Copolymerization of ethene with norbornene using palladium(II) α-diimine catalysts: Influence of feed composition, polymerization temperature, and ligand structure on copolymer properties and microstructure

Abstract

Four cationic palladium(II) α-diimine complexes, [{ArNC(R)–C(R)NAr}Pd(Me)(CH3CN)]BAr4f (1, R=H, Ar=2,6-Me2C6H3, BAr4f=B[3,5-C6H3(CF3)2]4; 2, R=CH3, Ar=2,6-iPr2C6H3; 3, R=CH3, Ar=2-tBuC6H4; 4, R,R=An, Ar=2,6-iPr2C6H3) were used for the copolymerization of ethene with norbornene. The copolymerization behavior of the catalysts and the influence of the polymerization temperature were investigated. The copolymers were characterized using 13C NMR spectroscopy, differential scanning calorimetry, and gel permeation chromatography techniques. Sterically demanding ortho -N-aryl substituents and rigid bulky bridge units increase the copolymer molar masses, while the incorporation level of norbornene is decreased. Microstructures with isolated norbornene units and alternating sequences are predominant. Less bulky substituted catalysts yield copolymers with higher norbornene contents and lower molar masses. Norbornene diblock sequences are dominant which are exclusively racemic connected, indicating that the insertion proceeds under chain end control. Optimal polymerization results are achieved at temperatures between 10 and 30°C, while temperatures below 0°C result in lower polymerization rates and molar masses. Above 30°C, activities, molar masses, and norbornene incorporation decreases due to catalyst decomposition.