Living Cationic Polymerization of P-Alkoxystyrenes by Free Ionic Species

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-me-

thoxy- and p-t-butoxystyrenes proceeded in polar solvents such as EtNO2/CH2Cl2 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 initiating system at Ϫ15°C in such polar solvents as CH Cl or EtNO / 2 2 2 ៮ 2 CH2Cl2 [1/1 (v/v)], as well as toluene. The number average molecular weight (Mn)ofthe polymers increased in direct proportion to the monomer conversion, even after sequen- tial monomer addition, and the molecular weight distribution (MWD) stayed very ៮ narrow throughout the reaction. In addition, the Mn 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 activator whereas SnCl4, which is a stronger Lewis acid, induces a polymerization accompa- In the cationic polymerization of vinyl monomers nied by chain transfer.4 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 Recent studies by various groups in turn sug- enough to suppress the dissociation of the grow- gest that the growing species in living cationic ing species would render the ␤-proton less acidic polymerization has the same nature as that in and prevent chain transfer but would still be able conventional cationic polymerization, but it is in 1 to induce polymerization. In fact, with HI as an an equilibrium between active and dormant spe- initiator, living cationic polymerization of isobu- cies, which lowers the concentration of the active tyl vinyl ether (IBVE) occurs when a weak Lewis ionic species to suppress undesired side reac- 2,3 acid such as I2 and ZnI2 is employed as an tions.5 Although these two general explanations differ in their mechanism, one can conclude that in living cationic processes dissociated or free Correspondence to: T. Higashimura ionic growing species would be absent or that Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 3694–3701 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-624X/99/193694-08 their concentration would be extremely low. 3694 LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3695

tBOS, tetrahydronaphthalene for pMOS) were washed by the usual methods and distilled at least twice over calcium hydride before use. Di- ethyl ether (anhydrous) was distilled once over There are in fact no reports of living cationic LiAlH4. polymerization mediated by a free ion, because ␤ the -proton of the free ionic carbocation was Procedures considered so acidic that chain transfer reactions would readily occur, as mentioned above. Accord- Polymerizations were carried out under dry nitro- ing to our recent study on the steric structure of gen in a baked glass tube with a three-way stop- poly(vinyl ethers),6 however, living cationic poly- cock. The reactions were initiated by sequentially merization apparently occurs even at a relatively adding a prechilled adduct 1 solution (0.50 mL) higher concentration of the free ionic species and a ZnCl2 solution (0.50 mL in diethyl ether) to when IBVE is polymerized with the HCl-IBVE a monomer solution (4.0 mL, 0.38M) at the polymerization temperature. After a certain period adduct (1)/ZnCl2 initiating system. Because we could deduce the possibility of liv- the polymerization was terminated with pre- ing cationic polymerization by free ions from the chilled methanol (ca. 2 mL) containing a small steric structure of polymers, the next direction of amount of ammonia. Monomer conversion was our study is to provide more direct evidence for determined from its residual concentration mea- the involvement of free ionic species. To this end, sured by gas chromatography. we turned our attention to the polymerization The quenched reaction mixtures were washed rate that gives direct information about the grow- with 10% aqueous sodium thiosulfate and then ing species (i.e., whether they are dissociated or with water. After the solvents were removed by not). The second phase should be to extend living evaporation under reduced pressure, the poly- polymerization by free ions to monomers other mers were dried under a vacuum overnight at than vinyl ethers, such as styrene derivatives. room temperature. Gel permeation chromatogra- The results would help us formulate general prin- phy (GPC) was performed in chloroform (1.0 mL/ ciples and/or new concepts for living cationic min ﬂow rate) at 40°C using a Shimadzu LC-10A polymerization. liquid chromatograph system equipped with a Therefore, the focus of this study is to examine Shimadzu LC-10AD pump, three polystyrene gel the feasibility of living cationic polymerization of columns (Shodex K-802, 803, and 804), and re- p-alkoxystyrenes mediated by dissociated species fractive index/UV dual detectors. The number average molecular weight (M៮ ) and polydispersity in polar media. p-Alkoxystyrenes were selected as ៮ ៮ n the monomer because the electron-donating ratio (Mw/Mn) of the polymers were calculated alkoxy group therein is expected to stabilize the from chromatographs relative to commercial poly- growing end and because their living polymeriza- styrene standards with molecular weights rang- 5 tions are feasible, at least in nonpolar media.7–9 ing from 800 to 4 ϫ 10 .

