Monodisperse Cyclic Polymer Mechanochemistry: Scission Kinetics and the Dynamic Memory Effect with Ultrasonication and Ball-Mill Grinding

A seriesof monodisperse cyclic and linear poly(d,l-lactide)s(c-PLA and l-PLA,respectively) were prepared with various degrees of polymerization(DP) using an iterative convergent synthesis approach. The absenceof a molecular weight distribution provided us a chance to study theirmechanochemical reactivity without obstructions arising from the sizedistribution. Additionally, we prepared l- and c-PLAs with identical DPs, which enabled us to attributedifferences in scission rates to the cyclic polymer architecture alone.The polymers were subjected to ultrasonication (US) and ball-millgrinding (BMG), and their degradation kinetics were explored. Up to9.0 times larger scission rates were observed for l-PLA (compared to c-PLA) with US, but the differencewas less than 1.9 times with BMG. Fragmentation requires two backbonescission events for c-PLA, and we were able to observelinear intermediates (formed after a single scission) for the firsttime. We also developed a new method of studying the dynamic memoryeffect in US by characterizing and comparing the daughter fragmentmolecular weight distributions of l- and c-PLAs. These results provide new insights into the influenceof the cyclic polymer architecture on mechanochemical reactions aswell as differences in reactivity observed with US and BMG.