Living Cationic Polymerization of p-alkoxystyrenes by Free Ionic Species SHOKYOKU KANAOKA, TOSHINOBU HIGASHIMURA Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka, Hikone 522-8533, Japan Received 17 May 1999; accepted 1 June 1999 ABSTRACT: In contrast to the common view, living cationic polymerization of p-methoxy- and p-t-butoxystyrenes proceeded in polar solvents such as EtNO 2 /CH 2 Cl 2 mixtures, and involvement of free ionic growing species therein was examined. For example, the two alkoxystyrenes were polymerized with the isobutyl vinyl ether-hcl adduct/zncl 2 initiating system at 15 C in such polar solvents as CH 2 Cl 2 or EtNO 2 / CH 2 Cl 2 [1/1 (v/v)], as well as toluene. The number average molecular weight (M n)ofthe polymers increased in direct proportion to the monomer conversion, even after sequential monomer addition, and the molecular weight distribution (MWD) stayed very narrow throughout the reaction. In addition, the M n agreed with the calculated values, assuming that one adduct molecule generates one living polymer chain. In these polar media the addition of a common ion salt retarded the polymerization, indicating that dissociated ionic species are involved in the propagating reaction. 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3694 3701, 1999 Keywords: cationic polymerization; living polymerization; free ion; p-t-butoxystyrene; p-methoxystyrene; zinc chloride; tin tetrachloride INTRODUCTION In the cationic polymerization of vinyl monomers the -proton of the growing carbocation is acidic enough to be abstracted by a counterion and/or a monomer to readily induce chain transfer reactions. Therefore, it was considered that living polymerization may be achieved when the -proton becomes much less acidic. We expected that a counteranion that was suitably nucleophilic enough to suppress the dissociation of the growing species would render the -proton less acidic and prevent chain transfer but would still be able to induce polymerization. 1 In fact, with HI as an initiator, living cationic polymerization of isobutyl vinyl ether (IBVE) occurs when a weak Lewis acid 2,3 such as I 2 and ZnI 2 is employed as an Correspondence to: T. Higashimura Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 3694 3701 (1999) 1999 John Wiley & Sons, Inc. CCC 0887-624X/99/193694-08 activator whereas SnCl 4, which is a stronger Lewis acid, induces a polymerization accompanied by chain transfer. 4 Recent studies by various groups in turn suggest that the growing species in living cationic polymerization has the same nature as that in conventional cationic polymerization, but it is in an equilibrium between active and dormant species, which lowers the concentration of the active ionic species to suppress undesired side reactions. 5 Although these two general explanations differ in their mechanism, one can conclude that in living cationic processes dissociated or free ionic growing species would be absent or that their concentration would be extremely low. 3694
LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3695 There are in fact no reports of living cationic polymerization mediated by a free ion, because the -proton of the free ionic carbocation was considered so acidic that chain transfer reactions would readily occur, as mentioned above. According to our recent study on the steric structure of poly(vinyl ethers), 6 however, living cationic polymerization apparently occurs even at a relatively higher concentration of the free ionic species when IBVE is polymerized with the HCl-IBVE adduct (1)/ZnCl 2 initiating system. Because we could deduce the possibility of living cationic polymerization by free ions from the steric structure of polymers, the next direction of our study is to provide more direct evidence for the involvement of free ionic species. To this end, we turned our attention to the polymerization rate that gives direct information about the growing species (i.e., whether they are dissociated or not). The second phase should be to extend living polymerization by free ions to monomers other than vinyl ethers, such as styrene derivatives. The results would help us formulate general principles and/or new concepts for living cationic polymerization. Therefore, the focus of this study is to examine the feasibility of living cationic polymerization of p-alkoxystyrenes mediated by dissociated species in polar media. p-alkoxystyrenes were selected as the monomer because the electron-donating alkoxy group therein is expected to stabilize the growing end and because their living polymerizations are feasible, at least in nonpolar media. 