A New Approach to Make Starch Graft Copolymers via Nitroxide-Mediated Polymerization
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Starch is one of the most abundant polysaccharides worldwide next to cellulose. Starch bio-polymers are sustainable, biodegradable, and abundant, but are often chemically modified to improve the physical or chemical properties. Cold water-soluble starch (CWS) is a starch variant of industrial relevance for applications including paper coatings, but the high viscosity of starch solutions limits the amount of starch that can be incorporated. It was hypothesized that the graft modification of CWS would make it more industrially useful. Starch was modified using both grafting from and grafting to approaches using nitroxide-mediated polymerization (NMP). NMP is a type of reversible-deactivation radical polymerization which allows greater control over molecular weight distribution and dispersity compared to free radical polymerization. In this work, bio-synthetic copolymers of starch were targeted with high starch contents to preserve the sustainability of the materials. The bio-synthetic polymers could have applications in sustainable paper coating latexes, in green emulsion formulations, or in wastewater treatment. In the grafting from approach, CWS was modified with 4-vinylbenzyl chloride (VBC) and 2-methyl-2-[N-tertbutyl-N-(diethoxy-phosphoryl-2,2-dimethylpropyl)-aminoxy] propionic acid initiator before graft polymerization with methyl methacrylate-co-styrene, methyl acrylate, and acrylic acid. The (co)polymerisations were well controlled showing linear reaction kinetics and relatively low dispersities (<1.5). Decreasing the monomer concentrations decreased the grafted polymer dispersity. By varying the polymerization time or monomer concentration, bio-synthetic polymeric materials with tunable graft polymer content were produced. In the grafting to approach, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) or poly(2-(diethylaminoethyl) methacrylate) (PDEAEMA) were synthesized via NMP and reacted with VBC modified CWS. Low grafting reaction substitutions and thereby the low grafting densities were observed which were inherently due to the grafting to method used. The final grafted materials had high starch contents. The grafted materials were shown to be pH-responsive by titration, switching from positive zeta-potential at low pH to negative zeta-potential at high pH. The measured zero zeta-potential intersect at the polymer pKaH corresponded well with literature for PDMAEMA but poorly for PDEAEMA, indicating precipitation of the PDEAEMA grafted CWS. PDMAEMA and PDEAEMA grafted CWS were shown to be good candidates for CO2-switchable bio-materials.