Macromol. Rapid Commun. 2001, 22, 1243–1248 1243

Copolymerization of N,N-Dimethylacrylamide withStyrene and Butadiene: The First Example of PolarGrowing Chain End/Nonpolar Monomer Cross-Initiation

Boris I. Nakhmanovich,1 Tatyana N. Prudskova,1 Alexander A. Arest-Yakubovich,* 1 Axel H. E. Müller 2

1 Karpov Institute of Physical Chemistry, Vorontsovo pole 10, Moscow 103064, RussiaFax: +7-095-975-2450; E-mail: arest@cc.nifhi.ac.ru

2 Makromolekulare Chemie II and Bayreuther Institut für Makromolekülforschung, Universität Bayreuth,D-95440 Bayreuth, Germany

IntroductionAs long as 50 years ago Mayo et al. showed that the anio-nic copolymerization of an equimolar mixture of methylmethacrylate (MMA) and styrene resulted in almost purePMMA.[1] Since then, the inability of MMA to copoly-merize with styrene, isoprene, and other nonpolar mono-mers has been well documented.[2–5] Graham et al. sug-gested that this phenomenon is due to the low basicity ofthe PMMA anion.[3] The pKa values for the conjugatedacids, corresponding to anions of acrylic esters and typi-cal nonpolar monomers, differ by more than 10 units(CH3COOC2H5 , pKa = 30–31; PhCH3, pKa = 43;CH22CHCH3 , pKa = 44[6]). The inability of nonpolarmonomers to add to esterenolate anions and to polar liv-ing chains in general is assumed, explicitly or implicitly,in all text books on the theory of anionic polymeriza-tion,[6–9] and there has been a lack of new experimentationin this direction.

Recently, the anionic polymerization of another acrylicmonomer, N,N-dimethylacrylamide (DMAA) has drawn

attention.[10–12] The basicity of its anion is several unitshigher than that of anions of acrylic esters (for the conju-gated acid CH3CONMe2 , pKa = 34–35[6]), which reducesthe difference between DMAA and nonpolar monomers.Besides, in a recent publication Xie and Hogen-Eschreported that fluorenyl caesium cannot polymerizeDMAA.[10] In contrast, the salts of the fluorenyl anion(fluorene, pKa = 22.6[6]) are known to initiate the poly-merization of MMA,[3, 6] but not that of nonpolar mono-mers.[6] These considerations prompted us to examine thepossibility of DMAA copolymerization with styrene andbutadiene and to perform test experiments that have ledto quite unexpected results.

Experimental Part

Reagents

Tetrahydrofuran (THF) and monomers were purified by stan-dard procedures[8] finally being treated with liquid K-Naalloy (THF) and BuLi (styrene, butadiene and 1,1-diphenyl-

Communication: Living potassium poly(N,N-dimethyl-acrylamide) initiates the polymerization of styrene andbutadiene, and adds 1,1-diphenylethylene in THF solu-tion. The model compound a-potassio-N,N-dimethylpro-pionamide also polymerizes styrene and butadiene in con-trast to esterenolates, which are known to be incapable ofsuch reactions. The IR spectra and SEC traces of the poly-mers obtained unequivocally prove that styrene and buta-diene initiation proceeds directly via the amidoenolateanion. Apparently, this is the first case observed where thepolymerization of a nonpolar monomer can be initiated bythe growing chain end of a polar polymer.

Macromol. Rapid Commun. 2001, 22, No. 15 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1336/2001/1510–1243$17.50+.50/0

1244 B. I. Nakhmanovich, T. N. Prudskova, A. A. Arest-Yakubovich, A. H. E. Müller

ethylene). DMAA (Aldrich) was dried over calcium hydrideand distilled under vacuum into ampoules equipped withbreak-seals.[10, 12]

