Reactions of Bifunctional Addition-Fragmentation Chain Transfer Agents for Synthesis of Polymer Bearing Unsaturated Moieties at Both Ends

Macromol. Chem. Phys. 2000, 201, 1565–1573 1565

Full Paper: Polymerizations of methyl methacrylate scopy that unsaturated and seven-membered cyclic end (MMA) and styrene (St) in the presence of bifunctional groups are formed by conventional AFCT and intramole- addition fragmentation chain transfer (AFCT) agents con- cular cyclization of the radical from the transfer agents. sisting of two a-(alkylthiomethyl)acryloyloxy groups con- However, formation of the cyclic end groups could be nected by an alkylene group are presented. The moieties suppressed by structural modification of the transfer from the transfer agent were introduced almost quantita- agent. PMMA bearing the unsaturated end group at one or tively at both ends and at the middle of PMMA under both chain ends was employed as a precursor for branched appropriate conditions. It was confirmed by NMR spectro- block copolymer preparation.

Reactions of bifunctional addition-fragmentation chain transfer agents for synthesis of polymer bearing unsaturated moieties at both ends

Kenta Tanaka, Bunichiro Yamada* Material Chemistry Laboratory, Faculty of Engineering, Osaka City University, Osaka 558-8585, Japan Fax: +81-6-6605-2797; E-mail: [email protected]

Introduction less reactive adduct radical which would readily couple a with PSt radical leading to a star-shaped polymer consist- Some -(substituted methyl)acrylic esters have been [10] known as effective chain transfer agents through an addi- ing of two PMMA chains and up to four PSt chains. tion fragmentation chain transfer (AFCT) mechanism to yield b-carboalkoxyallyl end groups depending on the type of the substituent.[1–4] The same unsaturated end group can be obtained by catalytic chain transfer polymerization of methyl methacrylate (MMA) using a Co(II) complex.[5] The addition of a PMMA radical to the carbo- methoxyallyl group is not a fast reaction and the adduct radical is known to readily fragment to regenerate the end group and PMMA radical. Consequently, an oligomer of MMA bearing an unsaturated end group can be utilized This paper deals with the polymerization of MMA in a as an AFCT agent and a mediator for block copolymer the presence of 2, 3, and 4 consisting of two -(alkylthio- formation through living free radical polymerization.[6, 7] methyl)acryloyl groups that are expected to function as Bi- and dual functional a-(substituted methyl)acrylate highly reactive AFCT agents similar to those in 1. The type AFCT agents have been used to synthesize branched unsaturated moieties resulting from the chain transfer MMA polymers, and MMA and styrene (St) block copo- were expected to be introduced at both ends of the poly- lymers utilizing almost quantitative bond formation mer. Quantitative introduction of the end groups and between polymer chain end and the resulting end further reaction with propagating radicals were investi- group.[8–10] When two a-(substituted methyl)acryloyl moi- gated as a novel procedure to attain branched block copo- eties are connected by an ester linkage such as 1, the lymer without gelation. bond formation as a result of AFCT could lead to PMMA bearing the unsaturated moiety at the middle and the Experimental other moieties at both ends of the polymer chain.[9] If the subsequent St polymerization was carried out in the pre- Materials sence of the PMMA, the addition of PSt radical to double MMA and St were commercially available and were purified bond of the unsaturated moiety was feasible to yield a by distillation under reduced pressure before use. Commer-

