Effects of a vanadium post-metallocene catalyst-induced polymer backbone inhomogeneity on UV oxidative degradation of the resulting polyethylene film

A Group 5 post-metallocene precatalyst, (ONO)VCl(THF)2 (ONO = a bis(phenolate)pyridine LX2 pincer ligand), activated with modified methylaluminoxane (MMAO-3A) produced a linear ethylene homopolymer (nm-HomoPE)and an unusual inhomogeneous copolymer (nm-CopolyPE) with 1-hexene having very low backbone unsaturation. The nm-CopolyPE inhomogeneity was reflected in the distributions of short chain branches, 1-hexene composition, and methylene sequence length. The 1-hexene incorporation into the polyethylene backbone strongly depended on the molecular weight of the growing polymer chain. (ONO)VCl(THF)2, because of site diversity and easier removal of a tertiary (vs. a secondary) hydrogen, produced a skewed short chain branching (SCB) profile, incorporating 1-hexene more efficiently in the low molecular weight region than in the high molecular weight region. The significant decrease in molecular weight by 1-hexene showed that the (ONO)VCl(THF)2 catalytic sites were also highly responsive to chain-transfer directly to 1-hexene itself, producing vinyl and trans-vinylene termini. Subsequently, the effect of backbone inhomogeneity on the UV oxidative degradation of films made from both polyethylenes was investigated. The major functional group accumulated in the branched nm-CopolyPE film was carbonyl followed by carboxyl, then vinyl/ester, whereas that in the linear nm-HomoPE film was carboxyl. However, (carbonyl, carboxyl, vinyl, and ester)nm-CopolyPE film >> (carboxyl)nm-HomoPE film). The distributions of the tertiary C–H sites and methylene sequence length in the branched nm-CopolyPE film enhanced abstraction of H, decomposition of hydroperoxide group ROOH, and generation of carbonyl compounds as compared with those in the linear nm-HomoPE film. This clearly establishes the role played by the backbone inhomogeneity. The effect of short chain branches and sequence length distributions on peak melting temperature Tpm, and most probably lamellar thickness Lo, was modeled from a nanoscopic viewpoint. The accumulation of the above oxygenated functionalities and its effect on % crystallinity are explained considering polyethylene UV autooxidation mechanism, and Norrish I and Norrish II chain scissions.