Synthesis of Graft Copolymers. III.Polystyrene-g-Poly(butyl acrylate)

GRACIELA MORALES, RAMIRO GUERRERO

Departamento de Quımica de Polımeros, Centro de Investigacion en Quımica Aplicada, Boulevard Enrique Reyna 140,Apdo Postal 379, 25100 Saltillo, Coahuila, Mexico

Received 17 October 2000; accepted 5 April 2001Published online 30 October 2001; DOI 10.1002/app.2227

ABSTRACT: Polystyrene (PS) chains functionalized with pendant 1,2-bis(trimethylsily-loxy)tetraphenylethane (TPSE) groups are used as macroinitiators to initiate the po-lymerization of n-butyl acrylate (BuA) to synthesize PS-g-poly(BuA) (PS-g-PBuA) co-polymers at 130°C. The TPSE groups are known to function as initers in the polymer-ization of several vinyl monomers. The homolytic decomposition of TPSE results in adiphenylmethyl (DPM) radical attached to the main chain and a free DPM radical. Theformer is responsible for the polymerization initiation and the latter momentarily stopsthe growth of the growing grafts by the formation of a dormant species. Unfortunately,side reactions like the combination between growing grafts take place and the poly-merization can only be controlled in a limited range of conversion. The most appropriateconditions for the synthesis of PS-g-PBuA are reported to present their potential use asthermoplastic elastomers with relatively controlled structures. © 2002 John Wiley & Sons,Inc. J Appl Polym Sci 83: 19–26, 2002

Key words: graft copolymers; tetraphenylethanes; controlled polymerization; macro-initiator

INTRODUCTION

It is known that poly(n-butyl acrylate) (PBuA) is anindustrially important polymer because of its lowglass-transition temperature (254°C) and durabil-ity. As a consequence of these properties it can beused as a soft segment in thermoplastic elastomers.However, it is also well known that in the synthesisof polyacrylate polymers and copolymers via anionicmethods, a number of side reactions take place1,2

and it is necessary for the polymerization to becarried out under strict conditions such as at verylow temperatures or in the presence of metal salts.Several methods have been applied to overcome

these problems3,4; however, many acrylate poly-mers and copolymers are still synthesized by free-radical polymerization. However, free-radical poly-merizations are difficult to control.

In the past decades several methods were de-veloped to control free-radical polymerizations,and they are essentially based on controlled or“living” free-radical mechanisms in which a dy-namic equilibrium between growing and dormantspecies is established. Different methods are re-ported in the literature for the synthesis of poly-acrylates such as the use of macromonomers,5

dithiocarbamates,6,7 alkoxyamines,8,9 and atomtransfer radical polymerization.10,11 The latterwas used to prepare ABA block copolymers withacrylonitrile, 2-ethylhexyl acrylate, and BuA.12

Rare earth metal complexes are used to synthe-size methyl methacrylate (MMA)/BuA/MMAtriblock copolymers,4 but there are very few re-

Correspondence to: G. Morales (gmorales@polimex.ciqa.mx).Journal of Applied Polymer Science, Vol. 83, 19–26 (2002)© 2002 John Wiley & Sons, Inc.

19

ports13 about the synthesis of graft copolymerswith BuA by controlled or living processes.

In this study we describe the graft controlledradical polymerization of BuA using polystyrene(PS)-based macroinitiators functionalized with1,2-bis(trimethylsilyloxy)tetraphenylethane(TPSE) groups. We report the effect of severalparameters on the synthesis of PS-g-PBuA in or-der to obtain graft copolymers that have a highmolecular weight and high content of PBuA andhave potential use as thermoplastic elastomers.

EXPERIMENTAL

Styrene was distilled over sodium under a vac-uum, and BuA and BuMA were distilled over

sodium hydroxide under a vacuum. All the mono-mers were distilled just before polymerization.Toluene and chlorobenzene were purified by dis-tillation and used immediately. The NMR spectrawere obtained with a Varian 200-MHz spectrom-eter. The size exclusion chromatography (SEC)analysis was made in THF on a Waters 150 LCwith PS standards.

The synthesis of the functional monomer 4-vinylbenzophenone (4-VBP), its copolymerizationwith styrene, and the synthesis of the macroini-tiators were carried out as described in an earlierarticle.14 Scheme 1 shows the general syntheticroute to prepare graft copolymers using TPSE-based macroinitiators. The characteristics of themacroinitiators used in this study are given inTable I.

Scheme 1 The general synthetic route to prepare PS-g-PBuA graft copolymers usingTPSE-based macroinitiators.

