Ring-expansion polymerization (REP) of l-lactide with cyclic tin(II) bisphenoxides

Lithium anthraquinoids as catalysts in the ROP of lactide and caprolactone into cyclic polymers

New lithium anthraquinoids 2b–d active in the synthesis of cyclic PLA and cyclic PCL have been synthesized and fully characterized.

Syntheses of polylactides by means of tin catalysts

Reaction mechanisms and synthetic methods used for the preparation of homo- and copolylactides based on tin(ii) and tin(iv) catalysts are reviewed.

Synthesis, Properties, and Applications of Bio-Based Cyclic Aliphatic Polyesters

Cyclic polymers have long been reported in the literature, but their development has often been stunted by synthetic difficulties such as the presence of linear contaminants. Research into the synthesis of these polymers has made great progress in the past decade, and this review covers the synthesis, properties, and applications of cyclic polymers, with an emphasis on bio-based aliphatic polyesters. Synthetic routes to cyclic polymers synthesized from bioderived monomers, alongside mechanistic descriptions for both ring closure and ring expansion polymerization approaches, are reviewed. The review also highlights some of the unique physical properties of cyclic polymers together with potential applications. The findings illustrate the substantial recent developments made in the syntheses of cyclic polymers, as well as the progress which can be made in the commercialization of bio-based polymers through the versatility this topology provides.

High T m linear poly(l-lactide)s prepared via alcohol-initiated ROPs of l-lactide

Alcohol-initiated ROPs of l-lactide were performed in bulk at 160 °C for 72 h with variation of the catalyst or with variation of the initiator (aliphatic alcohols). Spontaneous crystallization was only observed when cyclic Sn(ii) compounds were used as a catalyst. Regardless of initiator, high melting crystallites with melting temperatures (T m) of 189-193 °C were obtained in almost all experiments with Sn(ii) 2,2'-dioxybiphenyl (SnBiph) as catalyst, even when the time was shortened to 24 h. These HTm poly(lactide)s represent the thermodynamically most stable form of poly(l-lactide). Regardless of the reaction conditions, such high melting crystallites were never obtained when Sn(ii) 2-ethylhexanoate (SnOct2) was used as catalyst. SAXS measurements evidenced that formation of HTm poly(l-lactide) involves growth of the crystallite thickness, but chemical modification of the crystallite surface (smoothing) seems to be of greater importance. A hypothesis, why the "surface smoothing" is more effective for crystallites of linear chains than for crystallites composed of cycles is discussed.

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.

Cyclic polylactide synthesis initiated by a lithium anthraquinoid: understanding the selectivity through DFT and diffusion NMR

We present herein the application of a lithium anthraquinoid in the catalytic synthesis of cyclic PLA, showing that the aggregation plays a critical role in cyclic vs. linear selectivity.

Investigation of the influence of impurities on the ring-opening polymerisation of L-Lactide from biogenous feedstock

In order to use a L-lactide monomer that is derived from fermentation processes it is necessary to understand, how the polymerisation process is influenced by impurities which derive from the production process. We have selected a group of likely contaminants and added them at various concentrations to the polymerisation of L-lactide using tin (II)-2-ethylhexanoate as catalyst and 2-methoxyethanol as initiator. The effect of impurities onto the global properties of the polymers such as glass transition temperature, melting point and molecular mass distribution were investigated and NMR and MALDI mass spectrometry were used to identify structural changes within the polymers. Thus, it could be shown that in reference experiments cyclic polymers and linear polymers with different starting groups are formed. Addition of ethanol and sodium carbonate showed the strongest influence on molecular masses as well as polymer structures, which could be elucidated by interpretation of the MALDI mass spectra and NMR data.

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 Tm poly(L-lactide)s via REP or ROPPOC of l-lactide

A new kind of high melting (HT_m) pol(l-lactide) was discovered when cyclic poly(l-lactide)s were prepared by ring-expansion polymerization with cyclic tin catalysts at 130–160 °C in bulk.

Stereocomplexation of cyclic polylactides with each other and with linear poly(l-lactide)s

This study describes for the first time syntheses and properties of stereocomplexes based on cyclic polylactides.