Coordination–insertion polymerization of polar allylbenzene monomers

Abstract

Copolymerization of polar allyl monomers is challenging and suffers from a low catalytic activity, owing to the formation of the inert five-membered cyclic chelate and the deactivation of active species by β-X elimination. In this contribution, the issue of low activity (0.04–12.1 kg mol⁻¹ h⁻¹) is overcome by the use of polar allylbenzene as the polar allyl monomer. Under the activation of NaBArF, the classic α-diimine Pd(II) catalyst 1 [(Dipp-NC(Me)–C(Me)N-Dipp)PdMeCl] was inert to the copolymerization of ethylene and polar allylbenzene monomers. In contrast, the phosphine-sulfonate Pd(II) catalysts ({[κ²(P,O)-R₂P(C₆H₄SO₂O)]PdMe(DMSO)}) (2: R = 2-MeOC₆H₄; 3: R = cyclohexyl) enable the copolymerizations of ethylene with a rich variety of challenging polar allylbenzene monomers such as allylbenzene (A-H), 2-methoxy allylbenzene (A-OMe), 2-acetoxy allylbenzene (A-OAc), 2-allylphenol (A-OH), 2-allylbenzoaldehyde (A-CHO), 2-allylbromobenzene (A-Br) and 2-(N,N-dimethylamino) allylbenzene (A-NMe₂). Compared to those of the corresponding polar allyl monomers, the activities of polar allylbenzene monomers significantly enhanced by 9–6300 times (up to 252 kg mol⁻¹ h⁻¹). More remarkably, the activities of some polar allylbenzene monomers were even higher than that of non-polar allylbenzene, indicating a rare positive effect of the polar functional group toward the copolymerization reaction. The microstructures of all ethylene/polar allylbenzene copolymers obtained were comprehensively identified by ¹H NMR, ¹³C NMR, ¹H–¹H COSY, ¹H–¹³C HSQC and ¹H–¹³C HMBC experiments and GPC and DSC analysis.