Unusual Cross-Linked Polystyrene by Copper-Catalyzed ARGET ATRP Using a Bifunctional Initiator and No Cross-Linking Agent

An anomalous polystyrene gel was obtained during the copper-catalyzed "activators regenerated by electron transfer" "atom transfer radical polymerization" (ARGET ATRP) of styrene at 60-70 degrees C, using ascorbic acid/Na2CO3 as the reducing system and EtOAc/EtOH as the solvent mixture. The result is remarkable since no branching nor cross-linking reagents were added to the reaction mixture and their formation in situ was excluded. The anomalous PS branching, at the origin of the phenomenon, requires a generic bifunctional initiator and is mechanistically bound to termination reactions between bifunctional macroinitiators. As a matter of fact, the branching/cross-linking phenomenon loses intensity, or even disappears, under reaction conditions that cause the built-up of Cu-II or increase the chain polymerization rate. The temperature is also a critical variable since no branching was observed for temperatures higher than 90 degrees C. We believe that the route toward gelation starts with a controlled chain polymerization of styrene from the bifunctional initiator, soon integrated by a step-growth polymerization due to radical coupling of the terminal units. The progressive decrease in the number of chains and free radicals in the reaction mixture should make more and more probable the intramolecular coupling between the C-Cl ends of the remaining long and entangled chains, producing a polycatenane network.