The active oxidation state of Ti in the cyclopentadienyltributoxyltitanium methylaluminoxane (CpTi(OBu>3/MAO) catalyst has been determined by electron paramagnetic resonance (EPR) and redox titration. The results, together with previously reported active species concentrations measured by radiolabeling techniques, showed that all the catalytic species are trivalent Ti. The syndiospecific species (78% Ti) is characterized by a sharp EPR resonance at g = 1.989 (Ho = 7 G) which corresponds to a titanium(III) hydride. The nonspecific species (13% Ti) has a g value at 1.995.

SYNOPSIS Syndiospecific polymerization of styrene was catalyzed by monocyclopentadienyltributoxyThe atactic and syndiotactic polystyrenes were separated by extracting the former with refluxing 2-butanone. The activity and syndiospecificity of the catalyst were affected by changes in catalyst concentration and composition, polymerization temperature, and monomer concentration. Extremely high activity of 5 X lo7 g PS (mol T i mol S h ) -* with 99% yield of the syndiotactic product were achieved. The concentration of active species, [ C* 1, has been determined by radiolabeling. The amounts of the syndiospecific and nonspecific catalytic species, [ C: ] and [C:] respectively, correspond to 79 and 13% of the c p T i ( 0 B~)~. The rate constants of propagation for C: and C,* at 45OC are 10.8 and 2.0 ( M s)-', respectively, the corresponding rate constants for chain transfer to MA0 are 6.2 X 5 -l . There was no deactivation of the catalytic species during a batch polymerization. The rate constant of chain transfer with monomer is 6.7 X ( M s)-'; the spontaneous P-hydride transfer rate constant is 4.7 X lo-* s-'. The polymerization activity and stereospecificity of the catalyst are highest at 45"C, both decreasing with either higher or lower temperature. The stereoregular polymer have broad MW distributions, GW/M,, = 2.8-5.7, and up to three crystalline modifications. The T,,, of the s-PS polymerized at 0-90°C decreased from 261.8 to 241°C indicating thermally activated monomer insertion errors. The styrene polymerization behaviors were essentially insensitive to the dielectric constant of the medium.and 4.3 X ~~ * To whom a11 correspondence should be addressed. ' On leave from the Research Institute of Macromolecular Chemistry, BRNO, Czechoslovakia. EXPERIMENTALCpTi ( O B U )~ was synthesized by Wade's procedure' given for CpTi ( OPh)3. The methods for purification of materials, polymerization, active-site counting, and polymer characterizations were the same as detailed in Part I.' RESULTS Polymerization BehaviorsPolymerization of S (20 mL) in 40 mL of toluene at temperature (T,) of 60°C using 50 pmol of CpTi( O B U )~ and 0.29 g of MA0 ( [All / [ Ti] = 100) yielded 17.1 g of polymer for 94% conversion. The magnetic stirring bar ceased to function after just 5 min of polymerization time ( t p ) . In comparison, the Sci. Phys., B28,419 (1989). 20. V. Vittoria, F. de Candia, P. Iannelli, and A. Imminizi, Makromol. Chem. Rapid Commun., 9 , 765 ( 1988).

SYNOPSISSyndiospecific polymerization of styrene ( S ) was catalyzed by Bz,Ti/MAO (tetrabenzyltitanium/methylaluminoxane) . The product was separated into syndiotactic polystyrene (s-PS) and atactic polystyrene (a-PS) by extraction of the latter with boiling 2-butanone. Over the broad range of catalyst concentrations, compositions, and polymerization temperatures, the catalytic activity is 150 f 80 kg PS (mol Ti mol S h)-' with 89 +-5% yield of s-PS ( S Y ) . The concentration of active species has been determined by radiolabeling. Only about 1.7% of Bz,Ti initiates syndiospecific polymerization at 60°C with values of rate constants for propagation and for chain transfer to MA0 of 1.38 ( M s)-l and 5.2 X lo-, s-', respectively. Nonspecific polymerization was initiated by 16.8% of the Ti having values of 0.056 ( M s)-' and 6.5 X lo-, s-' for the rate constants of propagation and transfer, respectively. The effect of solvent polarity on the polymerization was studied using toluene mixed with chlorobenzene or o-dichlorobenzene as solvents. An increase of effective dielectric constant from 2.43 to 5.92 reduces the polymerization activity by a factor of two and lowers SY to mere 39%. In 1 : 1 toluene/chlorobenzene solvent mixture, it was found that 1.3% and 26% of the Bz4Ti initiate syndiospecific and nonspecific polymerizations of styrene, respectively. The Bz,Ti/MAO catalyst is poor in both productivity and stereoselectivity.Bz4Zr / M A 0 also catalyzed syndiospecific polymerization of S.7 Analogous Ni Compounds activated with M A 0 produced partially isotactic PS,8 but V, Nb, Ta, Cr, and Co compounds produced either a-PS or none.5 T h e central purpose of this work is t o determine the factors which affect the activity and syndiospecificity of the Bz4Ti /MA0 catalyst in styrene polymerization. EXPERIMENTAL MaterialsStyrene was distilled from calcium hydride and stored at -25°C under argon in darkness. Toluene, chlorobenzene, and o-dichlorobenzene were distilled from sodium under argon just before use. Bz4Ti was synthesized from benzyl magnesium chloride and TiC14 according t o the procedure of Zucchini e t aL9Trimethyl aluminum from Texas Alkyls was used t o prepare M A 0 as described previously." All the 30. K. Soga, H. Nakatani, and T. Monoi, Macromolecules, 23,953 (1990).