20 MORALES AND GUERRERO

Synthesis of PS-g-BuA

The graft polymerization of BuA initiated withstyrene-based macroinitiators was carried out asfollows: the macroinitiator (1.488 g, [TPSE] 5 6.43 1023 M) was dissolved in the monomer (5.94 g),and it was necessary to add toluene as a solvent(24-mL total volume) to prevent the increase ofviscosity in the early stages of the polymerization.Equal amounts of the solutions were placed in sixglass tubes. The tubes were degassed, sealed, andfinally immersed in an oil bath at 130°C for pre-determined periods of time. The conversion ofpure thermal BuA polymerization was measuredunder the same experimental conditions. Thepolymers were isolated by two precipitations froma large excess of methanol to eliminate the resid-ual homopolymer that was formed. After filtra-tion the polymers were dried under a vacuum at50°C to constant weight. In all cases the molecu-lar weight relative to linear PS was determinedby SEC.

RESULTS AND DISCUSSION

It was demonstrated in the synthesis of PS-g-poly(MMA) (PS-g-PMMA) using TPSE-function-alized macroinitiators15 that one of the principalparameters responsible for the formation of gel inthe synthesis of graft copolymers was the averagenumber of active sites (d) from the macroinitiatormolecule. If the d is #2 the synthesis of graftcopolymers is possible without the formation ofcrosslinked structures. It was also establishedthat a temperature of 130°C allows better condi-tions for this synthesis. At this temperature noinduction period exists as a consequence of the

instant decomposition of the TPSE groups (at130°C t1/2 5 5 s) and their capacity to initiate thepolymerization of a vinyl monomer.

In the synthesis of PS-g-PBuA copolymers,30% crosslinked products were obtained when aPS-based macroinitiator (d 4.81) was used to ini-tiate the polymerization, contrary to what Kolle-frath et al.13 reported when using poly(dimethyl-siloxane) macroinitiators. These authors only de-tected formation of gel when the macroinitiatorshad an average number of initiating sites of 33 at80°C. In our case, it was only possible to obtaintotal soluble graft copolymers when the averagenumber of active sites in the macroinitiator was#2. A macroinitiator with a d of 1.63 yielded totalsoluble graft copolymers even at 80% conversion.

To ascertain the optimal conditions for the syn-thesis of PS-g-PBuA to make it act like a thermo-plastic elastomer, the effect of several parameterssuch as the effect of the dilution media, concen-tration of TPSE, and initial concentration of theinitiator were studied. For this purpose, threemacroinitiators were synthesized that had simi-lar molecular weights but a different number ofTPSE groups in the macromolecular chain. Mac-roinitiators (XII), (XIII-1), and (XIII-2) were ob-tained with free benzophenone/chlorotrimethylsi-lane/magnesium molar ratios of 0.1:0.1:0.05,0.05:0.05:0.025, and 0.03:0.03:0.016, respectively.After the transformation reaction, the macroini-tiators were analyzed by SEC and 1H-NMR; theircharacteristics are listed in Table I.

Effect of Dilution in Synthesis of PS-g-PBuACopolymers

The three macroinitiators described in Table Iwere used in a series of experiments to initiate

Table I Characteristics of Macroinitiators Used in Synthesis of PS-g-PBuA Copolymers

MacroinitiatorA

(% mol Bz)Mn of a

(31023 g/mol)

TPSE1H-NMR(mol %) d

Mn

(31023 g/mol)aZ

(%)

XII 4-VBP-co-St(6.0)

9.4 1.72 1.63 11.0 27.7

XIII-1 4-VBP-co-St(2.6)

10.4 1.07 1.20 12.7 41.2

XIII-2 4-VBP-co-St(2.6)

10.4 0.63 0.72 12.6 24.9

The macroinitiators were synthesized according to Figure 1. A, precursor copolymer; Bz, benzophenone; d, the average numberof active sites (TPSE) by the polymeric chain; Z, the conversion of bonded benzophenone calculated by 1H-NMR; and St, styrene.

a The molecular weight of the macroinitiators determined by SEC.

SYNTHESIS OF GRAFT COPOLYMERS. III 21

the polymerization of BuA at 130°C. In all casesthe concentration of initiator with respect tomonomer was 20 wt % and the polymerizationswere carried out using toluene as a solvent. Theamounts of macroinitiator used correspond to aconcentration of TPSE of 6.4 3 1023 M. Afterpolymerization for predetermined periods of timethe polymers were isolated by two precipitationsfrom a large excess of methanol. As expected,there was no formation of gel as a consequence ofthe low value of d. The conversion was calculatedgravimetrically and the molecular weight was de-termined by SEC. The experimental conditionsand characteristics of the graft copolymers ob-tained are reported in Table II. The evolution ofthe conversion with time for the three macroini-tiators is presented in Figure 1.

