Degradable Polyisoprene by Radical Ring-Opening Polymerization and Application to Polymer Prodrug Nanoparticles

Radical ring-opening polymerization (rROP) has received renewed attention to incorporate cleavable linkages into the backbones of vinyl polymers, especially from cyclic ketene acetals (CKAs). Among the monomers that hardly copolymerize with CKAs are (1,3)-dienes such as isoprene (I). This is unfortunate since synthetic polyisoprene (PI) and its derivatives are the materials of choice for many applications, in particular as elastomers in the automotive, industrial, footwear, medical industry, but also nanomedicine. Thionolactones have been recently proposed as a new class of rROP-compatible monomers for insertion of thioester units in the main chain. Herein, we reported the synthesis of degradable PI by rROP via the copolymerization of I and DOT. Free-radical polymerization as well as two reversible deactivation radical polymerization techniques were successfully used for the synthesis of (well-defined) P(I-co-DOT) copolymers with adjustable molecular weight and DOT content (2.7-9.7 mol %). Reactivity ratios of rDOT = 6.600 and rI = 0.367 were determined, suggesting preferential incorporation of DOT in comparison to I. The resulting P(I-co-DOT) copolymers were successfully degraded (from -47% to -84% decrease in Mn) under basic conditions. As a proof of concept, the P(I-co-DOT) copolymers were formulated into stable and narrowly dispersed nanoparticles, showing similar cytocompatibility on J774.A1 and HUVEC cells than their PI counterparts. Furthermore, Gem-P(I-co-DOT) polymer prodrug were synthesized by the “drug-initiated” method and exhibited significant cytotoxicity on A549 cancer cells. The synthesis of P(I-co-DOT) copolymers may therefore open new perspectives in the field of degradable vinyl polymers, not only in nanomedicine but also for other applications involving PI.