Highly elastic and degradable vitrimeric elastomers using polycondensation

Introducing covalently crosslinked network to polymer matrix can merge the advantages in reprocessing and durability of polymers.In this contribution,a series of high-performance vitrimeric elastomers were achieved via polycondensation.The topological structures of polymers were tuned by varying the feeding ratios of bisacetoacetate,hex–substituted bisacetoacetate,bisamine and tris(2-aminoethyl)amine.With these structural manipulations,the vitrimeric elastomers presented great elastic recovery properties(strain recovery value up to 80%)benefiting from the introduction of long chain branch.Furthermore,the elastomers exhibited excellent reprocessing property,water vapor/oxygen barrier and adhesive properties.Specially,the elastomers could be degraded into monomer under acid conditions which enabled the elastomer synthesis again in closed loop recycling system.The ease of the polycondensation in this work to prepare highly elastic and recyclable vitrimeric elastomers demonstrated exciting opportunities for the synthesis of sustainable polymers.

From oxygenated monomers to well-defined low-carbon polymers

The synthesis of degradable polymers with easy-to-break in-chain carbon-oxygen bonds has attracted much attention.This minireview introduces the synthesis of a variety of degradable polymers from the(co)polymerizations of several typical oxygenated monomers such as epoxides,cyclic carbonates,cyclic esters,carbon dioxide(CO_(2)),carbonyl sulfide(COS),and cyclic anhydrides.We highlight the catalysts and mechanisms for these(co)polymerizations.The ring-opening copolymerization of five-membered carbonate with cyclic anhydride or COS has been introduced.We also highlight the synthesis of block copolymers and cyclic copolymers with well-defined sequences by the method of growing center switching.We hope that these new polymerization systems can provide new ideas for the development of degradable low-carbon polymers in the future.

Physical and degradation properties of PLGA scaffolds fabricated by salt fusion technique

Tissue engineering scaffolds require a controlled pore size and interconnected pore structures to support the host tissue growth. In the present study, three dimensional （3D） hybrid scaffolds of poly lactic acid （PLA） and poly glycolic acid （PGA） were fabricated using solvent casting/particulate leaching. In this case, partially fused NaCl particles were used as porogen （200-300μ） to improve the overall porosity （≥90%） and internal texture of scaffolds. Differential scanning calorimeter （DSC） analysis of these porous scaffolds revealed a gradual reduction in glass transition temperature （Tg） （from 48°C to 42.5°C） with increase in hydrophilic PGA content. The potential applications of these scaffolds as implants were further tested for their biocompatibility and biodegradability in four simulated body fluid （SBF） types in vitro. Whereas, simulated body fluid （SBF） Type1 with the optimal amount of HCO 3 ions was found to be more appropriate and sensible for testing the bioactivity of scaffolds. Among three combinations of polymer scaffolds, sample B with a ratio of 75：25 of PLA： PGA showed greater stability in body fluids （pH 7.2） with an optimum degradation rate （9% to 12% approx）. X-ray diffractogram also confirmed a thin layer of hydroxyapatite deposition over sample B with all SBF types in vitro.

TUNED MECHANICAL PROPERTIES ACHIEVED BY VARYING POLYMER STRUCTURE-KNOWLEDGE THAT GENERATES NEW MATERIALS FOR TISSUE ENGINEERING

By changing both the monomer composition and the polymer structure, we have varied the mechanical properties of resorbable polymers. The polymers were synthesized by ring-opening polymerization using L-lactide （LLA）, ε-caprolactone （εCL）, trimethylene carbonate （TMC） and 1,5-dioxepan-2-one （DXO） as monomers. Well-defined triblock copolymers, microblock copolymers and networks have been evaluated, and comparisons between them show that it is possible to tune the mechanical properties. Triblock copolymers with an amorphous middle block of poly（1,5-dioxepan-2- one） （PDXO） and semi-crystalline end-blocks of poly（ε-caprolactone） （PCL） were stronger and had a higher strain at break than triblock copolymers with poly（L-lactide） （PLLA） as end-blocks. Polymers with both DXO and TMC in the amorphous middle-block and PLLA as end-blocks showed a lower stress at break, but the material gained elasticity, a property which is very valuable in tissue engineering. Mechanical properties of networks, synthesized by a novel method, containing PDXO and PCL are also presented. Although it is difficult to compare them with the uncross-linked polymers, this is an additional way to modify and widen the properties.