From Figure 1 it can be observed that, in spiteof the concentration of the active sites being thesame in all three cases, the values of conversionare greater when the concentration of the mono-mer is higher. That is the case when macroinitia-tor PS (XIII-2) is used. The molar concentration ofthe monomer in this particular case is 3.3M,which is than in the other two cases, so the rate ofpolymerization is larger for the same polymeriza-

tion time. It can also be observed that, indepen-dent of the macroinitiator used, the conversionincreases in the first stage of the polymerizationas a consequence of the growth of the chains be-cause of the dissociation of TPSE groups attachedto the main chain and the insertion of more mono-mer molecules. After 3.5 h of polymerization, theconversion and the composition of PBuA in thecopolymers that are synthesized (Table II) re-mains the same. This indicates that after thatperiod of time the graft chains progressively de-activate and produce death graft chains.

However, the percentage of homopolymer in-creases with time as can be observed from thevalues reported in Table II. It can be attributed totwo factors: the capacity of the diphenylmethyl(DPM) radicals to initiate the polymerization andpure thermal polymerization of BuA, which isrelevant at 130°C (15%/h). These last two reac-tions are responsible for the decrease of the effi-ciency of the initiator that decreases the controlover the polymerization.

As mentioned previously, no crosslinked struc-tures were obtained in the synthesis of PS-g-PBuA, which was independent of the macroinitia-tor used, as a consequence of the low d presented

Table II Experimental Conditions and Characteristics of PS-g-PBuA Copolymers at DifferentPolymerization Times

Macroinitiator No.Polymerization

Time (h)

Conversion (%) Composition (%)Mn (31023

g/mol)b

PBuA (%)c

Total Partiala Copolymer PBuA Calculatedd 1H-NMR

PS (XII) (i) 1 1.5 9.1 1.8 81.38 18.61 21.1 7 222 2.5 24.2 6.9 64.10 35.89 21.0 22 353 3.5 34.4 9.8 57.98 42.01 20.2 29 364 4.5 41.5 18.2 64.66 35.33 19.0 43 455 5.5 38.6 19.9 70.50 29.50 19.5 45 41

PS (XIII-1) (i) 1 1.5 15.4 10.3 91.91 8.08 29.5 29 362 2.5 49.4 26.2 68.76 31.24 28.2 51 523 3.5 57.6 31.9 68.80 31.20 28.3 56 564 4.5 62.7 31.2 64.09 35.90 28.9 56 595 5.5 62.7 28.2 60.70 39.30 27.8 53 51

PS (XIII-2) (i) 1 1.5 59.4 48.2 86.74 13.26 33.4 66 592 2.5 72.9 58.5 85.29 14.71 32.8 70 803 3.5 71.8 62.9 90.80 9.20 33.8 72 774 4.5 73.1 64.5 91.22 8.78 32.2 72 785 5.5 — 64.2 — — 34.4 72 82

The conditions were 130°C, [TPSE] 5 6.4 3 1023M, and a concentration of initiator with respect to the monomer of 20 wt %.a The partial conversion is the waste of monomer in the graft formation.b The molecular weight determined by SEC.c The percentage of PBuA in the purified copolymer.d The composition determined gravimetrically.No. 5 experiment number.

22 MORALES AND GUERRERO

by them. However, macroinitiator PS (XIII-1) wasconsidered as the best one for studying the otherparameters that influence the synthesis of graftcopolymers because the values of conversion weremoderated (on the order of 30%), the molecularweights of the polymers obtained were doublethat of the initiator, and the composition of PBuAin the copolymers was approximately 50:50.

Effect of Average Number of Active Sites byMacroinitiator Molecule in Synthesis of PS-g-PBuACopolymers

In order to study the effect of the d in the synthe-sis of graft copolymers, equal amounts of macro-initiators (XII, XIII-1, XIII-2), monomer, and sol-vent were used in a series of experiments. Theestablishing of these conditions for the three mac-roinitiators implies variations in the molar con-centration of TPSE of 6.4 3 1023 M for PS (XIII-1), 3.7 3 1023 M for PS(XIII-2), and 9.6 3 1023 Mfor PS (XII). After polymerization for predeter-mined periods of time, the polymers were isolatedby twofold precipitation from methanol as in theearly example. Figure 2 shows the conversion

curves for the polymerization of BuA initiatedwith the three macroinitiators as a function oftime.