Living dipotassium-oligo(a-methylstyrene) (K-a-MSt) wassynthesized according to common procedure by the reactionof a-methylstyrene monomer with potassium mirror in THFat room temperature.[8] Sodium dihydrobis(2-methoxyeth-oxy)aluminate (NaAlH2(OC2H4OCH3)2 ; SDMA; Institute ofInorganic Chemistry, Prague) was obtained as a 70% toluenesolution, diluted with toluene to 1 mol N L–1 and partly hydro-lyzed (the extent of hydrolysis was 50%) in order to increaseits reactivity towards styrene polymerization.[13] The modelamidoenolate initiator, a-potassio-N,N-dimethylpropion-amide (CH3CH(K)CON(CH3)2 ; K-DMPA) was synthesizedsimilar to the procedure described previously.[14] The solu-tion of DMPA (6 mmol) in 10 mL of dry hexane was addeddropwise, with stirring, at –58C to the solution of potassiumhexamethyldisilazanate (6 mmol) in 20 mL toluene. Afterthe addition was finished, the reaction mixture was warmedto room temperature and the solvents were distilled off undervacuum. The solid product was washed with dry hexane anddried under vacuum up to constant weight. 0.8 g (96%) of K-DMPA were obtained as a white powder.

C5H10NOK (139.244): Calcd. C 43.13, H 7.24, N 10.06;Found C 43.02, H 7.18, N 9.95.

Polymerizations

All operations were carried out under high vacuum conditionsin an all-glass apparatus using break-seal techniques asdescribed earlier.[12, 15] The polymerization of DMAA was per-formed at –308C, and the polymerization of nonpolar mono-mers at ambient temperature. After completion, polymeriza-

tion was quenched with a drop of methanol, the polymer wasprecipitated by pouring the reaction mixture into a largeexcess of hexane and then dried in vacuo to constant weight.Typical monomer concentrations in polymerization experi-ments were ca. 1–2 mol N L–1, and initiator concentrationswere (5–10)610–3 mol N L–1. Monomer conversion wasdetermined by gravimetry and was taken into account in cal-culation of theoretically expected molecular weight (M

—n,theor).

Molecular weights and molecular weight distributions ofthe polymers were measured by means of size-exclusionchromatography (SEC) with a Waters-510 instrumentequipped with RI and UV (264 nm) detectors as describedearlier;[16] CHCl3 with 3% methanol was used as the eluent.Calibration was performed using polystyrene (PSt) stan-dards. IR spectra were obtained with a Perkin-Elmer FTIR-1710 spectrometer, polymers were deposited on KBr surfaceby evaporation of the solvent from CHCl3 solution.

Results and DiscussionSeveral series of experiments were performed using var-ious initiators and ways of introducing the reactants intothe system. The experimental conditions and results aregiven in Table 1.

In the first series (runs 1, 2) a thin-walled glass bulbcontaining the initiator solution was broken at –308Cinside an ampoule containing an equimolar mixture ofDMAA and styrene in THF. Both experiments showedthe following qualitative pattern: instantaneous polymeri-zation of DMAA on contact with the initiator, accompa-nied by heat evolution and a sharp increase in viscosity asdescribed in our preceding paper,[12] then a short quiet

Table 1. Summary of the experiments on DMAA (M1) copolymerization with nonpolar monomers (M2) in THF.

RunNo.

Initiator M2 ½M2�0mol NLÿ1

Conversion of M2

%Molar mass averages

103 N M—

n,theor 103 N M—

n,exp M—

w /M—

n

1a) SDMAb) styrenec) 1.0 100 50 93 3.12a) K-a-MStd) styrenec) 1.0 63 80 240 1.93a) K-a-MStd) DPEe) 0.2 f) 12 18 1.94g) K-a-MStd) styrene 2.4 60–70 30h) 43h) 2h)

85i) 162i) 2i)

5g) K-a-MStd) styrene 2.6 100 480 1.446103 –6g) K-a-MStd) butadiene 2.3 L30 36 – –7j) K-DMPAk) styrene 1.9 100 L1 3.6 28j) K-DMPAk) butadiene 2.2 L50 1.4 – –

a) Addition of initiator to the monomer mixture at –30 8C.b) Sodium dihydrobis(methoxyethoxy)aluminate, NaAlH2(OC2H4OCH3)2 , partly hydrolyzed (see Experimental Part).c) DMAA/styrene equimolar mixture.d) Living dipotassium-oligo(a-methylstyrene).e) DMAA/DPE mixture.f) DPE adds to PDMAA chain ends.g) Polymerization of DMAA at –308C, then the nonpolar monomer was added at room temperature.h) DMAA prepolymer.i) Total copolymer.j) Homopolymerization of nonpolar monomer at room temperature.k) a-Potassio-N,N-dimethylpropionamide.

