ATRP-RCM Synthesis of 8-Shaped Poly(methyl acrylate) Using a 4-Armed Star Telechelics

Abstract

Cyclic and multicyclic polymers have drawn increasing attention due to their unique architectures, i.e., free of chain ends. A number of protocols including, in particular, the ringexpansion polymerization and the end-to-end polymer cyclization, have now been developed for the selective and effective synthesis of single cyclic polymers to realize novel properties and functions unattainable by conventional linear or branched counterparts. We have also proposed a metathesis polymer cyclization (MPC) process, i.e., a ring-closing metathesis (RCM) reaction with a linear telechelic precursor having olefinic end groups under dilution. And the effective and selective synthesis of cyclic polyacrylate has been achieved through atom transfer radical polymerization (ATRP)-Keck allylation and the subsequent RCM reaction. In this process, the key telechelic prepolymer was produced by the ATRP using a difunctional initiator and the terminal bromide group was quantitatively converted to allyl group by the reaction with allyltributyltin. The subsequent intramolecular RCM reaction proceeded even under dilution in the presence of Grubbs catalyst to give a cyclic polymer in high efficiency. On the other hand, such a dicyclic as 8-shaped polymer has yet been an ongoing challenge in the synthetic polymer chemistry. We have shown that 8-shaped polymers are obtainable through an electrostatic self-assembly formed by two units of telechelic precursor having moderately strained cyclic ammonium salt end groups carrying one unit of tetracarboxylate counter anion. Alternatively, interand intramolecular metathesis condensation process have been applied between two units of a cyclic prepolymer having an allyloxy group (Scheme 1-a), and with a twin-tailed tadpole polymer precursor (Scheme 1-b), as well as with a cyclic polymer precursor having two allyloxy groups at the opposite positions (Scheme 1-c), respectively. The intramolecular metathesis condensation has also been applied with an H-shaped precursor having allyloxy end groups at each chain ends, to produce other types of dicyclic polymers having and manacle topologies (Scheme 1-d). As an extension of the preceding studies, we report in this paper on the synthesis of an 8-shaped polymer through the double ring-closing metathesis reaction using a 4-armed star polymer precursor, synthesized by means of the ATRP of methyl acrylate using a tetrafunctional initiator (Scheme 2). Since the ATRP has been recognized as a versatile means to give living star polymers upto hexafunctionalities by using multi-functional initiators, complex multicyclic polymers will be obtainable through the MPC process with end-functional star polymers.