Stereoselective zwitterionic ring-opening polymerization of rac-lactide

Understanding zwitterionic ring-expansion polymerization through mass spectrometry

Zwitterionic ring-expansion polymerization (ZREP) is a polymerization method in which a cyclic monomer is converted into a cyclic polymer through a zwitterionic intermediate. In this review, we explored the ZREP of various cyclic polymers and how mass spectrometry assists in identifying the product architectures and understanding their intricate reaction mechanism. For the majority of polymers (from a few thousand to a few million Da) matrix-assisted laser desorption/ionization time-of-flight mass spectrometry is the most effective mass spectrometry technique to determine the true molecular weight (MW) of the resultant product, but only when the dispersity is low (approximately below 1.2). The key topics covered in this study were the ZREP of cyclic polyesters, cyclic polyamides, and cyclic ethers. In addition, this study also addresses a number of other preliminary topics, including the ZREP of cyclic polycarbonates, cyclic polysiloxanes, and cyclic poly(alkylene phosphates). The purity and efficiency of those syntheses largely depend on the catalyst. Among several catalysts, N-heterocyclic carbenes have exhibited high efficiency in the synthesis of cyclic polyesters and polyamides, whereas tris(pentafluorophenyl)borane [B(C6F5)3] is the most optimal catalyst for cyclic polyether synthesis.

About the transformation of low T m into high T m poly(l-lactide)s by annealing under the influence of transesterification catalysts

Cyclic polylactides were prepared in bulk at 170 °C, crystallized at 120 °C and then annealed at temperatures between 130 and 170 °C with variation of catalyst, catalyst concentration and annealing time. The transformation of the initially formed low melting (LT m) crystallites, having melting temperatures (T m) < 180 °C into high melting (HT m) crystallites having T m values > 189 °C was monitored by means of DSC measurements and characterized in selected cases by SAXS measurements. It was confirmed that the formation of HT m crystallites involves a significant growth of the thickness of the lamellar crystallites along with smoothing of their surface. Annealing at 170 °C for 1 d or longer causes thermal degradation with lowering of the molecular weights, a gradual transition of cyclic into linear chains and a moderate decrease of lamellar thickness. An unexpected result revealed by MALDI TOF mass spectrometry is a partial reorganization of the molecular weight distribution driven by a gain of crystallization enthalpy.

Reversible polycondensations outside the Jacobson–Stockmayer theory and a new concept of reversible polycondensations

Reversible polycondensations exist involving intermolecular transesterification in the absence of back-biting, which may be understood as part of a so-called ‘polycondensation triangle’ based on the definition of three polycondensation classes.

Advancing macromolecular hoop construction: recent developments in synthetic cyclic polymer chemistry

Synthetic methodology to access cyclic macromolecules continues to develop via two distinct mechanistic classes: ring-expansion of macrocyclic initiators and ring-closure of functionalized linear polymers.

The Ring-Opening Polymerization-Polycondensation (ROPPOC) Approach to Cyclic Polymers

A new concept called ring-opening polymerization-polycondensation (ROPPOC) is presented and discussed. This synthetic strategy is based on the intermediate formation of chains having two end groups that can react with each other. The ROPPOC syntheses are subdivided into three groups according to the nature of the chain ends: two ionic end groups, one ionic and one covalent chain end, and a combination of two reactive covalent end groups may be involved, depending on the catalyst. The usefulness for the preparation of cyclic polymers is discussed with a review of numerous previously published examples. These examples concern the following classes of cyclic polymers: polypeptides, polyamides, and polyesters, including polycarbonates and cyclic polysiloxanes. It is demonstrated that the results of certain ROPPOC syntheses are in contradiction to the Jacobson-Stockmayer theory. Finally, the usefulness of ROPPOCs for the detection of polydisperse catenanes is discussed.

High molar mass cyclic poly(l-lactide) obtained by means of neat tin(ii) 2-ethylhexanoate

l-Lactide was polymerized in bulk at 120, 140, 160 and 180 °C with neat tin(ii) 2-ethylhexanoate (SnOct₂) as the catalyst and a cyclic topology was detected.

Isoselective ring-opening polymerization and asymmetric kinetic resolution polymerization of rac-lactide catalyzed by bifunctional iminophosphorane–thiourea/urea catalysts

The isoselective mechanism (CEC or ESC) for ring-opening polymerization of rac-LA was demonstrated using chiral and achiral IPTU/IPU organocatalysts.