Copolymerization of Ethylene and 6-tert-Butyl-2-(1,1-dimethylhept-6-enyl)-4-methylphenol over Three Different Metallocene−Alumoxane Catalyst Systems

Copolymers of ethylene and the polar monomer 6-tert-butyl-2-(1,1-dimethylhept-6-enyl)-4-methylphenol were synthesized using three different homogeneous metallocene−methylalumoxane catalyst systems, i.e. rac-[1,1‘-(dimethylsilylene)bis(η5-4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (Me2Si(IndH4)2ZrCl2)/methylalumoxane (MAO), rac-[ethylene-1,2-bis(η5-4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (Et(IndH4)2-ZrCl2)/MAO, and dicyclopentadienylzirconium dichloride (Cp2ZrCl2)/MAO. The initial polymerization rate, compared to that of ethylene homopolymerization, increased up to almost 3 times when the sterically hindered phenolic stabilizer was added during ethylene polymerization over one of the two chiral bridged metallocene catalysts. In contrast, the addition of the phenolic monomer during ethylene polymerization over the achiral Cp2ZrCl2 catalyst did not result in an appreciable change in polymerization activity. The dissimilarity in polymerization rate behavior of chiral versus achiral metallocene catalysts may be attributed to differences in the gap aperture between the π-ligands of the catalyst and to sterical and electronic factors. The level of comonomer incorporation was also found to be different with copolymers produced over chiral versus achiral metallocene catalyst. The comonomer content was 2−3 times lower for the copolymers produced over the achiral Cp2ZrCl2 catalyst compared to the copolymers prepared over either of the two chiral catalysts under similar conditions at low temperatures. As expected, the melting points and crystallinities of copolymers decreased with increasing phenol content. According to 13C NMR studies, the chemical shifts of the copolymer's methylene and methine backbone carbons correspond to those observed for random ethylene/1-octene copolymer with isolated hexyl branches. Thus, the produced copolymers are random copolymers, which contain isolated phenolic long chain branches. No detectable traces of phenolic homopolymer or blockcopolymer fragments were found by 13C NMR. The thermo-oxidative stability of the copolymers prepared was high even after prolonged extraction with a mixture of refluxing (50:50) 2-propanol/cyclohexane; the oxidation induction time at 200 °C ranged from 18 to 72 min for the copolymers whereas unstabilized polyethylene exhibited an oxidation induction time of only 1 min, as determined by differential scanning calorimetry (DSC). The numerical values of the ratio of weight-to-number average molecular weights of the copolymers were below 3 and thus characteristic of polymers produced by single-site catalysts. Furthermore, the copolymer molecular weights were similar to those of polyethylene prepared under similar conditions.