Macromol. Chem. Phys. 2000, 201, No. 14 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1352/2000/1409–1565$17.50+.50/0 1566 K. Tanaka, B. Yamada

cial 2,29-azobisisobutyronitrile (AIBN) was purified by ane or methanol was used to precipitate the resultant poly- recrystallization from methanol. MMA-d8 was obtained by mer. The polymer was dried and purified by reprecipitation. esterification of methacrylic acid-d5 prepared from acetone- [11] d6 cyanohydrin and purified by distillation. Measurements 1H and 13C NMR spectra and 2D(H-C) spectrum were taken — — Preparation of 2, 3, and 4 by a JEOR JNM a-400 spectrometer. Mn and Mw were obtained employing a Tosoh 8000 series HPLC equipped — 1,2-Ethanedithiol (3.2 g, 0.034 mol) was added dropwise to a with columns for GPC, and PSt standards (M = 5.06102 – a [12] n benzene solution of methyl -(choloromethyl)acrylate 1.096106) were used for calibration. (MCMA) (10.1 g, 0.075 mol) in the presence of triethyl- amine (7.6 g, 0.075 mol) with cooling. After stirring the mix- ture for two days at room temperature, the benzene solution Results and discussion was dried by MgSO4 after washing with water. The benzene was evaporated, and 2 was obtained as colorless crystals. Polymerizations of MMA and St Further purification of 2 was performed by recrystallization from n-hexane, and the structure of 2 was proven by 1H and The polymerizations of MMA and St in the absence and 13C NMR spectroscopies. 2: yield 6.27 g (0.022 mol, 64%), presence of 2 are compared in Tab. 1. Although 2 is a divi- mp 37–388C. nyl compound, all the polymers of St and MMA obtained 1 H NMR (CDCl3): d = 2.67 (s; SC2H4S), 3.42 (s; CH2S), were soluble in organic solvents such as benzene, chloro- 3.79 (s; OCH3), 5.68 (s; CH22), 6.23 (s; CH22). Intensity form, and acetone. Chain transfer constants (Ctr) of 2 were [13] ratio: 2:2:3:1:1. determined by Mayo plots; Ctr = 0.70 and 1.67 for 13 d 2 C NMR (CDCl3): = 31.0 (C2H4), 32.7 (CH2C ), 52.1 MMA and St polymerizations, respectively. These values 2 2 2 (OCH3), 126.2 (CH2 C), 136.6 (CH2 C), 166.4 (C O). are similar to those for the corresponding mono-functional 3 and 4 were also synthesized from the corresponding di- transfer agent, ethyl a-(tert-butylthiomethyl)acrylate, for thiols and MCMA at room temperature, and obtained as an oil the respective polymerizations.[1] A small amount of 2 and as crystals, respectively. 4 was recrystallized from n-hex- effectively reduced the molecular weight but caused only ane, and 3 was purified by a recycling preparative HPLC a slightly decrease in the polymerization rate. A plausible (Japan Analytical Industry LC-908). The structures of 3 and 4 — were confirmed by 1H and 13C NMR spectroscopies. 3: yield explanation for the decrease in Mn with increasing conver- 27%. sion is that the rate of conversion of chain transfer agent is 1 H NMR (CDCl3): d = 1.84 (q; SCH2CH2), 2.55 (t; SCH2), lower than that of MMA monomer, as indicated by the Ctr 3.38 (s; CH2C2), 3.79 (s; OCH3), 5.67 (s; CH22), 6.21 (s; value being less than unity. CH2). Intensity ratio: 1:2:2:3:1:1. 13 C NMR (CDCl3): d = 28.5 (SCH2CH2), 30.3 (SCH2CH2), 32.7 (CH2C2), 52.1 (OCH3), 126.0 (CH22C), 136.7 Structure of PMMA 2 2 8 (CH2 C), 166.6 (C O). 4: yield 46%, mp 42–43 C. According to the AFCT mechanism, addition of a poly- 1 d H NMR (CDCl3): = 1.38 (m; SCH2CH2CH2), 1.58 (m; mer radical to 2 yielding 5 followed by rapid b-fragmen- SCH2CH2), 2.45 (t; 4H, SCH2), 3.37 (s; CH2C2), 3.79 (s; OCH ), 5.65 (s; CH 2), 6.20 (s; CH ). Intensity ratio: 3 2 2 Tab. 1. Results of polymerization in the presence of 2 in ben- 2:2:2:2:3:1:1. zene at 608C.a) 13 C NMR (CDCl3): d = 28.3 (SCH2CH2CH2), 28.9 2 — –2 — — (SCH2CH2), 31.4 (SCH2CH2), 32.6 (CH2C ), 52.0 (OCH3), Monomer 2 Time Conv. Mn 610 Mw/Mn 2 2 62 125.7 (CH2 C), 136.8 (CH2 C), 166.6 (CO). M in h in wt.-% (GPC) (GPC)

