Core-crosslinked star copolymers as viscosity index improvers for lubricants

Core cross-linked star (CCS) copolymers were prepared by reversible addition-fragmentation chain-transfer polymerization according to the arm-first technique and studied as viscosity index improvers for the application in hydraulic lubricants. A one-pot two-step synthetic procedure was optimized: starting from PMA arms with a precisely controlled structure and three different molecular weights and using different amounts of either divinylbenzene or ethylene glycol dimethacrylate as cross-linking agents, CCS copolymers with a high star yield (63 to 88%) were obtained. CCS copolymers present weight average molecular weight values ranging from 130,000 to 420,000 g mol^-1and a number of arms comprised between 4 and 13. All samples were employed, without any purification step, in the preparation of model lubricant formulations for hydraulic fluid applications. A good compromise between viscosity index improvement and capability to resist chain breaking reactions was obtained for CCS copolymers with target molecular weight for the PMA arms of 20,000 g mol^-1. Improved performances with respect to a commercial additive were obtained.