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

Abstract
A Group 5 post-metallocene precatalyst, (ONO)VCl(THF) 2 (ONO = a bis(phenolate)pyridine LX 2 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. © 2012 Elsevier Ltd. All rights reserved.

Citation
Atiqullah M, Winston MS, Bercaw JE, Hussain I, Fazal A, et al. (2012) Effects of a vanadium post-metallocene catalyst-induced polymer backbone inhomogeneity on UV oxidative degradation of the resulting polyethylene film. Polymer Degradation and Stability 97: 1164–1177. Available: http://dx.doi.org/10.1016/j.polymdegradstab.2012.03.042.

Acknowledgements
The authors thank the King Abdullah University of Science & Technology (KAUST) Center-in-Development for Transformative Research in Petrochemicals and Polymers, established at King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia for supporting this research. The technical assistances provided by the Department of Chemistry and Chemical Engineering at California Institute of Technology (Caltech), Pasadena, USA; the King Fahd University of Petroleum & Minerals (KFUPM) Center of Refining & Petrochemicals (CRP) at the Research Institute, the Center of Research Excellence in Petroleum Refining & Petrochemicals (CoRE-PRP), and the Department of Chemical Engineering; Polymer Char, Spain; and KAUST are also gratefully acknowledged. The authors also thank Jubail United Petrochemical Company for donating 1-hexene.

Publisher
Elsevier BV

Journal
Polymer Degradation and Stability

DOI
10.1016/j.polymdegradstab.2012.03.042

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