MMA 0 6 37.6 857 2.3 Polymerization procedure 0.0025 6 36.6 419 2.5 0.025 6 32.7 54.4 1.8 MMA and St polymerizations in the presence of the bifunc- 0.100 1 4.4 26.0 1.3 tional chain transfer agent were carried out in glass tubes 0.100 4 19.3 23.1 1.4 sealed under vacuum. After polymerization completion, the 0.100 8 34.9 20.6 1.4 contents of the tube were poured into a large excess of n-hex- 0.100 20 66.7 21.3 1.4 ane or methanol to precipitate the polymeric product, which 0.100 24 76.1 18.5 1.5 was dried and purified by reprecipitation. 0.125 6 30.5 15.6 1.5 PMMA to be used as a prepolymer for the preparation of a 0.125 15 76.8 14.7 1.5 — 0.250 6 27.9 10.6 1.3 block copolymer (PP, Mn (GPC) = 1470) was obtained by St 0 24 28.1 344 1.5 8 MMA polymerization in the presence of 2 for 15 h at 60 C: 0.0025 24 20.6 147 1.8 [MMA] = 2.0 mol/L, [2] = 0.125 mol/L, [AIBN] = 5.0 0.020 24 17.5 45.0 1.7 mmol/L. The polymerizations of MMA and St in the pre- 0.100 24 5.9 22.0 1.3 sence of PP were carried out in glass tubes sealed under vacuum. After the polymerization, a large amount of n-hex- a) [M] = 2.0 mol/L, [AIBN] = 5.0 mmol/L. Reactions of bifunctional addition-fragmentation chain transfer ... 1567

Tab. 2. Polymerization of MMA in the presence of 2.

2 P(GPC)a) P(NMR) P GPC 62 MMA P NMR

0.025 51 86 0.59 0.100 18 34 0.53 0.250 8 16 0.50

a) P(GPC) = [Mn(GPC) – (molecular weight of 2)]/(molecular weight of MMA) = [Mn(GPC) – 290.04]/100.12.

yielding polymer radical is not negligible as discussed later. If the chain transfer is effectively fast, the contents of b-PMMA will be small and most of the polymer chains will contain a c moiety each. The 1H NMR spectrum of the PMMA is shown in Fig. 1. The methoxy protons of a (3.74 ppm) resonated at a similar chemical shift to that of the end group formed by AFCT using methyl a-(bromomethyl)acrylate (MBMA).[14] The methylene protons between the sulfur and the vinyl group of b exhibited resonance at 3.4 ppm. Signals in the range 5.4 to 6.3 ppm are assignable to the trans- and cis-vinyl protons of a and b. If the molecular weight is sufficiently low as a result of AFCT, the end groups of almost all the polymers should be a or b. In Scheme 1. Scheme 1, P denotes any polymer chain i.e. it may or may not contain moieties from 2. The intensity of the resonances due to the vinyl protons of a and b and the methoxy protons of the MMA units were employed for calculation of the degree of polymerization (P(NMR)). Tab. 2 summarizes the results of the calculation of P(NMR) with the approximation that all the a- and x- ends are formed by AFCT where P(GPC) was obtained — from Mn(GPC). The polymers used for comparison of P(NMR) and P(GPC) were obtained in similar conversions, 30.5–34.9%, and the change in [2]/[MMA] ratio within this conversion range would not significantly affect the degree of polymerization. P(NMR) is found to be approximately twice as high as P(GPC) irrespective of [2]/[MMA]. The presence of end groups arising from ordinary bimolecular termination by disproportionation (combination does not generate new end groups) will result in overestimation of P(NMR), although the discrepancy seems to be too large for this to be the only reason. The significant decrease in molecular weight (Tab. 1) resulting from the addition of 2 indicates that AFCT Fig. 1. 1H NMR spectra of PMMA obtained by polymerization in the presence of 2. should be the main chain-stopping event under these conditions. tation gives a polymer bearing the unsaturated end group, a-P, and a thiyl radical 6 as shown in Scheme 1. 6 is expected to reinitiate polymerization to form polymer Cyclization of expelled radical involving the 2-carbomethoxypropenyl-3-thioethylthiyl The significantly higher P(NMR) than P(GPC) strongly moiety, b-P. Further reactions of b-P as an AFCT agent suggests the occurrence of a reaction of 2 to form an end lead to formation of a-P and P-c-P. Although fragmenta- group other than a and b. In order to investigate this, 2- tion of 5 to a-P is fast, a different type of fragmentation cyano-2-propyl radicals generated from AIBN were 1568 K. Tanaka, B. Yamada

