Investigating the radical based graft copolymerization of chitosan with organohalide/metal complex redox coinitiating systems

The purpose of this research was to investigate the feasibility of grafting chitosan powders with different organohalide/metal complex redox coinitiating systems using a free radical mechanism. The two different systems chosen were a trichloroacetyl/Mn₂(CO)₁₀ photoinitiating method and various atom transfer radical polymerization (ATRP) methods. Graft copolymers have a wide range of end uses, unfortunately the synthesis of these derivatives suffers from the simultaneous formation of homopolymer. The two sources of homopolymer are nonspecific macroradical formation and chain transfer from growing grafted chain ends. The first initiating system chosen allows specific macroradical formation only, while ATRP (a “living” system) has the ability to promote specific macroradical formation while minimizing the chain transfer from the grafted chain ends. Grafting trichloroacetylated chitosan powder with methyl acrylate using Mn₂(CO)₁₀ photoinitiation yielded over a 600% weight increase (based on weight of chitosan powder) where approximately 20–30% of the polymer formed was homopolymer. Due to the strong chelating nature of chitosan, a bipyridine based ATRP system was found intolerant of highly deacetylated chitosan powder. However, an ATRP system utilizing an aliphatic amine ligand (tetramethylethylene diamine) was found to be highly compatible with highly deacetylated chitosan, and thus was employed for grafting experiments. Incorporating this ATRP method required the synthesis of a bromoisobutyrylated chitosan powder, which was obtained with an extremely low bromine yield. Corresponding grafting experiments with methyl methacrylate using this powder suggest grafting did occur, however yields were quite low. A second ATRP system was utilized; it was based on trichloroacetyl initiating sites using a hexamethyltriethylenetetraamine/CuBr complex to polymerize methyl methacrylate. Using the trichloroacetylated powder previously synthesized, an excellent grafting yield was obtained; a 925% weight increase was observed based on the original weight of trichloroacetylated chitosan where only 11% of the polymer formed was homopolymer.