Reactivity of Methacrylates in Insertion Polymerization

Polymerization of ethylene by complexes [{(P^O)PdMe(L)}] (P^O = κ2-(P,O)-2-(2-MeOC6H4)2PC6H4SO3)) affords homopolyethylene free of any methyl methacrylate (MMA)-derived units, even in the presence of substantial concentrations of MMA. In stoichiometric studies, reactive {(P^O)Pd(Me)L} fragments generated by halide abstraction from [({(P^O)Pd(Me)Cl}μ-Na)2] insert MMA in a 1,2- as well as 2,1-mode. The 1,2-insertion product forms a stable five-membered chelate by coordination of the carbonyl group. Thermodynamic parameters for MMA insertion are ΔH ⧧ = 69.0(3.1) kJ mol−1 and ΔS ⧧ = −103(10) J mol−1 K−1 (total average for 1,2- and 2,1-insertion), in comparison to ΔH ⧧ = 48.5(3.0) kJ mol−1 and ΔS ⧧ = −138(7) J mol−1 K−1 for methyl acrylate (MA) insertion. These data agree with an observed at least 102-fold preference for MA incorporation vs MMA incorporation (not detected) under polymerization conditions. Copolymerization of ethylene with a bifunctional acrylate−methacrylate monomer yields linear polyethylenes with intact methacrylate substituents. Post-polymerization modification of the latter was exemplified by free-radical thiol addition and by cross-metathesis.