Biobased Polymers: Synthesis of Graft Copolymers and Comb Polymers Using Lactic Acid Macromonomer and Properties of the Product Polymers

For developing broader application of biobased polymers, graft copolymers and comb polymers having poly­(lactic acid) (PLA) side chains have been synthesized by using a macromonomer technique. PLA macromonomers (MM<i>m</i>) having a methacryloyl polymerizable group with different PLA chain length with an average length <i>m</i> = 4, 6, 8, 12, 18, and 30 were prepared via ring-opening polymerization of l-lactide using hydroxyethyl methacrylate (HEMA) initiator catalyzed by Sn­(Oct)₂. Radical polymerization behaviors of these macromonomers were examined. Radical copolymerization of MM<i>m</i> (<i>m</i> = 4, 6, and 8), with vinyl monomers like <i>n</i>-butyl methacrylate (BMA) and <i>n</i>-butyl acrylate (BA) in water as the reaction medium, gave stable miniemulsions of poly­[<i>n</i>-butyl (meth)­acrylate-<i>graft</i>-lactic acid]­s [PB­(M)­A-<i>g</i>-PLA<i>m</i>]. MM<i>m</i> with <i>m</i> value higher than 12, however, gave aggregate products in a minor amount besides miniemulsions in a major amount, producing not a stable emulsion system of graft copolymers. The solution copolymerization, on the other hand, produced a wider variety of the graft copolymers, where a wider range of MM<i>m</i> (even <i>m</i> ≥ 12) can be employed. In a 1,4-dioxane solution, the radical copolymerization of MM<i>m</i> with BMA and methyl methacrylate (MMA) gave various graft copolymers [PB­(M)­MA-<i>g</i>-PLA<i>m</i>]. A new type of comb polymers (PMM<i>m</i>) having PLA<i>m</i> as pendant side chains were obtained by radical homopolymerization of MM<i>m</i> in a 1,4-dioxane solution. The graft copolymers and comb polymers obtained here are amorphous. Physical properties of the polymers from miniemulsions suggested them to be applicable for coatings or elastic materials which are environmentally desirable as a new class of biobased polymers. In addition, the present approach provided fundamental information on relationships between the length of PLA side chain and the bulk properties of the product polymers