Controlled Radical Polymerization of methyl methacrylate and styrene mediated by a cobalt(II)-based initiatior system

The role of cobalt(II) complexes containing 2-formylpyrrolyl chelating ligands and phosphine ligands combined with a bromo-alkyl ester, an alkyl bromide or an azo compound in the formation of suitable initiator systems for vinyl monomer controlled radical polymerization was studied. The formation of syndiotactic-rich (Pr>0.7) poly(methyl methacrylate) (PMMA) and atactic (Pr=0.49) poly(styrene) was achieved, in high yields, with higher reaction rates in the first case. Polymerization of styrene was successfully controlled by the initiator system [Co{κ 2 N,O-NC4H3-C(H)=O}2(PMe3)2] (1)/tertbutyl-α-bromoisobutyrate (2), at 50 and 70 oC, whereas for methyl methacrylate (MMA) control was only possible below 50 oC. Other sources of cobalt(II) were used, such as CoCl2(PMe3)2 and CoCl2(PPh3)2, the first being very active in the initiation of MMA, similar to the one observed in the previous case and able to control its polymerization below 50 oC. The use of a coordinating solvent (THF) reduces the polymerization rate mediated by CoCl2(PMe3)2, but does not affect the polymerization rate in the case of complex 1. MALDI-TOF mass spectrometry pointed to a chain-end bromination of the poly(styrene) obtained in a controlled way, as opposed to poly(methyl methacrylate). The controlled polymerization of MMA in the presence of a poly(styrene) macroinitiator and complex 1, prepared in molecular weight control conditions, was successful, resulting in the formation of the block-copolymer poly(styrene)-b-poly(MMA).