Living star polymer formation

The generation of star polymers via living polymerization protocols is well documented; however, the impact of midchain radicals (MCRs) on the precise formation pathways under operation in living acrylate star polymerizations is still poorly understood. In the present study, electrospray ionization-mass spectrometry (ESI-MS) technology has been applied to map the products generated in R-group approach reversible addition fragmentation chain transfer (RAFT) methyl acrylate (MA) star polymerizations in order to gain insight into the precise formation pathways under operation in such systems. The polymerizations were conducted at 65°C using the tetrafunctional RAFT agent 1,2,4,5-tetrakis(2-phenylthioacetylsulfanylmethyl)benzene and 2,2′-azobis(isobutyronitrile) (AIBN) as the thermally decomposing initiator. Initiator fragment derived linear chains, ideal stars, star-star couples, and other terminated star products formed as a result of combination and disproportionation reactions were successfully imaged. Additionally, MCR derived products that lie outside of the conventional R-group approach RAFT star polymerization mechanistic scheme were identified. Products associated with termination reactions involving intermolecularly formed MCRs on star arms and linear chains were observed; specifically, structures formed from MCR termination with propagating stars or radical carrying star cores, or with initiator fragments or propagating initiator derived linear chains. Additionally, structures produced via repropagation of intermolecularly formed MCRs on star arms were also identified. The products generated from MCR-derived reaction pathways were imaged from a degree of polymerization (DP_n) as low as one, indicating that MCRs can form upon molecules carrying only a single monomer unit.