EXPERIMENTAL RESULTS AND DISCUSSION Polymerization of tBOS Materials Effects of Solvent Polarity p-t-Butoxystyrene (tBOS, Hokko Chemicals) and p-methoxystyrene (pMOS, Aldrich) were puriﬁed Cationic polymerization of tBOS at Ϫ15°C was by double distillation over calcium hydride under examined with the 1/ZnCl2 initiating system, reduced pressure. The HCl-IBVE adduct (1) was which is known to induce the living polymeriza- prepared by bubbling dry HCl gas into an n-hex- tion of IBVE involving dissociated free ionic spe- 10 6 10 ane solution of IBVE. ZnCl2 (1.0M solution in cies, as well as nondissociated species. The diethyl ether, Aldrich) and nBu4NCl (purity polymerization solvents employed in this study Ͼ 98%, Tokyo Chemical Industry) were used as were nonpolar toluene, polar dichloromethane commercially supplied. Solvents (toluene, dichlo- (CH2Cl2), and a nitoroethane (EtNO2)/CH2Cl2 romethane, and nitroethane) and internal stan- mixture [1/1 (v/v)] as the most polar counterpart. dards for gas chromatography (bromobenzene for We ran the experiments at Ϫ15°C, which is a 3696 KANAOKA AND HIGASHIMURA

៮ ៮ ϭ very narrow (Mw/Mn 1.1–1.2) at any monomer conversion.

Living Polymerization by Free Ionic Growing Species The clearly greater polymerization rates in both

CH2Cl2 and EtNO2/CH2Cl2 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 EtNO2/CH2Cl2 the tBOS was thus polymerized with 1/ZnCl2 in the presence of nBu4NCl (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 Mn values were in direct proportion to the conversion [Fig. 2(B)] in the presence and absence of the salt. The re- tardation 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 EtNO2/ CH2Cl2 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 ϭ ϭ [tBOS]add 0.38M) was added to the reaction mixture when monomer conversion reached about

100% in both CH2Cl2 and the EtNO2/CH2Cl2 mixture. On the addition of the monomer the second- ៮ Figure 1. (A) Time-conversion curves and (B) Mn and phase polymerization occurred without an induc- ៮ ៮ Ϫ Mw/Mn of poly(tBOS) obtained with 1/ZnCl2 at 15°C: tion phase at a rate similar to that of the ﬁrst [M] ϭ 0.38M,[1] ϭ 10.0 mM, [ZnCl ] ϭ 5.0 mM. (B) 0 0 2 0 stage. In both media the MWDs of the polymers The diagonal solid line indicates the M៮ calculated ៮ ៮ n remained narrow (M /M Ͻ 1.1), and the rela- from the feed ratio of tBOS to 1 and monomer conver- w n tionship between conversion and the M៮ values sion. n was linear (Fig. 3). Figure 4 shows the MWD

curves of the polymers thus obtained in CH2Cl2 and the EtNO /CH Cl mixture. After the se- relatively higher temperature for cationic poly- 2 2 2 quential monomer addition, the MWDs shifted merization, at which living polymerization of toward a higher molecular weight, although a tBOS was achieved with HI/ZnI in a nonpolar 2 slight amount of lower molecular weight products solvent.8 was observed, which were probably due to partial As shown in Figure 1, the polymerization rate deactivation of the living ends in the ﬁrst stage. sharply increased with increasing solvent polar- However, it should be noted that almost all the ity and in the EtNO /CH Cl mixture it became 2 2 2 growing species initiated the second-stage poly- 70–80 times as large as that in toluene. In con- merization, establishing their living nature even trast, the M of the product polymers increased n in such a strongly polar solvent. proportionally to the monomer conversion inde- pendently of the solvent polarity, which agreed Effects of Counterions with the calculated values assuming that one polymer chain forms per molecule of 1, and the In the preceding section we demonstrated that molecular weight distributions (MWDs) were living polymerization can occur even in the pres- LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3697

