All-Acrylic Multigraft Copolymers: Synthesis and Structure-Property Relationship for Producing Next Generation Thermoplastic Elastomers

Polymer architecture and the advancement of molecular design using anionic and other controlled polymerization methods continues to be of significant research interest because of the tunable approach it provides, which can impact numerous applications ranging from thermoplastics to drug delivery systems. Among the numerous branched structures currently investigated, comb and graft copolymers continue to provide tailored materials which exhibit superior mechanical properties when compared to their di- and triblock linear counterparts. More specifically, the incorporation of two or more monomers into graft and multigraft constructions where the side chains are composed of a plastic (high T_g [glass transition temperature]) segment attached to a rubbery (low T_g) backbone has displayed much improved elastomeric properties for use in thermoplastic elastomer (TPEs) applications. These elastomeric materials continue to be dominated by compositions of styrene-isoprene or styrene-butadiene with little attention to all-acrylic systems in which both the soft and hard segments are made of acrylic monomers. By using anionic polymerization, methyl methacrylate macromonomers were synthesized and subsequently copolymerized with n-butyl acrylate using reversible addition-fragmentation chain transfer polymerization. In this manner we were able to construct the desired multigraft structures via a grafting-through methodology. The fundamental structure-property relationships were then studied to see how compositional changes such as branch point number, branch point functionality, side chain molecular weight, and volume percent of the glassy PMMA [poly(methyl methacrylate)] segments affects the overall mechanical performance of the branched material. This allowed us to show the ability to dramatically control the overall strength and elasticity of the all-acrylic multigraft copolymers, as well as to demonstrate a versatile synthetic technique that has the ability to be adapted for the synthesis of more complex architectures using a vast array of hard and soft segments