Binuclear Initiators for the Telechelic Synthesis of Elastomeric Polyolefins

Novel binuclear complexes, 4,4′-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidato-κ2-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1′-biphenyl (2a) and 4,4′-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-4-methylpentamidato-κ2-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1′-biphenyl (2b), were synthesized by linking two nickel centers through a bis(benzyl) fragment. When activated with nickel bis(1,5-cyclooctadiene) (Ni(COD)2), 2a and 2b are capable of polymerizing ethylene in a quasi-living fashion, producing polymers with approximately twice the molecular weights relative to those obtained by using a structurally related mononuclear system. In addition, 2b/Ni(COD)2 was utilized to synthesize a series of pseudo-triblock polyethylene (PE) macroinitiating copolymers, bearing atom-transfer radical polymerization (ATRP) initiators. Pseudo-pentablock copolymers were also prepared by taking advantage of a pressure-pulsing technique, wherein the ethylene pressure was increased from 100 to 500 psi in order to produce semicrystalline ethylene-rich end-blocks. Copolymers with elastomeric properties were synthesized by grafting n-butyl acrylate from the PE macroinitiators via ATRP. Examination using monotonic and step-cyclic stress−strain tests demonstrates that the materials exhibit large strains at break (1600−2000%) and excellent elastic recoveries at large strains (∼80%). That materials with such desirable properties could not be attained using a mononuclear initiator demonstrates the clear advantage of growing the polymer via a telechelic mechanism.