obtained by evaporation of the solvent were purified by recrystallization from n-hexane, and the NMR spectra of the isolated product are shown in Fig. 2. The 1H NMR chemical shifts of the vinyl protons (5.54 and 6.26 ppm) are similar to those of the end group formed by AFCT using MBMA.[14] Furthermore, the resonances due to two types of methoxy protons (3.66 and 3.73 ppm) and the ethylene protons (2.92 ppm) are observed. Correlations between the resonances of the protons and carbons were confirmed by 1H-13C COSY as summarized in Tab. 3. The four lines at 2.99, 3.03, 3.37 and 3.40 ppm in Fig. 2 are assigned to the equatorial and axial protons of the methylene groups because of four lines and one line in the 1H and 13C NMR spectra, respectively. The same table also includes 13C DEPT data indicating methylene (negative signal), methyl or methyne (positive signal), and others (disappearance of signal) carbons.[15] Based on the assignments of all the resonances in the NMR spectra, the product was deduced to be 7. Scheme 2 shows the mechanism of formation of 7 through addition-fragmentation and cyclization as presented by Crozet et al.[16] A radical structurally similar to [17] Fig. 2. 1H (upper) and 13C NMR (lower) spectra of the reaction 8 has been proposed by Evans and Rizzardo as the product of 2 and 2-cyano-2-propyl radical. intermediate of the ring opening polymerization of 6- methylene-1,4-dithiepane suggesting that the cyclization of the thiyl radical to 8 may be reversible. Addition of the allowed to react with 2. AIBN (0.82 g, 0.005 mol) and 2 cyanopropyl radical to 2 would yield 9 which was iso- (2.90 g, 0.01 mol) dissolved in 100 mL of benzene were lated as a by-product because a large amount of AIBN heated at 608C in a nitrogen atmosphere for two AIBN relative to 2 was used. The structure of 9 was confirmed half-lives. After the reaction, crystals (mp 81–828C) by 1H NMR.

Scheme 2. Reactions of bifunctional addition-fragmentation chain transfer ... 1569

Tab. 3. NMR chemical shifts and DEPT resonances of 7.

Type of carbona) d (ppm) DEPT

1H NMR 13C NMR

1 C H2 2.92 38.3 Negative 2 C H2 2.99 39.3 Negative 3.03 – 3.37 – 3.40 – C3 – 55.0 None C42O – 174.5 None 5 OC H3 3.66 52.0 Positive 6 C H2 2.66 39.0 Negative C72 – 136.1 None 8 C H22 5.54 129.1 Negative 6.26 – C92O – 167.1 None 10 OC H3 3.73 52.0 Positive a) Numbering of the carbon atoms are indicated by 7.

Fig. 3. PMMA-d8 (I, II, and III) and PMMA (IV and V) obtained by polymerization in the presence of 2.

Tab. 4. Chemical shift assignments for the moieties of 2 in the PMMA.

