Effects of a vanadium post-metallocene catalyst-induced polymer backbone inhomogeneity on UV oxidative degradation of the resulting polyethylene film M. Atiqullah a, b, c, * , M.S. Winston d , J.E. Bercaw d, ** , I. Hussain a, b, c , A. Fazal c , M.A. Al-Harthi e , A.-H.M. Emwas f , M.J. Khan d , A. Hossaen a, b, c a King Abdullah University of Science & Technology (KAUST) Center-in-Development for Transformative Research in Petrochemicals and Polymers, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia b Center for Refining & Petrochemicals (CRP), Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia c Center of Research Excellence in Petroleum Refining & Petrochemicals (CoRE-PRP), King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia d Department of Chemistry and Chemical Engineering, 1200 E. California Blvd., California Institute of Technology (Caltech), Pasadena, USA e Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia f NMR Core Laboratory, Thuwal, KAUST, Saudi Arabia article info Article history: Received 29 January 2012 Received in revised form 22 March 2012 Accepted 26 March 2012 Available online 16 April 2012 Keywords: Post-metallocene catalyst UV oxidative degradation Short chain branch Polyethylene abstract A Group 5 post-metallocene precatalyst, (ONO)VCl(THF) 2 (ONO ¼ 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 inho- mogeneity 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 ^CeH 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Polyethylenes are commercially produced using mainly ZieglereNatta and chromium catalysts. Here, the catalyst sites primarily direct the structure of the polyethylene structural back- bones. However, monomer feed composition, polymerization process type, and reactor configuration and operating condition, in parallel, may also assist the catalysts and thereby vary and regu- late the resulting backbone structures. The distributions of molec- ular weight, copolymer composition, methylene sequence length, branches, and chain unsaturation generally characterize and describe the backbone structures. These catalytic process-generated backbone properties eventually influence the density; the thermal, rheological, mechanical, and chemical properties; the processing characteristics; and the applications of the end-products [1e 7]. * Corresponding author. King Abdullah University of Science & Technology (KAUST) Center-in-Development for Transformative Research in Petrochemicals and Polymers, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia. Tel.: þ966 3 860 3898; fax: þ966 3 860 4509. ** Corresponding author. Tel.: þ1 626 395 6577. E-mail addresses: matiq@kfupm.edu.sa (M. Atiqullah), bercaw@caltech.edu (J.E. Bercaw). Contents lists available at SciVerse ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.042 Polymer Degradation and Stability 97 (2012) 1164e1177