From the results presented in Figure 2 it canbe observed that the values of conversion increaseaccording to the number of active sites by themacroinitiator molecule and with the concentra-tion of TPSE. In a similar way, the pendant of thecurves increases with the initial concentration ofTPSE, which means that the polymerization rateincreases as well. Taking into account that thevalues of conversion for using the PS (XII) and PS(XIII-1) macroinitiators are more relevant thanusing PS (XIII-2), the former were analyzed bySEC and the results are presented in Figure 3.The curves of this figure reveal that, when theconcentration of TPSE increases, the molecularweights decrease for a certain value of conversion.

The copolymers were analyzed by 1H-NMR todetermine their composition, which was found tobe on the order of 20, 50, and 60 wt % of PBuA forPS (XIII-2), PS (XIII-1), and PS (XII), respec-tively. Once again the graft copolymers synthe-sized from PS (XIII-1) with a d of 1.21 and a TPSEconcentration of 6.4 3 1023 M present acceptablevalues of conversion (30%) and the highest molec-ular weights.

Figure 1 The evolution of the conversion with timefor the polymerization of BuA initiated with PS (XII),PS (XIII-1), and PS (XIII-2) macroinitiators under theexperimental conditions cited in Table II.

Figure 2 The conversion versus the polymerizationtime for the synthesis of PS-g-PBuA with differentmacroinitiators and variable TPSE concentrations(3 1023 M) of (l) 6.4, (■) 3.7, and (F) 9.6 at 130°C.

SYNTHESIS OF GRAFT COPOLYMERS. III 23

Effect of Initiator Concentration in Synthesis ofPS-g-PBuA Copolymers

Taking into account the previous results, we de-cided to use the PS (XIII-1) macroinitiator to ini-tiate the polymerization of BuA with a TPSE con-centration of 6.4 3 1023 M; however, in this casethe concentration of macroinitiator with respectto monomer increases to 10 wt %. This changewas made in order to have a larger concentrationof monomer (4.36M compared to 1.93M with a 20wt % concentration of initiator with respect tomonomer) and to reach higher values of conver-sion. Consequently, the copolymers synthesizedwould have higher molecular weights and thecontent of PBuA in the copolymers would increaseas well, allowing their behavior as thermoplasticelastomers to be increased as reported for othersimilar systems.16,17 Under the experimental con-ditions cited earlier, the copolymers obtainedwere isolated by twofold precipitation from alarge excess of methanol. The conversion wasgravimetrically determined and the results areshown in Figure 4.

As can be seen from Figure 4 and as expected,the values of conversion obtained in this case are

substantially higher, reaching 72.5% of conver-sion without the formation of gel. Once again, theconversion increases in the first 60 min of poly-merization as a consequence of the DPM radicalsformed upon homolytic cleavage of the TPSEgroups attached to the main chain, allowing thegrowth of the grafts. However, as explained, theDPM radicals can either recombine with thegrowing chains to generate a dormant species orinitiate a new polymeric chain. This latter reac-tion is responsible for the slow decline of the con-trolled character of the polymerization that wasnoticed from 60 min of polymerization. BecauseDPM radicals can be irreversibly consumed in theformation of new chains, they progressively tendto give up their role as scavengers of the growingradicals and the probability for a bimolecular ter-mination reaction is therefore bound to increase,provoking the deactivation of the grafts. A reallinear variation of conversion with time can beobserved during the first stage of polymerization,but as the conversion increases the system pro-gressively loses its controlled character because ofsubstantial irreversible termination.

The variation of the molecular weight with con-version was also followed (Fig. 5). Observe in the

Figure 4 The conversion versus time for the synthe-sis of PS-g-PBuA with different initiator concentrationsat 130°C.

Figure 3 The molecular weight versus conversion forthe synthesis of PS-g-PBuA with different macroinitia-tors and variable TPSE concentrations (3 1023 M) of(l) 6.4 and (F) 9.6 at 130°C.

24 MORALES AND GUERRERO

figure that the molecular weight increases withconversion, which attests that a certain degree ofcontrol was achieved with the use of TPSE radi-cals. The amount of PBuA in the copolymers wascalculated by 1H-NMR and found to be 79.7, 80.9,81.4, 82.4, 84.3, 85.8, 86.7, 86.3, and 86.7 wt % for30, 45, 60, 75, 90, 150, 210, 300, and 360 min,respectively.