7 9 tbos, tetrahydronaphthalene for pmos) were washed by the usual methods and distilled at least twice over calcium hydride before use. Diethyl ether (anhydrous) was distilled once over LiAlH 4. Procedures Polymerizations were carried out under dry nitrogen in a baked glass tube with a three-way stopcock. The reactions were initiated by sequentially adding a prechilled adduct 1 solution (0.50 ml) and a ZnCl 2 solution (0.50 ml in diethyl ether) to a monomer solution (4.0 ml, 0.38M) at the polymerization temperature. After a certain period the polymerization was terminated with prechilled methanol (ca. 2 ml) containing a small amount of ammonia. Monomer conversion was determined from its residual concentration measured by gas chromatography. The quenched reaction mixtures were washed with 10% aqueous sodium thiosulfate and then with water. After the solvents were removed by evaporation under reduced pressure, the polymers were dried under a vacuum overnight at room temperature. Gel permeation chromatography (GPC) was performed in chloroform (1.0 ml/ min flow rate) at 40 C using a Shimadzu LC-10A liquid chromatograph system equipped with a Shimadzu LC-10AD pump, three polystyrene gel columns (Shodex K-802, 803, and 804), and refractive index/uv dual detectors. The number average molecular weight (M n) and polydispersity ratio (M w/m n) of the polymers were calculated from chromatographs relative to commercial polystyrene standards with molecular weights ranging from 800 to 4 10 5. EXPERIMENTAL Materials p-t-butoxystyrene (tbos, Hokko Chemicals) and p-methoxystyrene (pmos, Aldrich) were purified by double distillation over calcium hydride under reduced pressure. The HCl-IBVE adduct (1) was prepared by bubbling dry HCl gas into an n-hexane solution of IBVE. 10 ZnCl 2 (1.0M solution in diethyl ether, Aldrich) and nbu 4 NCl (purity 98%, Tokyo Chemical Industry) were used as commercially supplied. Solvents (toluene, dichloromethane, and nitroethane) and internal standards for gas chromatography (bromobenzene for RESULTS AND DISCUSSION Polymerization of tbos Effects of Solvent Polarity Cationic polymerization of tbos at 15 C was examined with the 1/ZnCl 2 initiating system, which is known to induce the living polymerization of IBVE involving dissociated free ionic species, 6 as well as nondissociated species. 10 The polymerization solvents employed in this study were nonpolar toluene, polar dichloromethane (CH 2 Cl 2 ), and a nitoroethane (EtNO 2 )/CH 2 Cl 2 mixture [1/1 (v/v)] as the most polar counterpart. We ran the experiments at 15 C, which is a
3696 KANAOKA AND HIGASHIMURA very narrow (M w/m n 1.1 1.2) at any monomer conversion. Figure 1. (A) Time-conversion curves and (B) M n and M w/m n of poly(tbos) obtained with 1/ZnCl 2 at 15 C: [M] 0 0.38M, [1] 0 10.0 mm, [ZnCl 2 ] 0 5.0 mm. (B) The diagonal solid line indicates the M n calculated from the feed ratio of tbos to 1 and monomer conversion. relatively higher temperature for cationic polymerization, at which living polymerization of tbos was achieved with HI/ZnI 2 in a nonpolar solvent. 