culated values, assuming that one polymer chain forms per molecule of 1, whereas those in EtNO / ៮ 2 CH2Cl2 showed higher Mn. However, the polymers obtained in both polar media showed broad MWDs. In addition, the reaction mixture turned

reddish orange as soon as SnCl4 was added, indicating side reactions including ␤-proton abstrac- tion. Monomer-addition experiments were carried out to examine whether long-lived species exist in

the polymerization with SnCl4 in EtNO2/CH2Cl2

៮ Figure 2. (A) Time-conversion curves and (B) Mn and ៮ ៮ Ϫ Mw/Mn of poly(tBOS) obtained with 1/ZnCl2 at 15°C in the absence or presence of a common ion salt: [M]0 ϭ ϭ ϭ 0.38M,[1]0 10.0 mM, [ZnCl2]0 5.0 mM, [nBu NCl] ϭ 1.0 mM. (B) The diagonal solid line in- 4 0 ៮ dicates the Mn calculated from the feed ratio of tBOS to 1 and monomer conversion.

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 conjunc-

tion with SnCl4, a Lewis acid stronger than ៮ ៮ ៮ Figure 3. The Mn and Mw/Mn of the polymers ob- ZnCl2. The SnCl4-mediated polymerization was tained in (A) CH2Cl2 and (B) EtNO2/CH2Cl2 in the very fast and completed in a few seconds, which is monomer-addition experiments in the polymerization much faster than that with ZnCl , and the reac- Ϫ ϭ ϭ 2 of tBOS with 1/ZnCl2 at 15°C: [M]0 [M]add 0.38M, tion was not reproducible. Table I summarizes [1] ϭ 10.0 mM, [ZnCl ] ϭ 5.0 mM. (A,B) The diagonal ៮ 0 2 0 ៮ the results of typical experiments. The Mn of the solid lines indicate the Mn calculated from the feed polymers obtained in CH2Cl2 agreed with the cal- ratio of tBOS to 1 and monomer conversion. 3698 KANAOKA AND HIGASHIMURA

Figure 4. MWD curves of poly(tBOS) obtained with

1/ZnCl2 in the monomer addition experiments at Ϫ15°C in (A) CH Cl and (B) EtNO /CH Cl : [M] Figure 5. MWD curves of poly(tBOS) obtained with 2 2 2 2 2 0 Ϫ ϭ [M] ϭ 0.38M,[1] ϭ 10.0 mM, [ZnCl ] ϭ 5.0 mM. 1/SnCl4 in a monomer addition experiment at 15°C in add 0 2 0 ϭ ϭ ϭ EtNO2/CH2Cl2: [M]0 [M]add 0.38M,[1]0 10.0 ϭ mM, [SnCl4]0 5.0 mM. (Fig. 5). A second-stage polymerization was in- deed initiated without an induction phase, and the second tBOS feed was consumed quantita- ular weight and MWD were less controllable than tively in a few seconds. As shown in Figure 5, in its absence. however, only a part of the growing ends appar- The dependence of tBOS polymerization on the ently initiated the second-stage polymerization to activators (ZnCl2, SnCl4) suggests that different give the higher molecular weight fraction. These types of termination and/or chain transfer reac- results imply that at least a part of the growing tions occur with the two activators. These marked ends is long-lived in the polymerization with effects of the activators further suggest the inter-

1/SnCl4. action of a counterion with the carbocation, even In separate studies we recently found that the if the latter is free ionic or dissociated.

