A Chemoselective Approach to Grafting Biodegradable Polyesters

Post-polymerisation approaches for the rapid modification of conjugated polymer properties

Post-polymerisation functionalisation provides a facile and efficient way for the introduction of functional groups on the backbone of conjugated polymers. Using post-polymerisation functionalisation approaches, the polymer chain length is usually not affected, meaning that the resulting polymers only differ in their attached functional groups or side chains, which makes them particularly interesting for investigating the influence of the different groups on the polymer properties. For such functionalisations, highly efficient and selective reactions are needed to avoid the formation of complex mixtures or permanent defects in the polymer backbone. A variety of suitable synthetic approaches and reactions that fulfil these criteria have been identified and reported. In this review, a thorough overview is given of the post-polymerisation functionalisations reported to date, with the methods grouped based on the type of reaction used: cycloaddition, oxidation/reduction, nucleophilic aromatic substitution, or halogenation and subsequent cross-coupling reaction. Instead of modifications on the aliphatic side chains of the conjugated polymers, we focus on modifications directly on the conjugated backbones, as these have the most pronounced effect on the optical and electronic properties. Some of the discussed materials have been used in applications, ranging from solar cells to bioelectronics. By providing an overview of this versatile and expanding field for the first time, we showcase post-polymerisation functionalisation as an exciting pathway for the creation of new conjugated materials for a range of applications.

A New Approach Utilizing Aza-Michael Addition for Hydrolysis-Resistance Non-Ionic Waterborne Polyester

This work first synthesized a series of linear polyesters by step-growth polycondensation, then an amino-terminated hydrophilic polyether was grafted to the polyester as side-chains through aza-Michael addition to prepare a self-dispersible, non-ionic waterborne comb-like polyester (NWCPE). In contrast to traditional functionalization methods that usually require harsh reaction conditions and complex catalysts, the aza-Michael addition proceeds efficiently at room temperature without a catalyst. In this facile and mild way, the NWCPE samples with number-average molecular weight (Mn) of about 8000 g mol−1 were obtained. All dispersions showed excellent storage stability, reflected by no delamination observed after 6 months of storage. The NWCPE dispersion displayed better hydrolysis resistance than an ionic waterborne polyester, as was indicated by a more slight change in pH value and Mn after a period of storage. In addition, the film obtained after the NWCPE dispersion was cross-linked with the curing agent, it exhibited good water resistance, adhesion, and mechanical properties.

Lipase-catalyzed synthesis and post-polymerization modification of new fully bio-based poly(hexamethylene γ-ketopimelate) and poly(hexamethylene γ-ketopimelate-co-hexamethylene adipate) copolyesters

A novel full bio-based ketone-containing aliphatic polyester was prepared by enzyme-catalyzed polycondensation of diethyl γ-ketopimelate (DEK) with 1,6-hexanediol (HDO) using immobilized lipase B from Candida antarctica (CALB). The influences of polymerization conditions such as temperature, time, enzyme amount, and solvent amount on the molecular weight of poly(hexamethylene γ-ketopimelate) (PHK) were investigated. New fully bio-based poly(hexamethylene γ-ketopimelate-co-hexamethylene adipate) (poly(HK-co-HA)) copolymers with narrow polydispersity and well-defined composition were synthesized by copolymerization of DEK, HDO, and diethyl adipate. The structures of PHK and poly(HK-co-HA) copolymers were characterized by nuclear magnetic resonance, and their thermal characterization was examined by thermogravimetric analysis and differential scanning calorimetry. The degradation of PHK and poly(HK-co-HA) copolymers was studied. The post-polymerization modification of these polyketoesters via oxime click chemistry was further demonstrated.

Facile preparation of stereochemistry-controllable biobased poly(butylene maleate-co-butylene fumarate) unsaturated copolyesters: a chemoselective polymer platform for versatile functionalization via aza-Michael addition

Biobased stereochemistry-controllable poly(butylene maleate-co-butylene fumarate) unsaturated copolyesters were facilely prepared as a chemoselective polymer platform for versatile functionalization via aza-Michael addition.

