Syndiotactic-Specific Polymerization of Methacrylates by Tertiary Phosphine-Triethylaluminum

Polymer Journal, Vol. 24, No. 8, pp 817-827 (1992) Syndiotactic-Specific Polymerization of Methacrylates by Tertiary Phosphine-Triethylaluminum Tatsuki KITAYAMA, Eiji MASUDA, Minoru YAMAGUCHI, Takafumi NISHIURA, and Koichi HATADA* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received March 2, 1992) ABSTRACT: Polymerization of MMA with R 3 P-R;Al in toluene at low temperatures was reinvestigated and was found to give highly syndiotactic PMMA. Among the initiators examined Ph 3 P-Et 3 Al gave the PMMA with highest syndiotacticity and the polymerization at - 93oC yielded PMMA with rr content of 95%. PMMAs prepared with Et 3 P-Et 3 Al and Ph 3 P-Et 3 Al contained the structural unit comprised of the phosphine components at the chain end as evidenced by 31 P and 1 H NMR analyses. The results clearly indicate that R 3 P takes an important role in the initiation process, though R 3 P alone does not polymerize MMA. Methacrylic acid esters of primary and secondary alcohols were also polymerized by these initiators to give highly syndiotactic polymers. Block and random copolymers of MMA with several methacrylates were also obtained. The effect of syndiotacticity on the glass transition temperature (T.) of PMMA was examined and T• for 100% syndiotactic PMMA with infinite molecular weight was estimated to be 141 oc. KEY WORDS Stereospecific Polymerization I Tacticity I Poly(methyl methacrylate) I Polymethacrylate I Trialkylaluminum I Phosphine I Anionic Polymerization I Block Copolymer I Glass Transition Temperature I Highly syndiotactic poly(methyl methacrylate) (PMMA) has high glass transition temperature (Tg) and is expected to be used as optical materials having high thermostability. PMMAs with syndiotacticity in triad over 90% have been prepared by the polymerizations with Ziegler catalysts, 1,2 aluminum amide, 3 .4 magnesium amide, 5 Grignard reagents, 6 - 8 and organocalcium compound. 9 • 10 Syndiotactic PMMA with narrow molecular weight distribution (MWD) can be prepared by the living polymerization with bulky alkyllithium initiators such as 1, 1-diphenylhexyllithium in tetrahydrofuran (THF)Y - 13 However, the syndiotacticity in triad of the PMMA does usually not exceed around 85%. Recently, it was found that t-C 4 H 9 Li gave highly syndiotactic (over 90% in triad) poly- (methacrylate)s with narrow MWD in toluene at low temperatures in the presence of trialkylaluminums such as triethylaluminum. 14 • 15 Polymerization by t-C 4 H 9 Li alone gave the isotactic polymers with broad MWD and initiator efficiency was low. These results indicate that alkylaluminum dissociates the aggregate of t-C 4 H 9 Li into monomeric species, leading to high initiator efficiency, and changes of the properties of the propagating species from isotactic-specific to syndiotactic-specific. 14,15 In 1960, Murahashi and his coworkers reported that the mixture of triphenylphosphine (Ph 3 P) and triethylaluminum (Et 3 Al) initiates the polymerization of methyl methacrylate (MMA). 16 l Later, Ikeda and his coworkers 17 reported that the Al-C bond of * To whom correspondence should be addressed. 817 T. KITAYAMA eta!. Et 3 Al is activated in the presence of Lewis base, such as 2,2'-bipyridine and Ph 3 P, enough to initiate the polymerization of MMA. In the course of survey for syndiotactic polymerization of methacrylates we realized that most of the initiators containing aluminum compounds initiate syndiotactic-specific polymerization, indicating that aluminum compounds play an important role in syndiotactic-specific polymerization of methacrylates. 18 The idea prompted us to reinvestigate the polymerization of MMA with R 3 P-Et 3 Al in toluene at low temperatures. The polymerization was found to proceed in a living manner to give highly syndiotactic polymer although the initiator efficiency was less than unity. The preliminary results were reported previously. 19 The present paper describes the detailed study of the polymerization system and its utilization to the preparation of highly syndiotactic block and random copolymers of methacrylates. Tacticity dependence of glass transition temperature of PMMA is also discussed. EXPERIMENTAL Methacrylate monomers were purified in usual manner, dried over calcium dihydride (CaH 2 ) and vacuum-distilled just before use. Et 3 Al was obtained commercially and used as a heptane solution. Triethylphosphine (Et 3 P) was used as a toluene solution. Commercially obtained Ph 3 P and tricyclohexylphosphine were purified by recrystallization from hexane and the purified phosphines were used as toluene solutions. Toluene and heptane were purified in usual manner, dried over metallic sodium and distilled. The purified solvents were mixed with a small amount of n-butyllithium to remove a trace amount of water and vacuum distilled just before use. THF was refluxed under nitrogen over CaH 2 , distilled into the flask containing LiAIH 4 , refluxed over the LiAIH 4 , and distilled again into the flask containing LiAlH 4 . The THF was vacuumdistilled just before use. 818 Polymerization was carried out in a glass ampoule filled with dried nitrogen. After the prescribed period of time the reaction was terminated with a small amount of methanol. The reaction mixture was poured into a large amount of hexane to precipitate the polymeric product. The precipitate was collected by filtration, washed with hexane and dried under vacuum at room temperature. Block copolymerization was carried out in a three-necked flask equipped with a mechanical stirrer. For tacticity determination, t-butyl ester groups of poly(t-butyl methacrylate) (poly(tBMA)) and block copolymer of MMA and t-butyl methacrylate (t-BMA) were selectively hydrolyzed in a mixture of benzene and methanol (3/2 voljvol) containing 1.0 vjv% of concentrated hydrochloric acid (12N) under reflux for 2 days. Complete elimination of the t-butyl group was confirmed by 1 H NMR spectroscopy. The spectra also indicated that the content of MMA in the block copolymer did not change from the original copolymer and thus the ester groups of MMA were not hydrolyzed during this procedure. Tacticity of the homopolymer was determined from 1 H NMR spectra of PMMA derived from poly(methacrylic acid) by methylation with diazomethane. 20 Tacticity of the poly(methacrylic acid) block and PMMA block in the block copolymer could be determined from the corresponding carbonyl carbon NMR signals. 21 1 H and 13 C NMR spectra were measured on a JEOL JNM-MHlOO, JEOL JNM-FX100, JEOL JNM GSX-270, or JEOL JNM GX-500 spectrometer using tetramethy1silane as an internal standard. 31 P NMR spectra were measured on a JEOL JNM-FX100 spectrometer at 40.3 MHz and the chemical shift was referred to Ph 3 P in CDC1 3 ( - 5.6 ppm). Number-average molecular weights (Mn) were measured on a Hitachi 117 vapor pressure osmometer in toluene at 60.0°C or on a JASCO FLC-A 10 GPC chromatograph equipped with Shodex GPC (...truncated)