Titanocene−Methylaluminoxane Catalysts for Copolymerization of Styrene and Ethylene: Synthesis and Characterization of Styrene−Ethylene Copolymers

The copolymerization of styrene and ethylene has been performed using a titanocene-based catalytic system of cyclopentadienyltitanium triphenoxide (CpTi(OPh) 3 ) and methylaluminoxane (MAO). The catalyst system exhibited a high catalytic activity of 10 4 −10 5 g (mof Ti·h) -1 and was found to selectively (more than 90 wt %) give an elastoplastic and amorphous styrene−ethylene (S−E) copolymer with a well-defined random/alternating microstructure and a single glass transition ( T g ) as thoroughly characterized by solvent fractionation, GPC, 13 C NMR, DSC, and WAXD. Under the reaction conditions employed, up to 54.5 mol % of styrene could be introduced into the copolymer chains. The composition, microstructure, molecular weight of the copolymers, and catalytic activity of the copolymerization are strongly dependent upon the comonomer feed ratio, polymerization temperature ( T p ), CpTi(OPh) 3 /MAO mole ratio, and trimethylaluminum (TMA) content or structure in MAO. For 300 ⩽ Al/Ti < 1000, the copolymerization product was essentially the random S−E copolymer. For 1000 < Al/Ti ⩽ 2000, the copolymerization product was significantly SPS homopolymer. The results showed that 24.5−28.2 mol % TMA content in MAO (oligomerization degree ( n ) ≈ 18−20) was optimum for the copolymerization, but 30.2−35.7 mol % TMA in MAO (oligomerization degree ( n ) ≈ 24−28) for styrene syndiospecific homopolymerization, even in the presence of ethylene feed monomer. The external addition of TMA or triisobutylaluminum (TIBA) inhibited the copolymerization but promoted the styrene homopolymerization. ESR spectroscopic analysis combined with copolymerization results suggests the presence of a Ti(IV) active center which is responsible for the formation of polyethylene, a Ti(III) species which is active in the syndiospecific polymerization of styrene, and, moreover, the presence of a third intermediate which contributes to promoting the copolymerization of styrene with ethylene to produce S−E copolymer.