Copolymerization of N,N-Dimethylacrylamide with Styrene and Butadiene ... 1245

period (10–15 min at room temperature), and after that asecond exothermic polymerization accompanied byfoaming of the reaction mixture. A high total yield ofpolymer (Table 1) confirms the participation of styrene atthis stage. The instantaneous disappearance of the charac-teristic UV absorption band of a-methylstyryl anions at330 nm on contact with DMAA (run 2) excludes the pos-sibility of a participation of residual initiator in the initia-tion of the second stage of the process.

The polymers obtained in runs 1 and 2 are soluble inCHCl3 . The SEC traces are multimodal (Figure 1).Because the absorption coefficient of PDMAA at 264 nmis very low (see Figure 2a and Table 2) one can concludethat the low-molecular weight part of the copolymer iscomposed mainly of PDMAA, whereas the high-molecu-lar weight fraction undoubtedly contains a markedamount of styrene. This conclusion agrees well with thelow nucleophilicity of the DMAA anion leading to a lowinitiation efficiency in the second stage and, conse-quently, to the presence of unreacted DMAA homopoly-mer. Other reasons for multimodal MWD of the copoly-mer will be discussed in more detail below.

The polymerization of a mixture of DMAA with 1,1-diphenylethylene (DPE; run 3) proceeds similarly to runs1 and 2. After the initial stage of violent DMAA polymer-ization the solution gradually acquired the red color char-acteristic of DPE anions. According to the optical densityof the UV band at 480 nm, approximately 20% of grow-ing PDMAA chains added DPE. The SEC curve obtained

with the use of the UV detector (not shown) confirms thepresence of DPE in macromolecules.

The behavior of the system resembles the well-knownstyrene-butadiene copolymerization initiated by alkyl-lithium compounds in hydrocarbon solvents where buta-diene polymerizes first, and only after its consumptiondoes styrene start to polymerize, resulting in the forma-tion of a tapered copolymer.[8, 9] The rate constant of theaddition of styrene to the living poly(butadienyl lithium)is about 4 orders of magnitude lower than the rate con-stant of the reverse process.[17] For the DMAA-styrenesystem in THF this pattern seems to be similar and con-sistent with the large difference in the nucleophilicity ofthe corresponding anions.

The next series (runs 4–6) was performed as follows.First, a living PDMAA prepolymer was obtained by thereaction of DMAA with the dipotassium salt of oligo(a-

Figure 1. SEC traces of DMAA-styrene copolymer obtainedby simultaneous addition of monomers (run 2).

Figure 2. SEC traces of PDMAA prepolymer (a) and the totalcopolymer (b) resulting from the sequential copolymerization ofDMAA and styrene (run 4).

Table 2. Analysis of copolymer from run 4 using the ratio ofintegral intensities of signals from UV (k = 264 nm) and RIdetectors.

Polymer ðUV=RIÞ ratioarbitrary units

DMAA weight fraction%

polystyrene(reference sample)

38 0

prepolymer (PDMAA) 0.46 100total copolymer 27 30fractions:

methanol-soluble 3.5 92methanol-insoluble 35 8

1246 B. I. Nakhmanovich, T. N. Prudskova, A. A. Arest-Yakubovich, A. H. E. Müller

methylstyrene) (K-a-MSt) in THF at –308C at a ratio[DMAA]/[K-a-MSt] of L150 mol/g-equiv. A part of theprepolymer was taken for analysis, and the rest was addedat room temperature to the second monomer (styrene orbutadiene). The prepolymer and copolymers obtained inrun 4 were dissolved in CHCl3 and examined by means ofSEC (Figure 2). As one can see from Table 1, the averagemolar mass of the prepolymer (based on PSt standards) isclose to the theoretical value whereas the molar mass ofthe copolymer is higher than the theoretical value. Appar-ently, this is due to the occurrence of some secondaryreactions resulting in the formation of graft copolymersas will be discussed further.