Chemical shift Type of proton Scheme 3. (ppm)

1 H NMR (CDCl3): d = 1.35 (CH3), 2.60 (CH2), 3.78 (OCH3), and 5.86 and 6.43 (CH22).

Structure of end group bound to PMMA-d8 2.44–3.40 The formation of d, shown in Scheme 3 (PMMA means PMMA chain which may contain moieties from 2), was unambiguously confirmed by comparison of the 1H NMR spectrum of the PMMA with that of the PMMA-d8 prepared in the presence of 2. In the spectrum of the PMMA- d8 in CDCl3 shown in Fig. 3 some of signals can be assigned to protons of d: d = 2.66, 2.76, 3.33, and 3.36 5.50 (CH2), 2.89 (CH2CH2), and 3.68 and 3.73 (CH3O). These chemical shifts are quite similar to those of the corresponding protons of 7, confirming the formation of d-

PMMA-d8. Apparently, the cyclization of 6 to 8 takes place during the polymerizations of MMA and MMA-d8.

5.69 Fate of 2 in MMA polymerization The intensities of the 1H NMR resonances of the PMMA shown in Tab. 4 were used for calculation of the contents of the products from 2. Using a 750 MHz 1H NMR spec- intensities of the individual resonances cannot be deter- trometer, Hatada et al. assigned the signal at 2.51 ppm to mined at 2.44–3.40 ppm, and instead the total intensity the methylene protons of a in racemo-configuration.[18] of the resonances in this range was used in the calcula- They showed that the methylene protons in meso-config- tions. When the contents of b and c are denoted by x and uration appear at lower magnetic field.[18] Therefore, the y, respectively, the content of a should be (x + 2y). The 1570 K. Tanaka, B. Yamada

Fig. 5. 1H NMR spectrum of PMMA obtained by polymeriza- Fig. 4. Ratio of contents of a (0), b (H), c (f), and d (G) to that tion in the presence of 4: 26[4]/[MMA] = 0.10 (molar ratio). of MMA units of the PMMA obtained by the polymerization in the presence of 2 in benzene at 608C: [MMA] = 2.0 mol/L, 6 2 [2]/[MMA] = 0.10 (molar ratio), [AIBN] = 5.0 mmol/L. Tab. 5. Polymerization of MMA in the presence of 3.

3 Conversion P(GPC)a) P(NMR) P GPC value of (x + 2y) and x were obtained from the 1H NMR 62 MMA in wt.-% P NMR intensities at 5.50 and 5.69 ppm characteristic of a and b, respectively, enabling evaluation of y. The signal at 0.02 77.0 77 103 0.75 2.44–3.40 ppm consisted of some of the protons of a, b, 0.05 85.0 43 57 0.76 c, and d, of which the intensities can be used to estimate 0.10 75.7 25 29 0.86