Effect of Nature of Propagating Radical

The studies of different parameters like the onesanalyzed previously allowed us to establish theoptimal experimental conditions for the synthesisof PS-g-PBuA copolymers with the desired mac-romolecular parameters (i.e., high molecularweights and high contents of PBuA in the copol-ymers). However, instead of these results it wasobserved in all the cases that the polymerizationmechanism is controlled for a limited range ofconversion and molecular weight.

It was discussed earlier for this particular sys-tem and others that the main reason for the gelformation was the multifunctionality of the mac-roinitiator used. Another factor of no less impor-tance is the mode of termination of the polymer-izable monomer: when MMA is the monomer to begrafted, the fact that the termination reactiontakes place preferentially by disproportionationmakes it possible to avoid recombination between

growing chains to the same extent. Moreover, it isknown that carbon-centered radicals lead to atermination reaction by recombination and/or dis-proportionation. The disproportionation/recombi-nation ratio depends on the structures of the im-plied radicals. In this way, as the substitution onthe radical center increases, a persistent radicalcan be formed18 and the diminishing of the spinelectronic density in the a-carbon delays recom-bination reactions in favor of the disproportion-ation ones.

Having these facts in mind for our particularcase, BuA was replaced with methyl BuA (Me-BuA). This was done not only to lower the recom-bination reactions, but also to observe if in thepresence of a more stable radical like the MeBuone (a tertiary radical) the termination reactionbetween growing and primary radicals is favoredin such a way that the polymerization mechanismwould be controlled for a longer period of time.

In this way MeBuA was polymerized in thepresence of PS (XIII-1) under the identical exper-imental conditions used for the synthesis of PS-g-PBuA copolymers. Table III summarizes the ex-perimental conditions and the characteristics ofthe copolymers obtained in this manner.

The results reported in Table III reveal similarbehavior than those for BuA. However, contraryto the former, the polymerization can be con-trolled for longer periods of time. With MeBuAthe conversion and the molecular weight increaseduring the first 60 min of polymerization. Afterthis time it is observed once again that there is aloss in control as a consequence of irreversibletermination reactions instead of the more stableradical. These reactions are responsible for thedecrease in functionality of the growing chainsand the destruction of the DPM active radicals.Therefore, it can be pointed out that the polymer-ization presents a controlled character in the firststages and then this character is progressivelylost due to secondary reactions that eventuallystop the polymerization before the total consump-tion of the monomer.

CONCLUSION

For the synthesis of PS-g-PBuA copolymers initi-ated with PS-based macroinitiators functional-ized with TPSE the formation of gel depends ex-clusively on the d of the macroinitiator molecule.A d of #2 allows the synthesis of totally solublegraft copolymers. With respect to the polymeriza-

Figure 5 The molecular weight versus conversion forthe synthesis of PS-g-PBuA [PS(III-1)] at 10 wt % withrespect to the monomer at 130°C.

SYNTHESIS OF GRAFT COPOLYMERS. III 25

tion mechanism, it can be pointed out that itpresents a certain degree of control for low valuesof conversion (#30%). In this interval of conver-sion, the conversion and molecular weight areboth linearly increased with the polymerizationtime. Outside of this interval the polymerizationis affected by secondary reactions that decreasethe controlled character because of the loss offunctionality in the growing chains. In themethacrylic monomers such as MeBuA it is pos-sible to access total soluble graft copolymersabove 100°C with a better degree of control.

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Table III Experimental Conditions and Characteristics of PS (XIII-1)-g-PMeBuA Copolymers atDifferent Polymerization Times

ExperimentPolymerization

Time (min)Conversion

(%)a

Composition (%) PMeBuA (% P/P)b

Mn

(31023 g/mol)Graft

Copolymer PMeBuA 1H-NMR Gravimetrically

1 15 28.0 96.7 3.3 75 72 35.62 30 47.3 97.5 2.5 82 81 44.03 60 66.2 98.1 1.9 87 86 53.34 75 68.2 98.4 1.6 86 86 55.55 90 72.5 95.9 4.1 88 87 52.66 150 81.2 97.8 2.2 90 88 56.07 210 85.3 97.7 2.3 88 88 51.18 270 88.1 98.0 2.0 90 89 56.19 330 88.0 97.8 2.2 90 89 56.0

The conditions were 130°C, [TPSE] 5 6.4 3 1023M, and a concentration of initiator with respect to the monomer of 10 wt %.a The consumption of monomer in the graft formation.b PMeBuA composition in the purified copolymer.

26 MORALES AND GUERRERO