesis of triple-combPBd, triple-graft copolymers, 3-arms star PBdwhere each arm is a comb, along with triple mo-lecular brushes. The in situ homo- and copolymer-ization of triple-tailed macromonomers, withouttheir isolation, is the key factor for the successfulsynthesis of complex macromolecular architec-tures. This methodology can be further extendedto the synthesis of well-defined complex architec-tures bearing more than three branches at everybranching point, possible leading to polymeric

materials with unprecedented properties andfunctions.

REFERENCES AND NOTES

1. Schulz, G. O.; Milkovich, R. J Polym Sci Part A:Polym Chem 1984, 22, 1633.

2. Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.;Iatrou, H. Chem Rev 2001, 101, 3747.

3. Hadjichristidis, N.; Pitsikalis, M.; Iatrou, H.; Pis-pas, S. Macromol Rapid Commun 2003, 24, 979.

4. Iatrou, H.; Mays, J. W.; Hadjichristidis, N. Macro-molecules 1998, 31, 6697.

5. Uhrig, D.; Mays, J. W. Macromolecules 2002, 35,7182.

6. Xenidou, M.; Hadjichristidis, N. Macromolecules1998, 31, 5690.

7. Velis, G.; Hadjichristidis, N. J Polym Sci Part A:Polym Chem 2000, 38, 1136.

8. Hirao, A.; Ryu, S. W. Macromol Symp 2003, 192,31.

9. Driva, P.; Iatrou, H.; Lohse, D. J.; Hadjichristidis,N. J Polym Sci Part A: Polym Chem 2005, 43, 4070.

10. Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsika-lis, M. J. Polym Sci Part A: Polym Chem 2000, 38,3211.

11. Knauss, M. D.; Al-Muallen, A. H.; Huang, T.; Wu,T. D. Macromolecules 2000, 33, 3557.

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Table 4. Molecular Characteristics of Living Tails, Styrenic Triple-Tailed Macromonomers (tcPBd) andCorresponding Polymacromonomers (tmbPBd)

Sample

Tail sTMMB tmbPBdDegree of

Polymerization

Mwa

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

bMw

a

(kg/mol)Mw/Mn

b Xn

TPM1 9.9 1.03 29.5 1.05 90.7 1.07 3.05TPM2 6.7 1.03 20.1 1.06 80.0 1.07 3.98TPM3 8.3 1.02 29.2 1.04 96.8 1.05 3.32TPM4 3.2 1.07 12.6 1.09 22.5 1.06 1.78




Anionic homo- and copolymerization of styrenic triple-tailed polybutadiene macromonomers

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