SYNTHESIS AND POLYMERIZATION OF LIGNIN MACROMONOMERS .2. EFFECTS OF LIGNIN FRAGMENTS ON METHYL-METHACRYLATE RADICAL POLYMERIZATION

In the course of a study on the synthesis and polymerization of lignin macromonomers, the effect of lignin fragments and acetylated lignin ones on the radical polymerization of methyl methacrylate has been investigated. Kinetics show that lignin moieties act first as inhibitors of the MMA polymerization. The inhibition process has been attributed to the phenolic groups, which primarily react through hydrogen transfer with free radicals issued from benzoyl peroxide dissociation, thus leading to a series of lignin free radicals inactive towards ethylenic groups. The derivatization of lignin hydroxyl groups into ester groups totally suppresses the inhibition process. Besides, it was also shown that both lignin and acetylated lignin units act as chain limiting agents towards MMA polymerization. As indicated both by the chemical composition of the polymers produced and the kinetic data, the latter process might involve the formation of a new type of lignin free radicals, able to initiate MMA polymerization. A reaction scheme is proposed.