Short-chain branching structures in ethylene copolymers prepared by high-pressure free-radical polymerization: An NMR analysis

It is well-known that short-chain branching (SCB) reactions (intramolecular H-abstraction) play an important role in determining the properties of ethylene homopolymers produced under high pressure by free-radical polymerization. There is little information, however, regarding SCB mechanisms that occur during the synthesis of ethylene copolymers under similar reaction conditions. This work describes SCB structures for a wide range of ethylene copolymers of varying composition (ethylene with n-butyl acrylate (nBA), methyl acrylate (MA), vinyl acetate (VAc), n-butyl methacrylate (nBMA), acrylic acid (AA), and methacrylic acid (MAA)), as determined by proton, C-13, and 2D NMR techniques. Close examination of the resonances reveals that for many (if not all) of these copolymers, a significant fraction of the SCBs contain comonomer as a result of CH2-radical to CH2 backbiting around a comonomer unit. In addition, SCBs are formed not only by hydrogen abstraction from CH2 polyethylene units but also by abstraction of hydrogen from the comonomer methine units. This latter mechanism does not occur during production of E/VAc, E/nBMA, or E/MAA but is important for E/AA and acrylate (E/MA and E/nBA) copolymers; for these systems 10-20% of the comonomer groups in the polymer are alkylated. Implications of these findings to the polymerization kinetics are discussed.