the content of d according to Scheme 1. The estimated a) P(GPC) = [Mn(GPC) – (molecular weight of 3)]/(molecular ratio of [b]:[d] was approximately 1:2 at 4% conversion weight of MMA) = [Mn(GPC) – 304.43]/100.12. of MMA, suggesting that cyclization of 6 is faster than reinitiation by 6 by a factor of 2. Tab. 6. Polymerization of MMA in the presence of 4. The contents of the moieties from 2 are plotted against conversion of MMA in Fig. 4. The content of a increased 4 Conversion P(GPC)a) P(NMR) P GPC 62 slightly with increasing MMA conversion, probably due MMA in wt.-% P NMR to the conversion of b-PMMA to a-PMMA. Only a small 0.02 81.0 75 93 0.81 amount of b was present, and the content of b decreased 0.05 78.3 34 46 0.75 with the conversion of MMA indicating that b-PMMA is 0.10 78.5 20 26 0.76 an effective AFCT agent to yield a-PMMA. The contents a) of c remained almost constant, and further reactions of c P(GPC) = [Mn(GPC) – (molecular weight of 4)]/(molecular with PMMA radicals can thus be ruled out. The content weight of MMA) = [Mn(GPC) – 346.51]/100.12. of d, which was higher than that of b and c, increased with increasing conversion. A decrease in monomer con- to unity than for 2, indicating that the alkene groups centration by the proceeding polymerization facilitated between the sulfur atoms of 3 and 4 are too long for cycli- the unimolecular cyclization to d-PMMA, because the zation to occur. The discrepancy is similar to that for the rate of reinitiation with 6 would be reduced with decreas- polymerization of PMMA in the presence of MBMA.[14] ing monomer concentration. The use of PSt standards for GPC measurement and bimolecular termination by disproportionation might be the cause of this discrepancy. Polymerization of MMA in the presence of 3 and 4 3 and 4 were also synthesized and used as bifunctional AFCT agents for MMA polymerization. The 1H NMR Polymerization of St in the presence of 2 spectrum of the PMMA obtained in the presence of 4 in The 1H NMR spectrum of PSt obtained by polymerization Fig. 5 shows no signal in the range of 2.8–3.4 ppm indi- in the presence of 2 is shown in Fig. 6. The signals at 5.1

cating that the polymer does not contain any end group and 5.9 ppm are due to the CH2C2CH2 of a-PSt. The similar to d. The molecular weight of the resultant weaker signals at 5.5 and 6.2 ppm correspond to

PMMA was similar to those in Tab. 2 as displayed in CH2C2CH2 of b-PSt. The reaction of a-PSt would result Tab. 5 and 6. The ratios of P(GPC) to P(NMR) are closer in branching point e through addition of a St propagating Reactions of bifunctional addition-fragmentation chain transfer ... 1571

Fig. 6. 1H NMR spectrum of PSt obtained by polymerization in the presence of 2.

Fig. 7. 1H NMR spectra of obtained polymers before (I) and after polymerizations of MMA for 27 h (II) and St for 72 h (III) in the presence of PP in benzene at 608C: [M] = 2.0 mol/L, [a] + [b] = 0.1 mol/L, [AIBN] = 5.0 mmol/L.

Scheme 4. radical followed by coupling of the adduct radical with a PSt radical as shown in Scheme 4,[10] in which –PSt may or may not contain any group from 2. Formation of d-PSt was confirmed by the resonances at 3.0, 3.1, 3.5, and 3.6 ppm due to the equatorial and axial protons of the

SCH2CH2S group, and the resonance at 3.8 ppm was ascribed to the methylene group of St unit bound to d. 6 can add to St in competition with the cyclization, and the intensities of the respective resonances indicate that cyclization is favored. Estimation of the contents of the units from 2 in the PSt using 1H NMR spectroscopy seems to be difficult because of the possibility of the con- Fig. 8. Change in the conversion for St polymerization in the 8 sumption of a by further reactions as can be seen from presence of PP in benzene at 60 C: [St] = 2.0 mol/L, [a] + [b] = 0.1 mol/L, [AIBN] = 5.0 (f), and 50 mmol/L (9). Scheme 4.[14] (0.4%), c (0.5%), and d (3.2%) were carried out to con- firm that a was not consumed in the MMA polymeriza- Reactivity of end group tion. Fig. 7 shows the 1H NMR spectrum of PP before The polymer obtained by polymerization in the presence and after the polymerization at 75% conversion of MMA. of 2 can be used for preparation of a branched polymer, These spectra were drawn to have the same spectral and estimation of the reactivity of the unsaturated end intensity at 3.3–3.4 ppm (SC2H4S of d), which is not groups toward propagating radicals is therefore of great shown in the figure. The reaction of b-PMMA with pro- interest. Polymerizations of MMA in the presence of PP pagating radicals via AFCT produces a-PMMA. Consis- which consisted of MMA units (91.4%), a (4.5%), b tently, the spectral intensity at 5.50 ppm assignable to the 1572 K. Tanaka, B. Yamada

