Reactivity of methacrylates in insertion polymerization

Polymerization of ethylene by complexes [{(P∧O)PdMe(L)}] (P∧O = κ²-(P,O)-2-(2-MeOC₆H₄)₂PC ₆H₄SO₃)) 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.