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

Abstract

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 8C 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-2pentyl 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.

Synthesis of transparent block copolymers consisting of poly(diisopropyl fumarate) and poly(2‐ethylhexyl acrylate) segments by reversible addition‐fragmentation chain transfer polymerization using trithiocarbonates as the chain transfer agents

“…This was probably due to the limited initiation rate of the R 1 fragment produced by the chain transfer to T2 as the CTA. A similar unavoidable problem was already pointed out for the polymerization systems using DB1 and the other related CTAs . The results of the chain end group analysis suggested the formation of deactivated PDiPF chains during the polymerization of DiPF.…”

“…In this study, the introduction efficiency of the MAIB fragment was evaluated to be approximately 10%, but an accurate efficiency could not be determined due to the significant overlap of the peak with large peaks at 2.8 to 4 ppm for the methine hydrogen of the main chain. The contribution of the MAIB fragment was also observed for the polymerization using DB1 in the literature …”

“…We carried out the RAFT polymerization of DiPF in the bulk at 80 °C in the presence of T1 as the monofunctional CTA and MAIB as the radical initiator at the concentration of [DiPF]/[MAIB]/[T1] = 200/0.35/1.0 in molar ratio. The results are shown in Figure a and Table , in which the results obtained for the polymerization with DB1 as the most effective benzoate compound in our previous study was also shown for comparison. The polymerization with T1 proceeded and reached a high conversion by the use of a small amount of MAIB ([MAIB]/[T1] = 0.35).…”

“…We clarified that 2‐(ethoxycarbonyl)‐2‐propyl dithiobenzoate (DB1 in Fig. ) exhibited adequate fragmentation and initiation rates and it consequently provided PDiPF with well‐controlled chain structures …”

“…In typical RAFT polymerization processes, it has been pointed out that an increase in the steric and resonance factors accelerates the fragmentation rate of the intermediate radical, which is produced by the addition of any radical to chain transfer agents (CTAs). In our previous study, it was reported that the reactivity of the radical formed by the fragmentation of the CTAs was important for the controlled polymerization of DiPF using various dithiobenzoates as the CTAs. The steric bulkiness and the stabilization of the radical suppressed the initiation of the DiPF polymerization, while such structures were advantageous for the fast fragmentation of the CTA adduct radicals.…”