Spontaneous Gradients by ATRP and RAFT: Interchangeable Polymerization Methods?

The spontaneous gradient polymerization was investigated by ATRP and RAFT. The model monomers for both systems were methyl methacrylate (MMA) and ethyl acrylate (EA). Spontaneous gradient polymers were generated; however, the control over a polymer structure varied with both the polymerization method and the initiator structure. RAFT polymerization was found to have the best control while using a chain transfer agent (CTA) compatible with both the methacrylic and acrylic monomers. The use of this dual compatible CTA allowed for efficient RAFT polymerization and gave excellent correlation of experimental and theoretical molecular weights, dispersities below 1.3, and reactivity ratios consistent with spontaneous gradient formation. In contrast, the RAFT CTA with a less effective homolytic leaving group led to poor correlation between experimental and theoretical molecular weights and dispersity typically above 1.5. In general, ATRP led to somewhat higher dispersity polymers in the order of 1.4-1.5 with decreased correlation of experimental and theoretical molecular weights despite the formation of gradients. Reactivity ratios for both MMA and EA were determined by using a terminal reactivity model. The reactivity ratios were similar to those in conventional radical polymerization as the propagating radicals were identical. Although similar, RAFT and ATRP are not fully interchangeable in gradient polymerization with RAFT offering a simpler approach to gradient polymers when the CTA compatible with both monomers is selected.