trans-vinyl protons of a-PMMA increased with proceeding MMA polymerization and the increase in the intensity corresponded to the decrease in that of the trans-vinyl protons of b-PMMA at 5.69 ppm. Addition of the radical to the vinyl group of a-PMMA would be followed by AFCT to regenerate a-PMMA. It can therefore be con- cluded that the reactivity of a PMMA radical towards the adduct radical is quite low. St polymerization in the presence of PP was carried out. The unsaturated end group concentration employed was higher than that of the previous paper using 1[9] for two reasons; (i) to obtain a high signal-to-noise ratio for the vinyl protons in the NMR spectrum even at high St conversion and, (ii) to establish whether a high rate of polymerization can be achieved also at a high end group concentration, thus enabling us to control the ratio [MMA unit]/[St unit] of the produced polymer over a wide range. The styrene conversion as a function of time and the 1H NMR spectrum of the obtained polymer are presented in Fig. 7 and 8, respectively. The spectral intensity at 3.4

ppm (OCH3) was adjusted to be equal to that of PP before the polymerization. When the initial concentration of AIBN was 5.0 mmol/L, the content of a-PMMA decreased by 36 mol-% within 72 h. The resonances due to the vinyl protons of b in PP diminished and new signals due to those of a-PSt appeared at 5.1 and 5.9 ppm. These findings indicate that the additions of MMA and St propagating radicals to a-PMMA are followed by b- fragmentation and reinitiation. The fragmentation of the Fig. 9. Change in the ratio of [a-PMMA] (I), [b-PMMA] (II), adduct radical of PSt radical to a-PMMA would regener- [a-PSt] (III), and [PMMA-a-PSt2] (IV) to [MMA unit] for St ate the same end group connected to a PSt chain. polymerization in the presence of PP in benzene at 608C: [St] = f Although the coupling of the adduct radical with a 2.0 mol/L, [a] + [b] = 0.1 mol/L, [AIBN] = 5.0 ( ), and 50 mmol/L (9). PMMA radical is not as fast as with a PSt radical, the reaction would yield branching group e. The contents of

a-PMMA, b-PMMA, a-PSt, and PMMA-e-PSt2 at different conversions of St were obtained by 1H NMR spectroscopy as shown in Fig. 9. The contents of a-PMMA and b-PMMA as functions of the conversion form master curves for the two initiator concentrations investigated. Higher radical concentration (as a result of higher initiator concentration) leads to a higher rate of formation of e. The rate of fragmentation, however, is not dependent on the radical concentration, hence no master curve is

obtained in the case of PMMA-e-PSt2 and a-PSt (Fig. 9 (III) and (IV)). The carbon-carbon double bonds of the PMMA bearing the moiety from 1[9] disappeared comple- tely in the polymerization of St: [unsaturated group] = –3 8.6610 mol/L, [AIBN] = 52 mmol/L, t = 40 h. In this Fig. 10. Dependence of [PMMA-e-PSt2]/([a-PSt] + [PMMA-e- study, [unsaturated group] = 0.1 mol/L, [AIBN] = 50 PSt2]) on initiator concentration in St polymerization in the pre- 8 mmol/L, t = 43 h, and the conversion of a was only 68% sence of PP in benzene at 60 C: [St] = 2.0 mol/L, [a] + [b] = 0.1 mol/L, [AIBN] = 5.0 (f) and 50 mmol/L (9). as estimated from the ratio of [a-PMMA] / [MMA unit] in Fig. 9. From the experimental data available, it is not possible to determine which of the unsaturated moieties PSt9 with the adduct radical yielding e). A higher [initia- from 1 or 2 is more reactive toward propagating radicals tor] to [a] ratio would however result in complete conver- (addition of PSt9 to a-PMMA followed by coupling of sion of a. Reactions of bifunctional addition-fragmentation chain transfer ... 1573

Scheme 5.