Segmental long bone regeneration guided by degradable synthetic polymeric scaffolds

Recent developments in synthetic bone grafting materials and adjuvant therapeutic agents have opened the door to the regenerative reconstruction of critical-size long bone segmental defects resulting from trauma,osteoporotic fractures or tumour resections.Polymeric scaffolds with controlled macroporosities,degradability,useful surgical handling characteristics,and the ability to deliver biotherapeutics to promote new bone ingrowth have been developed for this challenging orthopaedic application.This review highlights major classes of degradable synthetic polymers and their biomineral composites,including conventional and amphiphilic polyesters,polyanhydrides,polycarbonates,and polyethylene glycol-based hydrogels,that have been explored for the regenerative reconstruction of critical-size long bone segmental defects over the past two decades.The pros and cons of these synthetic scaffold materials are presented in the context of enabling or impeding the functional(mechanical and radiographic)repair of a long bone segmental defect,with the long bone regeneration outcomes compared with healthy long bone controls or results achieved with current grafting standards.

A GENERAL APPROACH TO DETERMINING THE EVOLUTION OF MOLECULAR WEIGHT DISTRIBUTION CURVES OF LINEAR POLYMERS UNDERGOING CHAIN SCISSION

A numerical method is developed to compute the development of molecular weight distribution （MWD） curves of linear polymers undergoing chain scission. The method can be applied to complex chain scission kinetics and for arbitrarily complex initial MWD curves. Our method is based on the method of lines （MoL）. Different from the existing numerical scheme, we propose the use of logarithmically spaced points. This development ensures the accuracy of the computed MWD curves at low molecular weights, and it does not require a very fine discretization to produce an accurate result.

Recycling and Upgrading Utilization of Polymer Plastics

Due to the current situation of massive waste consumption and accumulation,the recycling and upgrading utilization of polymer materials is an effective technology to solve environmental pollution.In this work,the recycling and upgrading methods of polymers are summarized,and the latest progress in polymer upcycling is discussed from the perspective of upgrading materials.The common polymer recovery methods,including mechanical recovery,chemical recovery,biocatalysis,and photocatalytic recovery,are discussed based on their mechanism and industrialized application.The upgrading products of polymers are divided into monomers,fuels and fine chemicals.The challenges and prospects of polymer degradation technology are discussed.

A ring-controlled fluorescent platform for visualizing polymer degradation

The creation of a new photoluminescent platform to study their photo physics and further applications is important in the fields of chemistry,material,physics,etc.In this work,we developed benzene-based fluorophores generated by chemical reactions on conjugate acceptor precursors.Their optical properties have been studied,and the large Stokes shifts,high molar extinction coefficients and quantum yields have been revealed.The starting precursors containing bis-intercalated vinyl thioester shows no luminescence,which is turned on by the chemically triggered cyclization.From spectroscopic,crystallographic,and computational studies,cyclization-induced emission enhancement is proposed as a mean to explain the photoluminescent performances,a principle extended in the substituent incorporated fluorophores.Furthermore,they were applied to the chemically triggered degradation of a polymer and achieved visual tracking,quantifying,and downcycling of the processes.The method for designing and developing series fluorophores and visually tracking polymer degradation represents a new photoluminescent platform,allowing its further applications.

End-life Prediction of Commercial PLA Used for Food Packaging through Short Term TGA Experiments: Real Chance or Low Reliability?

A long-term （about nine months） isothermal degradation experiment of two different commercial polylactide （PLA） samples used for food packaging was carried out at a relatively low temperature （423 K）. Thermooxidative degradations of the same polymers were carried out in a thermogravimetric （TG） analyser, at higher temperatures （453 K 〈 T 〈 523 K）, under isothermal heating conditions. The obtained set of experimental TG data was used to determine the apparent activation energy （Ea） of degradation through two isothermal kinetic methods. The results from long-term experiment evidenced considerable mass loss for both PLA samples in the investigated period, but the experimental data were not in agreement with those from the short-term degradations at higher temperatures, thus suggesting a different degradation kinetics, and, then a low reliability of the lifetime predictions for polymers in service or degradation forecasts for the end of their life based on experiments at higher temperatures.

Synthesis of Well-defined Telechelic Trans-1,4-polyisoprene by Oxidative Cleavage

Synthesis of telechelic trans-1,4-polyisoprenes（TPI： trans-structure ＆gt; 95%） was evaluated based on two different methods of oxidative cleavage（indirect cleavage： first epoxidation of TPI, then the selective cleavage of epoxidized units in epoxidized trans-1,4-polyisoprene（ETPI） and direct cleavage of isoprene units in TPI）. The influence of solvents and the ratio of oxidative agents was investigated by 1H-NMR and 13C-NMR. A series of well-defined telechelic TPI with double terminated functional groups and less side reaction（molecular weight distribution range： 1.96？2.26） were synthesized by indirect cleavage in chloroform. Telechelic TPI showed similar crystallization behavior with TPI and interesting cold crystallization behavior characterized by DSC.