Reversible addition-fragmentation chain transfer polymerization of diisopropyl fumarate using various dithiobenzoates as chain transfer agents

Dialkyl fumarates as 1,2-disubstituted ethylenes exhibit unique features of radical polymerization kinetics due to their significant steric hindrance in propagation and termination processes and provide polymers with a rigid chain structure different from conventional vinyl polymers. In this study, we carried out reversible addition-fragmentation chain transfer polymerization of diisopropyl fumarate (DiPF) in bulk at 80 degrees C using various dithiobenzoates with different leaving R groups as chain transfer agents to reveal their performance for control of molecular weight, molecular weight distribution, and chain end functionality of the resulting poly(DiPF) (PDiPF). 2-(Ethoxycarbonyl)-2-propyl dithiobenzoate (DB1) and 2,4,4-trimethyl-2-pentyl dithiobenzoate (DB2) underwent fragmentation and reinitiation at a moderate rate and consequently led to the formation of PDiPF with well-controlled chain structures. It was confirmed that molecular weight of PDiPF produced by controlled polymerization with DB1 and DB2 agreed with theoretical one and molecular weight distribution was narrow. Dithiobenzoate and R fragments were introduced into the polymer chain ends with high functionality as 95% by the use of DB1. In contrast, polymerizations using 1-(ethoxycarbonyl)benzyl dithiobenzoate (DB3), 1-phenylethyl dithiobenzoate (DB4), and 2-phenyl-2-propyl dithiobenzoate (DB5) resulted in poor control of molecular weight, molecular weight distribution, and chain end structures of PDiPF. Fragmentation and reinitiation rates of the used benzoates as chain transfer agents significantly varied depending on the R structures in an opposite fashion; that is, introduction of bulky and conjugating substituents accelerated fragmentation, but it retarded initiation of DiPF polymerization. It was revealed that balance of fragmentation and reinitiation was important for controlled polymerization of DiPF. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3266-3275