8 As shown in Figure 1, the polymerization rate sharply increased with increasing solvent polarity and in the EtNO 2 /CH 2 Cl 2 mixture it became 70 80 times as large as that in toluene. In contrast, the M n of the product polymers increased proportionally to the monomer conversion independently of the solvent polarity, which agreed with the calculated values assuming that one polymer chain forms per molecule of 1, and the molecular weight distributions (MWDs) were Living Polymerization by Free Ionic Growing Species The clearly greater polymerization rates in both CH 2 Cl 2 and EtNO 2 /CH 2 Cl 2 suggest that free ionic growing species with higher reactivity would be involved. To confirm their involvement in these polar media, the effects of common ion salts were examined. In EtNO 2 /CH 2 Cl 2 the tbos was thus polymerized with 1/ZnCl 2 in the presence of nbu 4 NCl (0.10 equivalent to 1). The addition of the salt retarded the polymerization [Fig. 2(A)], while the MWD of the polymers stayed narrow throughout the reaction and the M n values were in direct proportion to the conversion [Fig. 2(B)] in the presence and absence of the salt. The retardation by the salt indicated that the concentration of the dissociated ionic species was reduced in the salt-containing polar solvent. These results, in turn, demonstrated that the living polymerization of tbos in the salt-free EtNO 2 / CH 2 Cl 2 mixture was mediated by the free ionic growing species, at least in part. To further confirm the occurrence of living polymerization in the presence of the dissociated or free ionic species, a fresh feed of tbos ([tbos] 0 [tbos] add 0.38M) was added to the reaction mixture when monomer conversion reached about 100% in both CH 2 Cl 2 and the EtNO 2 /CH 2 Cl 2 mixture. On the addition of the monomer the secondphase polymerization occurred without an induction phase at a rate similar to that of the first stage. In both media the MWDs of the polymers remained narrow (M w/m n 1.1), and the relationship between conversion and the M n values was linear (Fig. 3). Figure 4 shows the MWD curves of the polymers thus obtained in CH 2 Cl 2 and the EtNO 2 /CH 2 Cl 2 mixture. After the sequential monomer addition, the MWDs shifted toward a higher molecular weight, although a slight amount of lower molecular weight products was observed, which were probably due to partial deactivation of the living ends in the first stage. However, it should be noted that almost all the growing species initiated the second-stage polymerization, establishing their living nature even in such a strongly polar solvent. Effects of Counterions In the preceding section we demonstrated that living polymerization can occur even in the pres-
LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3697 ence of a certain amount of free ionic species, but the mechanism still remains unfound. Herein the interaction of a counterion with the growing carbocations may play a key roll; to investigate the effects of counteranions, polymerization of tbos was examined in polar media with 1 in conjunction with SnCl 4, a Lewis acid stronger than ZnCl 2. The SnCl 4 -mediated polymerization was very fast and completed in a few seconds, which is much faster than that with ZnCl 2, and the reaction was not reproducible. Table I summarizes the results of typical experiments. The M n of the polymers obtained in CH 2 Cl 2 agreed with the calculated values, assuming that one polymer chain forms per molecule of 1, whereas those in EtNO 2 / CH 2 Cl 2 showed higher M n. However, the polymers obtained in both polar media showed broad MWDs. In addition, the reaction mixture turned reddish orange as soon as SnCl 4 was added, indicating side reactions including -proton abstraction. Monomer-addition experiments were carried out to examine whether long-lived species exist in the polymerization with SnCl 4 in EtNO 2 /CH 2 Cl 2 Figure 2. (A) Time-conversion curves and (B) M n and M w/m n of poly(tbos) obtained with 1/ZnCl 2 at 15 C in the absence or presence of a common ion salt: [M] 0 0.38M, [1] 0 10.0 mm, [ZnCl 2 ] 0 5.0 mm, [nbu 4 NCl] 0 1.0 mm. (B) The diagonal solid line indicates the M n calculated from the feed ratio of tbos to 1 and monomer conversion. Figure 3. The M n and M w/m n of the polymers obtained in (A) CH 2 Cl 2 and (B) EtNO 2 /CH 2 Cl 2 in the monomer-addition experiments in the polymerization of tbos with 1/ZnCl 2 at 15 C: [M] 0 [M] add 0.38M, [1] 0 10.0 mm, [ZnCl 2 ] 0 5.0 mm. (A,B) The diagonal solid lines indicate the M n calculated from the feed ratio of tbos to 1 and monomer conversion.