Next, the copolymer was fractionated by precipitationfrom CHCl3 into methanol, which is a precipitant for PSt,but a good solvent for PDMAA. The large differencebetween the UV absorption coefficients of DMAA andstyrene units enabled the estimation of the composition ofproducts from the ratio of integral intensities of the sig-nals of the UV and RI detectors. The results of the meas-urements are given in Table 2. These data strongly sug-gest that the methanol-soluble fraction contains mostlyunreacted PDMAA prepolymer, with some copolymersof styrene and DMAA, whereas the methanol-insolublefraction mostly contains copolymer and, possibly, somestyrene homopolymer. The IR spectra definitely indicatethe presence of PSt (characteristic bands of the monosub-stituted aromatic ring at 697 and 745 cm–1) in the metha-nol-soluble fraction and the presence of PDMAA (charac-teristic band of the carbonyl group at 1640 cm–1) in theinsoluble one (Figure 3). In a similar experiment (run 5)the precipitate obtained after the precipitation of the totalcopolymer from CHCl3 into methanol, after thorough

washing with hot methanol, contains 3% DMAA (accord-ing to NMR data). As methanol is a poor solvent for PStand a good one for PDMAA, these results prove the pre-sence of styrene units in the methanol solution, andDMAA units in the precipitate in the form of blocks orgrafted branches, but in no case as homopolymers.

A similar experiment with butadiene (run 6) gave prin-cipally the same result. In spite of a relatively low buta-diene conversion, the reasons for which will be discussedbelow, butadiene also adds to PDMAA anions. This isshown not only by the increase in the total weight of theproduct of polymerization, but also by properties of thecopolymer obtained. The copolymer was rubber-like,well soluble in THF and THF/CHCl3 mixture (solventsfor both components) but insoluble in heptane (solventfor polybutadiene), or in water and water-methanol mix-ture (solvents for PDMAA). The IR spectrum of thecopolymer shows strong bands of the carbonyl group(1640 cm–1) and the 1,2-butadiene unit (910 cm–1) and asmaller one of the trans-1,4-butadiene unit (967 cm–1)(Figure 4).

For an additional confirmation of the ability of thePDMAA anion to initiate the polymerization of nonpolarmonomers, K-DMPA was synthesized as its model.When this compound was added to a styrene solution inTHF at room temperature (run 7), the polymerizationstarted after 5 min, as was indicated by a warming andfoaming of the reaction mixture and the appearance of ared color characteristic of PSt anions. The polymer yieldwas close to quantitative (see Table 1). The low initiatorefficiency again indicates slow initiation. Butadiene poly-merizes more slowly than styrene, the polymer yield after3 h of polymerization at room temperature was ca. 50%

Figure 3. IR spectrum of DMAA-styrene copolymer (run 4):(a) methanol-soluble, (b) methanol-insoluble fraction.

Figure 4. IR spectrum of DMAA-butadiene copolymer (run 6).

Copolymerization of N,N-Dimethylacrylamide with Styrene and Butadiene ... 1247

(run 8). It is essential that the IR spectra of both poly-mers, purified from possible traces of unreacted initiatorby the reprecipitation from THF into methanol, containthe characteristic band of the carbonyl group, which con-firms the initiation by amidoenolate anions.

