Switching from Controlled Ring-Opening Polymerization (cROP) to Controlled Ring-Closing Depolymerization (cRCDP) by Adjusting the Reaction Parameters That Determine the Ceiling Temperature

Abstract

Full control over the ceiling temperature (Tc) enables a selective transition between the monomeric and polymeric state. This is exemplified by the conversion of the monomer 2-allyloxymethyl-2-ethyl-trimethylene carbonate (AOMEC) to poly(AOMEC) and back to AOMEC within 10 h by controlling the reaction from conditions that favor ring-opening polymerization (Tc > T0) (where T0 is the reaction temperature) to conditions that favor ring-closing depolymerization (Tc < T0). The ring-closing depolymerization (RCDP) mirrors the polymerization behavior with a clear relation between the monomer concentration and the molecular weight of the polymer, indicating that RCDP occurs at the chain end. The Tc of the polymerization system is highly dependent on the nature of the solvent, for example, in toluene, the Tc of AOMEC is 234 °C and in acetonitrile Tc = 142 °C at the same initial monomer concentration of 2 M. The control over the monomer to polymer equilibrium sets new standards for the selective degradation of polymers, the controlled release of active components, monomer synthesis and material recycling. In particular, the knowledge of the monomer to polymer equilibrium of polymers in solution under selected environmental conditions is of paramount importance for in vivo applications, where the polymer chain is subjected to both high dilution and a high polarity medium in the presence of catalysts, that is, very different conditions from which the polymer was formed.

Reversibility in Controlled Polymerization Systems: Recent Progress and New Opportunities

“…We begin our exploration of reversible polymerization approaches in ROP systems. [48,49,[51][52][53][54][55][56] As one of the most popular controlled polymerization strategies, ROP of cyclic monomers has emerged as a useful synthetic route to prepare technologically interesting polymers with desirable architectures and specific properties. In particular, nucleophile-initiated ROP in the presence of metal-or organocatalyst allows the polymerization to proceed in a controlled manner, affording polymers with pre-determinable molecular weights and narrow molecular weight distributions.…”

“…Inspired by this pioneering work, Albertsson's team continued the study on AOMEC and first demonstrated the reversible ROP of this six-membered cyclic carbonate monomer in 2016. [49] It was concluded that the equilibrium between controlled ROP of AOMEC and controlled ring-closing depolymerization (RCDP) of poly(AOMEC) were dictated by various reaction parameters such as monomer concentration, reaction temperature, and even solvents. In their approach, AOMEC was polymerized by ROP in the presence of organocatalyst, that is, 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) in either bulk or different solvents (Figure 1a).…”

“…Moreover, the molecular weight distribution of all the evolving polymers remained as low as 1.1, which further verified the exceptional control over both the course of polymerization and depolymerization. [49] As one of the best suitable biomass-derived compounds to replace petroleum-derived chemicals, γ-butyrolactone (γ-BL) and its polymer PγBL are of great potential as sustainable materials. [53,55] However, γ-BL was commonly considered as "non-polymerizable" due to its low strain energy, which thereby rendered the ROP of γ-BL extremely challenging.…”

“…To fully achieve sustainability in polymer materials, recent attentions have been shifted to the development of new methods to depolymerize polymers back into original monomers under accessible conditions. [45,[47][48][49][50][51][52][53][54][55][56] On the other hand, those regenerated monomers can be recycled and further repolymerized to obtain new polymer materials. In this perspective, we aim to first critically assess the state-of-the-art toolbox for achieving reversible polymerization in controlled polymerization systems (Schemes 1 and 2).…”

Anionic polycondensation and equilibrium driven monomer formation of cyclic aliphatic carbonates

“…[9][10][11][12][13] The equilibrium behavior is clearly seen during the ring-opening polymerization of cyclic monomers, where, residual monomer will be present regardless of the reaction time and catalytic system employed. [14][15][16][17] The implications of residual monomer in the material or in the reaction mixture depend on the end-use. Obviously, for polymer synthesis the amount of residual monomer should be as low as possible; however, for monomer synthesis the opposite is advantageous.…”