Scalable Bimolecular Ring-Closure Method for Cyclic Polymers

Scalable formation of cyclic polymers is a long-term challenge for bimolecular ring-closure methods, although it has been demonstrated as one of the oldest strategies for preparing well-defined cyclic polymers. An efficient bimolecular ring-closure method was developed in this study using a fast double strain-promoted azide-alkyne click reaction (DSPAAC) as a ring-closing reaction, in which sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) was used as small linkers to ring-close linear polymer precursors with 2,6-diisopropylphenyl azide end groups connected by para-ether bonds. The kinetic investigation of this DSPAAC ring-closing reaction produced the rate constants of 0.712 and 128.16 M(-1)s(-1) for the azide cycloaddition reaction with the first (k(1)) and the second (k(2)) alkyne moieties of DIBOD. A 180 times larger k(2) than k(1) endowed the DSPAAC ring-closing reaction with a self-accelerating property, which facilitated the bimolecular method to efficiently ring-close the linear polymers and prepare the corresponding cyclic polymers with a high purity in the presence of excessive DIBOD small linkers. The large k(1) and k(2) values supported the DSPAAC ring-closing reaction with a fast reaction rate, which allowed the bimolecular method to combine with the slow feeding technique to prepare cyclic polymers in large scales. This is the first study to achieve the scalable formation of cyclic polymers based on the combination of the bimolecular ring-closure method and slow feeding technique.