3698 KANAOKA AND HIGASHIMURA Figure 4. MWD curves of poly(tbos) obtained with 1/ZnCl 2 in the monomer addition experiments at 15 C in (A) CH 2 Cl 2 and (B) EtNO 2 /CH 2 Cl 2 : [M] 0 [M] add 0.38M, [1] 0 10.0 mm, [ZnCl 2 ] 0 5.0 mm. Figure 5. MWD curves of poly(tbos) obtained with 1/SnCl 4 in a monomer addition experiment at 15 C in EtNO 2 /CH 2 Cl 2 : [M] 0 [M] add 0.38M, [1] 0 10.0 mm, [SnCl 4 ] 0 5.0 mm. (Fig. 5). A second-stage polymerization was indeed initiated without an induction phase, and the second tbos feed was consumed quantitatively in a few seconds. As shown in Figure 5, however, only a part of the growing ends apparently initiated the second-stage polymerization to give the higher molecular weight fraction. These results imply that at least a part of the growing ends is long-lived in the polymerization with 1/SnCl 4. In separate studies we recently found that the addition of nbu 4 NCl is crucial in achieving living polymerization of styrene 11 and IBVE 12 with SnCl 4. The salt is also expected to exhibit a similar effect on the tbos polymerization, because the reactivity of tbos lies between IBVE and styrene. In fact, as observed with styrene, 11 the polymerization slowed down in the presence of the salt (0.10 equivalent to SnCl 4 ), but the molecular weight and MWD were less controllable than in its absence. The dependence of tbos polymerization on the activators (ZnCl 2, SnCl 4 ) suggests that different types of termination and/or chain transfer reactions occur with the two activators. These marked effects of the activators further suggest the interaction of a counterion with the carbocation, even if the latter is free ionic or dissociated. Polymerization of pmos We found that pmos, which has an electrondonating alkoxy group on its phenyl ring as tbos does, can be polymerized in a living fashion with HI/ZnI 2 at 25 C in a nonpolar solvent such as toluene. 7 On the other hand, in CH 2 Cl 2 the prod- Table I. Cationic Polymerization of tbos with 1/SnCl 4 at 15 C M n Solvent Time (s) Conversion (%) Obs Calcd CH 2 Cl 2 5.0 100 6,710 6,700 EtNO 2 /CH 2 Cl 2 5.0 100 8080 6,700 6.0 6.0 a 200 Bimodal 13,400 [M] 0 0.38M, [1] 0 10.0 mm, [SnCl 4 ] 0 5.0 mm. a Monomer-addition experiment: [M] 0 [M] add 0.38M. The values combined by the plus sign represent the reaction time in the first and the second stage, respectively.
LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3699 uct polymers had bimodal MWDs, although the average molecular weight increased with monomer conversion. 9 The lower molecular weight fraction of the product was generated from a longlived growing species because its molecular weight increased with monomer conversion, whereas that of the higher polymer fraction stayed unchanged during the polymerization. In CH 2 Cl 2, however, the addition of a common ion salt (nbu 4 NI) led to a living pmos polymerization that gave polymers with narrow MWDs. This fact suggested that only nondissociated growing species can lead to living polymers, which is not consistent with the occurrence of living tbos polymerization with 1/ZnCl 2 in a polar solvent, as already discussed. In this regard, it Figure 7. The M n and M w/m n of the polymers obtained in (A) CH 2 Cl 2 and (B) EtNO 2 /CH 2 Cl 2 in the monomer-addition experiments in the polymerization of pmos with 1/ZnCl 2 at 15 C: [M] 0 [M] add 0.38M, [1] 0 10.0 mm, [ZnCl 2 ] 0 5.0 mm. is of interest whether the structure of the monomers or the type of initiating systems primarily affects the polymerization behavior. Thus, we examined the polymerization of pmos with the HCl-IBVE adduct (1)/ZnCl 2 instead of HI/ZnI 2 in various solvents and the feasibility of living polymerization mediated by a free ionic growing species. Figure 6. (A) Time-conversion curves and (B) M n and M w/m n of poly(pmos) obtained with 1/ZnCl 2 at 15 C: [M] 0 0.38M, [1] 0 10 mm, [ZnCl 2 ] 0 5.0 mm. Effects of Solvent Polarity on Polymerization pmos was polymerized with 1/ZnCl 2 in polar solvents at 15 C. As shown in Figure 6(A), the