addition of nBu4NCl is crucial in achieving living polymerization of styrene11 and IBVE12 with Polymerization of pMOS SnCl4. The salt is also expected to exhibit a similar effect on the tBOS polymerization, because We found that pMOS, which has an electron- the reactivity of tBOS lies between IBVE and donating alkoxy group on its phenyl ring as tBOS styrene. In fact, as observed with styrene,11 the does, can be polymerized in a living fashion with

polymerization slowed down in the presence of HI/ZnI2 at 25°C in a nonpolar solvent such as 7 the salt (0.10 equivalent to SnCl4), but the molec- toluene. On the other hand, in CH2Cl2 the prod-

Ϫ Table I. Cationic Polymerization of tBOS with 1/SnCl4 at 15°C ៮ Mn

Solvent Time (s) Conversion (%) Obs Calcd

CH2Cl2 5.0 100 6,710 6,700 EtNO2/CH2Cl2 5.0 100 8080 6,700 6.0 ϩ 6.0a 200 Bimodal 13,400

ϭ ϭ ϭ [M]0 0.38M, [1]0 10.0 mM, [SnCl4]0 5.0 mM. a ϭ ϭ 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 long- lived growing species because its molecular weight increased with monomer conversion, whereas that of the higher polymer fraction stayed unchanged during the polymerization.

In CH2Cl2, however, the addition of a common ion salt (nBu4NI) 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/ZnCl2 in a polar solvent, as already discussed. In this regard, it

៮ ៮ ៮ Figure 7. The Mn and Mw/Mn of the polymers obtained in (A) CH2Cl2 and (B) EtNO2/CH2Cl2 in the monomer-addition experiments in the polymerization Ϫ ϭ of pMOS with 1/ZnCl2 at 15°C: [M]0 [M]add ϭ ϭ ϭ 0.38M,[1]0 10.0 mM, [ZnCl2]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)/ZnCl2 instead of HI/ZnI2 in various solvents and the feasibility of living polymerization mediated by a free ionic growing species.

៮ Effects of Solvent Polarity on Polymerization Figure 6. (A) Time-conversion curves and (B) Mn and ៮ ៮ Ϫ Mw/Mn of poly(pMOS) obtained with 1/ZnCl2 at 15°C: pMOS was polymerized with 1/ZnCl2 in polar sol- ϭ ϭ ϭ Ϫ [M]0 0.38M,[1]0 10 mM, [ZnCl2]0 5.0 mM. vents at 15°C. As shown in Figure 6(A), the 3700 KANAOKA AND HIGASHIMURA

Figure 8. MWD curves of poly(pMOS) obtained with 1/ZnCl2 in the monomer-addi- Ϫ ϭ tion experiments at 15°C in (A) CH2Cl2 and (B) EtNO2/CH2Cl2: [M]0 [M]add ϭ ϭ ϭ 0.38M,[1]0 10.0 mM, [ZnCl2]0 5.0 mM.

polymerization rate sharply increased with sol- Living Polymerization in Polar Solvent vent polarity, which was most likely due to an To conﬁrm the living nature of the pMOS poly- increased concentration of free ionic species, as merization by 1/ZnCl in polar media, a fresh feed observed with tBOS. 2 of pMOS ([pMOS] ϭ [pMOS] ϭ 0.38 M) was Inspection of Figures 1 and 6 reveals that the 0 add added to the reaction mixture just before the ini- polymerization rate is greater with pMOS than tial charge of pMOS was completely polymerized. with tBOS under the same reaction conditions. The added pMOS was smoothly polymerized Similar results were obtained in the polymeriza- without an induction phase at nearly the same 8 tion with HI/ZnI2. However, pMOS is polymer- rate as in the initial stage. Figure 7 shows that ized slightly faster in CH Cl with HI/ZnI than ៮ 2 2 2 the Mn of the polymer further increased in direct with 1/ZnCl2 [e.g., conversion in 10 min: HI/ZnI2, proportion to monomer conversion. In addition, Ϸ ϭ Ϸ 80% ([M]0 0.50M); 1/ZnCl2, 60% ([M]0 even after the monomer addition the MWD of the ϭ 0.38M)]. The COCl bond is harder to ionize polymer remained narrow and clearly shifted to- than the COI counterpart in generating fewer ward a higher molecular weight without a shoul- ionic species. Thus, the polymerization rate with der in the lower molecular weight region (Fig. 8).

