Identity of Low-Molecular-Weight Species Formed in End-To-End Cyclization Reactions Performed in THF.

Ching Pan,Samantha Parker,Eric Tillman,Katherine Xia

doi:10.3390/polym10080844

Abstract

Cyclic polymers were produced by end-to-end coupling of telechelic linear polymers under dilute conditions in THF, using intramolecular atom transfer radical coupling or click chemistry. In addition to the expected shift to longer elution times on gel permeation chromatography (GPC) indicative of the formation of cyclic product, lower molecular weight species were consistently observed upon analysis of the unpurified cyclization reaction mixture. By systematically removing or altering single reaction components in the highly dilute cyclization reaction, it was found that THF itself was responsible for the low-molecular-weight material, forming oligomeric chains of poly(THF) regardless of the other reaction components. When the reactions were performed at higher concentrations and for shorter time intervals, conducive to intermolecular chain-end-joining reactions, the low-molecular-weight peaks were absent. Isolation of the material and analysis by 1H NMR confirmed that poly(THF) was being formed in the highly dilute conditions required for cyclization by end-to-end coupling. When a series of mock cyclization reactions were performed with molar ratios of the reactants held constant, but concentrations changed, it was found that lower concentrations of reactants led to higher amounts of poly(THF) side product.

Highlights

Methods such as ring opening metathesis [1] and polyhomologation [2] can lead to macrocycles, yet perhaps the most common technique involves the chain-end closure of telechelic linear polymer precursors
A common solvent for both polymerizations, leading to linear precursors and post-polymerization transformation reactions, is tetrahydrofuran (THF). This solvent is compatible with all the cyclization reactions listed above, while being a compatible polymerization medium for anionic techniques [13,14] and reversible activation/deactivation radical polymerization (RDRP) methods such as atom transfer radical polymerization (ATRP) [15,16] and nitroxide-mediated polymerization (NMP) [17]
The results presented in this paper are focused not on assessing the yields or purity of the cyclic polymers, as the methods used have already been reported and proven capable of producing

Summary

Introduction

Methods such as ring opening metathesis [1] and polyhomologation [2] can lead to macrocycles, yet perhaps the most common technique involves the chain-end closure of telechelic linear polymer precursors This much-used sequence will often follow the same general plan: synthesize α,ω-telechelic polymers with reactive chain ends, and coerce an intramolecular coupling reaction by appropriate choice of chain ends and dilution (Scheme 1). This approach is conceptually simple, with the overall process involving end-group transformations of one or both of the chain ends to prepare the linear precursors capable of cyclization. The success of cyclization reactions is almost always evaluated, at least in part, using gel Polymers 2018, 10, 844; doi:10.3390/polym10080844 www.mdpi.com/journal/polymers

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Conclusion