Methacrylate Star Synthesis by GTP

Abstract

Abstract All process conditions were picked to minimize the back-biting termination reaction so the maximum level of living arm could be carried into the core-forming step. Major contributors to control of termination were: 1. Using 0.2% (mole basis) of catalyst on initiator instead of higher concentrations. Within the constraints defined below, this produces a robust process that is insensitive to minor changes in materials, temperatures, and rates of addition. 2. Minimizing the time between arm formation and core formation. Polymerization of arm monomethacrylate to about 96 to 98% before core formation was the aim because conversion to >99% would have allowed too much time for termination to proceed. 3. Adding core-forming monomer (ethylene glycol dimethacrylate) in about 5 min to keep monomer concentration high in this step. Faster add times may not be desirable because of mixing difficulties in this extremely fast polymerization. 4. Reaction solvent mixture containing 50% tetrahydrofuran. This allowed the reflux temperature to be about 85°C. 5. Keeping monomer concentration high in the arm-forming step by starting the polymerization with half of the monomer in the reactor. This forced the polymerization past the trimer stage, where termination is fastest. 6. Starting the reaction at room temperature so the exotherm could be dissipated by the heat capacity of the system. Two lots of trimethylsiloxyethyl methacrylate used to introduce hydroxyl groups into the product (after unblocking) gave different rates of polymerization, with one lot reaching the core-forming step in about half the time of the other. The monomer lot with the higher reactivity also led to higher levels of free arm. Stars made at high (2.0%) catalyst levels were quite unstable to molecular weight and viscosity increase. Mechanistic studies show that the increase is probably because of star/ star free-radical methacrylate polymerization involving pendant residual methacrylate unsaturation from the core-forming step. Stars made under the recommended conditions using 0.2% catalyst and the above process steps are probably stable enough for commercial use.