1/ZnCl2 became smaller than that with HI/ZnI2, Thus, the living nature of the pMOS polymeriza- although ZnCl2 is more Lewis acidic than ZnI2. tion with 1/ZnCl2 was demonstrated even in a Figure 6(B) shows the conversion dependence strongly polar solvent where the propagating spe- of M៮ and M៮ /M៮ for the pMOS polymers formed cies contain free ionic carbocations. n w n ៮ in the polar solvents. In both cases the Mn increased proportionally to conversion and the poly- ៮ ៮ Ϸ mers had very narrow MWDs (Mw/Mn 1.1), CONCLUSION ៮ although the Mn determined by GPC deviated from the calculated values, assuming that one 1 Living cationic polymerization of the p-alkoxysty- molecule generates one polymer chain. The devi- renes tBOS and pMOS with 1/ZnCl2 were observed ation of the observed M៮ is because it was calcu- n even in such polar solvents as CH2Cl2 or EtNO2/ lated on the basis of a polystyrene calibration (see CH2Cl2 where the free ionic species is involved in the Experimental section). Thus, living polymers the propagation as is suggested with vinyl ethers.6 can be obtained in a strongly polar solvent, not In general, the dissociated or free ionic carbocations only with tBOS, but also with pMOS. are highly reactive but unstable; thus, living poly- LIVING CATIONIC POLYMERIZATION VIA FREE IONS 3701 merization mediated by free ionic species had been 3. Sawamoto, M.; Okamoto, C.; Higashimura, T. Mac- considered difﬁcult for a long time. However, the romolecules 1987, 20, 2693. results of our study showed that by utilizing a suit- 4. Higashimura, T.; Aoshima, S.; Sawamoto, M. ably nucleophilic counteranion even free ionic grow- Makromol Chem Macromol Symp 1988, 13/14, ing species can induce living polymerization of the 457. monomers such as vinyl ethers and p-alkoxysty- 5. See the review of Matyjaszewski, K.; Sawamoto, M. In Cationic Polymerization: Mechanisms, Synthe- renes whose substituents can stabilize the growing sis and Applications; Matyjaszewski, K., Ed.; Mar- carbocations. It should be emphasized that the cel Dekker: New York, 1996; Chap. 4. choice of a counterion (Cl vs. I, ZnCl2 vs. SnCl4)is 6. Kanaoka, S.; Sawamoto, M.; Higashimura, T. Mac- crucial to achieving living cationic polymerization romol Symp 1998, 132, 75. via free ionic species. 7. Higashimura, T.; Kojima, K.; Sawamoto, M. Polym Bull 1988, 19, 7. This work was supported in part by a Grant-in-Aid for 8. Higashimura, T.; Kojima, K.; Sawamoto, M. Mak- Scientiﬁc Research (C) (No. 08651056) from the Minis- romol Chem 1989, 15, 127. try of Education, Science, Sports, and Culture, Japan, 9. Kojima, K.; Sawamoto, M.; Higashimura, T. Mac- for which T. H. is grateful. We thank Yasuko Kubota romolecules 1990, 23, 948. for her assistance in the polymerization experiments. 10. Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1992, 25, 2587. 11. (a) Ishihama, Y.; Sawamoto, M.; Higashimura, T. REFERENCES AND NOTES Polym Bull 1990, 23, 361; (b) Ishihama, Y.; Sawamoto, M.; Higashimura, T. Polym Bull 1990, 1. See the review of Higashimura, T.; Sawamoto, M. 24, 201; (c) Higashimura, T.; Ishihama, Y.; Adv Polym Sci 1984, 62, 49. Sawamoto, M. Macromolecules 1993, 26, 744. 2. Miyamoto, M.; Sawamoto, M.; Higashimura, T. 12. Kamigaito, M.; Maeda, Y.; Sawamoto, M.; Higashi- Macromolecules 1984, 17, 265. mura, T. Macromolecules 1993, 26, 1643.