3700 KANAOKA AND HIGASHIMURA Figure 8. MWD curves of poly(pmos) obtained with 1/ZnCl 2 in the monomer-addition experiments at 15 C in (A) CH 2 Cl 2 and (B) EtNO 2 /CH 2 Cl 2 : [M] 0 [M] add 0.38M, [1] 0 10.0 mm, [ZnCl 2 ] 0 5.0 mm. polymerization rate sharply increased with solvent polarity, which was most likely due to an increased concentration of free ionic species, as observed with tbos. Inspection of Figures 1 and 6 reveals that the polymerization rate is greater with pmos than with tbos under the same reaction conditions. Similar results were obtained in the polymerization with HI/ZnI 2. 8 However, pmos is polymerized slightly faster in CH 2 Cl 2 with HI/ZnI 2 than with 1/ZnCl 2 [e.g., conversion in 10 min: HI/ZnI 2, 80% ([M] 0 0.50M); 1/ZnCl 2, 60% ([M] 0 0.38M)]. The COCl bond is harder to ionize than the COI counterpart in generating fewer ionic species. Thus, the polymerization rate with 1/ZnCl 2 became smaller than that with HI/ZnI 2, although ZnCl 2 is more Lewis acidic than ZnI 2. Figure 6(B) shows the conversion dependence of M n and M w/m n for the pmos polymers formed in the polar solvents. In both cases the M n increased proportionally to conversion and the polymers had very narrow MWDs (M w/m n 1.1), although the M n determined by GPC deviated from the calculated values, assuming that one 1 molecule generates one polymer chain. The deviation of the observed M n is because it was calculated on the basis of a polystyrene calibration (see the Experimental section). Thus, living polymers can be obtained in a strongly polar solvent, not only with tbos, but also with pmos. Living Polymerization in Polar Solvent To confirm the living nature of the pmos polymerization by 1/ZnCl 2 in polar media, a fresh feed of pmos ([pmos] 0 [pmos] add 0.38 M) was added to the reaction mixture just before the initial charge of pmos was completely polymerized. The added pmos was smoothly polymerized without an induction phase at nearly the same rate as in the initial stage. Figure 7 shows that the M n of the polymer further increased in direct proportion to monomer conversion. In addition, even after the monomer addition the MWD of the polymer remained narrow and clearly shifted toward a higher molecular weight without a shoulder in the lower molecular weight region (Fig. 8). Thus, the living nature of the pmos polymerization with 1/ZnCl 2 was demonstrated even in a strongly polar solvent where the propagating species contain free ionic carbocations. CONCLUSION Living cationic polymerization of the p-alkoxystyrenes tbos and pmos with 1/ZnCl 2 were observed even in such polar solvents as CH 2 Cl 2 or EtNO 2 / CH 2 Cl 2 where the free ionic species is involved in the propagation as is suggested with vinyl ethers. 6 In general, the dissociated or free ionic carbocations are highly reactive but unstable; thus, living poly-
LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3701 merization mediated by free ionic species had been considered difficult for a long time. However, the results of our study showed that by utilizing a suitably nucleophilic counteranion even free ionic growing species can induce living polymerization of the monomers such as vinyl ethers and p-alkoxystyrenes whose substituents can stabilize the growing carbocations. It should be emphasized that the choice of a counterion (Cl vs. I, ZnCl 2 vs. SnCl 4 )is crucial to achieving living cationic polymerization via free ionic species. This work was supported in part by a Grant-in-Aid for Scientific Research (C) (No. 08651056) from the Ministry of Education, Science, Sports, and Culture, Japan, for which T. H. is grateful. We thank Yasuko Kubota for her assistance in the polymerization experiments. REFERENCES AND NOTES 1. See the review of Higashimura, T.; Sawamoto, M. Adv Polym Sci 1984, 62, 49. 2. Miyamoto, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1984, 17, 265. 3. Sawamoto, M.; Okamoto, C.; Higashimura, T. Macromolecules 1987, 20, 2693. 4. Higashimura, T.; Aoshima, S.; Sawamoto, M. Makromol Chem Macromol Symp 1988, 13/14, 457. 5. See the review of Matyjaszewski, K.; Sawamoto, M. In Cationic Polymerization: Mechanisms, Synthesis and Applications; Matyjaszewski, K., Ed.; Marcel Dekker: New York, 1996; Chap. 4. 6. Kanaoka, S.; Sawamoto, M.; Higashimura, T. Macromol Symp 1998, 132, 75. 7. Higashimura, T.; Kojima, K.; Sawamoto, M. Polym Bull 1988, 19, 7. 8. Higashimura, T.; Kojima, K.; Sawamoto, M. Makromol Chem 1989, 15, 127. 9. Kojima, K.; Sawamoto, M.; Higashimura, T. Macromolecules 1990, 23, 948. 10. Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1992, 25, 2587. 11. (a) Ishihama, Y.; Sawamoto, M.; Higashimura, T. Polym Bull 1990, 23, 361; (b) Ishihama, Y.; Sawamoto, M.; Higashimura, T. Polym Bull 1990, 24, 201; (c) Higashimura, T.; Ishihama, Y.; Sawamoto, M. Macromolecules 1993, 26, 744. 12. Kamigaito, M.; Maeda, Y.; Sawamoto, M.; Higashimura, T. Macromolecules 1993, 26, 1643.