To meet the rapidly growing demand for materials made from renewable resources, biopolymers like starch can be used to modify it into performance polymers. Such modifications are needed to add functionalities that are not there in native starch. An important method is to attach chains of functional polymers to the starch backbone by graft copolymerization. The most usual method to synthesize such a composite material is by free radical add-on polymerization with vinyl monomers. In our laboratory, acrylic acid has been the monomer of choice since composite polymers of starch with side chains of polyacrylic acid are a promising candidate to replace a spectrum of performance polymers from petrochemicals.
The most relevant side reaction of such graft copolymerization systems is the formation of homopolymer polyacrylic acid. A high selectivity of the grafting reaction is therefore wanted. The monomer acrylic acid is water-soluble which presents a greater challenge in this respect than water-insoluble monomers. The mechanisms behind this effect will be explained and some methods to improve on the graft selectivity will be discussed. Most promising is a method where not all monomer is added at the start but in portions over certain time intervals. Experimental data show that both important result parameters, grafting percentage (18 -> 28%) and graft selectivity (20 -> 31%), are improved. The method may need further refinement, also to make it applicable for use in a larger scale continuous reactor, but the principle breakthrough towards better graft selectivity is clearly demonstrated.