Therefore, the results described above prove the abilityof living PDMAA chain ends to add nonpolar monomers.The most convincing evidence is given in the following:(i) a high total yield of polymer when both monomerswere simultaneously mixed with the initiator (runs 1, 2)with spectrophotometric evidence for complete initiatorconsumption well before the beginning of the secondstage of polymerization; (ii) the addition of DPE toPDMAA growing chains (run 3) as confirmed by SECand UV spectroscopy; (iii) the ability of living PDMAAprepolymer to initiate styrene and butadiene polymeriza-tion (runs 4–6), the simultaneous presence of polar (car-bonyl group) and nonpolar (aromatic ring in the case ofstyrene and vinyl bond in the case of butadiene) frag-ments in the thoroughly purified polymerization productsbeing proven by spectroscopy; (iv) finally, the ability ofthe model potassium amidoenolate, K-DMAP, to initiatethe polymerization of styrene and butadiene, the presenceof the carbonyl group of the initiator in both polymersbeing proven by IR spectroscopy (runs 7 and 8). More-over, as we have shown in preliminary experiments,potassium hexamethyldisilazanate, used in the synthesisof the model amidoenolate, does not initiate styrene poly-merization.

Apparently, this is the first observation of the polymer-ization of nonpolar monomers initiated by a polar chainend. The exact chemistry of the process is not clear. Inthe case when both monomers are introduced simulta-neously, ideally one might expect the formation ofdiblock or tapered copolymers. However, as can be pre-dicted from the large difference in nucleophilicities ofpolar and nonpolar living chain ends, and from the pre-sence of highly nucleophilic nonpolar anions and reactivepolar functional groups, the copolymer composition canbe much more complex. Firstly, the former factor resultsin a low initiation efficiency and therefore in the presenceof PDMAA homopolymer. Secondly, the attack of grow-ing polystyrene (polybutadiene) chain ends on thePDMAA side groups seems to be plausible; this wouldresult in the formation of graft copolymer under theexpulsion of N,N-dimethylamide anions (Scheme 1).Such a reaction is well known for the interaction of livingpolystyrene with PMMA;[18, 19] a similar reaction wasrecently suggested by Hogen-Esch at al. for the anionicpolymerization of DMAA.[10] The ketone carbonyl groupformed according to Scheme 1 is more reactive than thatof the initial DMAA unit.[10] Therefore, it can be furtherattacked by the next growing polystyrene (polybutadiene)chain. Thirdly, Xie and Hogen-Esch suggested the possi-ble deprotonation of the PDMAA methine protons by

strong bases.[10] This reaction, as well as the deprotona-tion of methine protons of newly formed ketone-contain-ing monomer units, seem to be probable in our case dueto the high nucleophilicity of polystyrene (polybutadiene)anions (Scheme 2).

Depending on the relative reactivities of the resultingpotassium N,N-dimethylamide (Scheme 1) or amidoeno-late anions (Scheme 2) these reactions may have the char-acter of chain transfer and/or chain termination. Chaintermination – explaining the incomplete conversion ofthe nonpolar monomer – may proceed according toScheme 2 because the initiation ability of the stericallyhindered anion formed on a PDMAA chain should beeven lower than that of the living PDMAA anion. On theother hand, as follows from several publications (see,e.g., the paper by Lawson et al.[20] and references citedtherein), alkylamide anions are capable of initiating buta-diene and styrene polymerization in hydrocarbon solventsand in THF. Therefore, one cannot exclude chain transferaccording to Scheme 1. Butadiene seems to be more reac-tive in deprotonation but less reactive in reinitiation thanstyrene, which explains its lower conversion.

The considerations given above enable us to qualita-tively describe the results obtained. The exact mechanismof the process and, especially, quantitative characteristicsof reactions need to be investigated in more detail. How-ever, the reactions observed may be useful for the syn-thesis of new types of block and graft copolymers.

Acknowledgement: This work was supported by the RussianFoundation of Basic Research (RFBR), project No. 00-03-33209, and joint RFBR-INTAS Grant No. IR-97-278. Theauthors would like to thank Prof. T. E. Hogen-Esch, LosAngeles, for fruitful discussion and Dr. L. Lochmann, Prague,for helpful advice on the K-DMPA synthesis.

Received: April 24, 2001Revised: July 30, 2001

Scheme 1. Grafting of PSt anions onto PDMAA.

Scheme 2. Proton transfer between PSt anions and PDMAAchains.

1248 B. I. Nakhmanovich, T. N. Prudskova, A. A. Arest-Yakubovich, A. H. E. Müller

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