COPOLYMERIZATION OF ETHYLENE AND T-BUTYL-2-(1,1-DIMETHYLHEPT-6-ENYL)-4-METHYLPHENOL OVER 3 DIFFERENT METALLOCENE-ALUMOXANE CATALYST SYSTEMS

Copolymers of ethylene and the polar monomer t-butyl-2-(1,1-dimethylhe pt-6-enyl)-4-methylphenol were synthesized using three different homog eneous metallocene-methylalumoxane catalyst systems, i.e. bis(eta(5)-4 ,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride (Me(2)Si-(IndH(4))(2 )ZrCl2)/methylalumoxane (MAO), bis(eta(5)-4,5,6,7-tetrahydro-1-indenyl )]zirconium dichloride (Et(IndH(4))(2)-ZrCl2)/MAO, and dicyclopentadie nylzirconium dichloride (Cp(2)ZrCl(2))/MAO. The initial polymerization rate, compared to that of ethylene homopolymerization, increased up t o almost 3 times when the sterically hindered phenolic stabilizer was added during ethylene polymerization over one of the two chiral bridge d metallocene catalysts. In contrast, the addition of the phenolic mon omer during ethylene polymerization over the achiral Cp(2)ZrCl(2) cata lyst did not result in an appreciable change in polymerization activit y. The dissimilarity in polymerization rate behavior of chiral versus achiral metallocene catalysts may be attributed to differences in the gap aperture between the pi-ligands of the catalyst and to sterical an d electronic factors. The level of comonomer incorporation was also fo und to be different with copolymers produced over chiral versus achira l metallocene catalyst. The comonomer content was 2-3 times lower for the copolymers produced over the achiral Cp(2)ZrCl(2) catalyst compare d to the copolymers prepared over either of the two chiral catalysts u nder similar conditions at low temperatures. As expected, the melting points and crystallinities of copolymers decreased with increasing phe nol content. According to C-13 NMR studies, the chemical shifts of the copolymer's methylene and methine backbone carbons correspond to thos e observed for random ethylene/1-octene copolymer with isolated hexyl branches. Thus, the produced copolymers are random copolymers, which c ontain isolated phenolic long chain branches. No detectable traces of phenolic homopolymer or blockcopolymer fragments were found by C-13 NM R. 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 degrees C ra nged from 18 to 72 min for the copolymers whereas unstabilized polyeth ylene exhibited an oxidation induction time of only 1 min, as determin ed by differential scanning calorimetry (DSC). The numerical values of the ratio of weight-to-number average molecular weights of the copoly mers were below 3 and thus characteristic of polymers produced by sing le-site catalysts. Furthermore, the copolymer molecular weights were s imilar to those of polyethylene prepared under similar conditions.