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 T-pm, and most probably lamellar thickness L-o, 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. (C) 2012 Elsevier Ltd. All rights reserved.