The radical concentration decreases considerably dur- Acknowledgement: The authors are grateful to Dr. Per B. Zet- ing the three initiator half-lives and it follows that the terlund, Department of Applied Chemistry, Faculty of Engineer- ing, Osaka City University, for valuable discussions. ratio of the contents of PMMA-e-PSt2 to that of a-PSt should decrease with conversion. As shown in Fig. 10, Received: January 24, 2000 the ratio remains constant however, presumably a result Revised: March 31, 2000 of a lower radical concentration leading to a decrease in the rate of consumption of a-PSt by the reactions outlined in Scheme 4. All the possible reactions of a-PMMA and [1] G. F. Meijs, E. Rizzardo, S. H. Thang, Macromolecules subsequent reactions of the adduct radicals are shown in 1988, 21, 3122. Scheme 5. In this scheme, –PMMA are polymer chains [2] G. F. Meijs, E. Rizzardo, S. H. Thang, Polym. Bull. 1990, 24, 504. which may contain moieties from 2. [3] G. F. Meijs, T. C. Morton, E. Rizzardo, S. H. Thang, Macromolecules 1991, 24, 3689. [4] B. Yamada, S. Kobatake, Prog. Polym. Sci. 1994, 19, 1089. [5] J. Krstina, G. Moad, E. Rizzardo, C. L. Winzor, C. T. Conclusions Berge, M. Fryd, Macromolecules 1995, 28, 5381. The polymerization of MMA in the presence of 2 gave [6] N. S. Enikolopyan, B. R. Smirnov, G. V. Ponomarev, I. M. polymer containing a novel cyclic end group together with Belgouskii, J. Polym. Sci., Part A: Polym. Chem. 1981, 19, 879. the unsaturated end group formed by AFTC. Cyclization [7] C. L. Moad, G. Moad, E. Rizzardo, S. H. Thang, Macromo- of the incipient radical from 2 was revealed by comparison lecules 1996, 29, 7717. 1 of the H NMR spectra of the PMMA and PMMA-d8. A [8] B. Yamada, O. Konosu, Polym. Prepr. (ACS) 1998, 39 (1), similar cyclization product was found to be produced in 371. the reaction of 2 with 2-cyano-2 propyl radical. The num- [9] B. Yamada, O. Konosu, Kobunshi Ronbunshu 1997, 54, 723. ber of methylene groups connecting the chain transfer [10] B. Yamada, O. Konosu, K. Tanaka, F. Oku, Polymer 2000, moieties is crucial in determining the rate of cyclization. in press. Polymerization in the presence of 2 gave 72% unsaturated [11] R. F. B. Cox, R. T. Stormont, Org. Syn., Coll. Vol. II, p. 7. end groups, whereas the use of 3 and 4 resulted in a negli- [12] B. Yamada, S. Kobatake, S. Aoki, Macromolecules 1993, gible amount of cyclized end groups. Addition of the PSt 26, 5099. [13] G. Odian, “Principles of Polymerization”, 3rd edition, J. propagating radical to the unsaturated end group is feasi- Wiley & Sons, New York 1991, p. 253. ble, and the adduct radical readily couples with the propa- [14] B. Yamada, S. Kobatake, T. Otsu, Polym. J. 1992, 24, 281. gating radical to yield a branched structure. Branch forma- [15] D. M. Doddrell, D. T. Pegg, M. R. Bendall, J. Magn. tion relative to fragmentation is facilitated at higher initia- Reson. 1982, 48, 323. tor concentration. The synthetic route to branched polymer [16] M.-P. Crozet, J.-M. Surzur, C. Dupuy, Tetrahedron Lett. 1971, 25, 2031. developed here for St and MMA is advantageous in its [17] R. A. Evans, E. Rizzardo, Macromolecules 1996, 29, 6983. simplicity and lack of gelation, and could be applicable to [18] K. Hatada, T. Kitayama, K. Ute, Y. Terawaki, T. Yanagida, a wide variety of monomers. Macromolecules 1997, 30, 6754.