Substituted Polyesters by Thiol-Ene Modification: Rapid Diversification for Therapeutic Protein Stabilization

Many proteins, especially those used as therapeutics, are unstable to storage and shipping temperatures, leading to increased costs in research and industry. Therefore, the design and synthesis of novel stabilizers is an important area of investigation. Herein we report new degradable polymers that stabilize proteins to environmental stressors such as refrigeration and elevated temperature. Specifically, polycaprolactones with different pendant groups were synthesized and surveyed for their ability to stabilize an important therapeutic protein to storage and shipping conditions. Ring-opening polymerization (ROP) of an allyl-substituted caprolactone monomer was carried out using the organocatalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) to yield a well-defined, alkene-substituted degradable polymer, which was used as a common backbone to control for the degree of polymerization. Relevant side chains such as trehalose, lactose, glucose, carboxybetaine, and oligo(ethylene glycol) were installed via postpolymerization thiol-ene reactions. These degradable polymers were then employed as excipients for the stabilization of the therapeutic protein granulocyte colony-stimulating factor (G-CSF) against storage at 4 °C and shipping temperatures of 60 °C. The best stabilization was observed using the trehalose- and zwitterion- substituted polyesters. Both the trehalose- and carboxybetaine-substituted pCL were further investigated with regard to molecular weight dependence, and it was found that the molecular weight was minimally important for stabilization to refrigeration, but critical for G-CSF stabilization at elevated temperatures. Both high performing zwitterionic and trehalose polyesters were also degraded, and the polymers and degradation products were shown to be noncytotoxic. This work provides potential biocompatible polymers for stabilization of the important therapeutic G-CSF, as well as a general platform for the future discovery of new polymeric protein stabilizers.

One-pot Synthesis of Functional Poly(amino ester sulfide)s and Utility in Delivering pDNA and siRNA

The development of efficacious carriers is an important long-standing challenge in gene therapy. In the past few decades, tremendous progress has been made toward non-viral vectors for gene delivery including cationic lipids and polymers. However, there continues to be a need for clinically translatable polymer-based delivery carriers because they offer tunable degradation profiles and functional groups, diverse structures/morphologies, and scalability in preparation. Herein, we developed a library of 144 degradable polymers with varying amine and hydrophobic content via a facile method that involves thiobutyrolactone aminolysis and consequent thiol-(meth)acrylate or acrylamide addition in one-pot. The polymer platform was evaluated for pDNA and siRNA delivery to HeLa cells in vitro. Hydrophobically modified 5S, 2E1, 6CY1, 5CY2, and 2M1 grafted HEMATL polymers are capable of delivering pDNA depending on the chemical composition and the size of the polyplexes. Hydrophobically modified 5S and 2B grafted HEMATL and 5S grafted ATL polymers exhibit capability for siRNA delivery that approaches the efficacy of commercially available transfection reagents. Due to tunable functionality and scalable preparation, this synthetic approach may have broad applicability in the design of delivery materials for gene therapy.

Facile synthesis of biodegradable and clickable polymer

We report a facile approach to synthesize a biodegradable and clickable polymer consisting of ε-caprolactone (CL) and allyl methacrylate (AMA) with phosphazene base as the catalyst via hybrid copolymerization.

Methoxy-Poly(ethylene glycol)-block-Poly(ε-caprolactone) Bearing Pendant Aldehyde Groups as pH-Responsive Drug Delivery Carrier

A biodegradable amphiphilic block copolymer of methoxy poly(ethylene glycol)-block-poly(ϵ-caprolactone) bearing pendant aldehyde groups was synthesised by a combination of ring-opening polymerisation and thio-bromo ‘click’ chemistry. The free aldehyde groups on the copolymer were reacted with hydrophobic payloads (p-methoxylaniline as a model drug) by a benzoic–imine linker, which was responsive to pH change. NMR, FTIR, and gel permeation chromatography analysis confirmed the copolymer structures. In vitro release studies revealed that under acid stimulus, hydrolysis of the benzoic–imine bond resulted in a rapid drug release. This new amphiphilic block copolymer is expected to have promising applications in biodegradable controlled drug delivery systems.

Low-Temperature Processable Degradable Polyesters

Block copolymers composed of a low-T(g) and high-T(g) block, with suitable pressure miscibility characteristics, can be formed at low-temperature through the application of pressure. Aliphatic block copolyesters composed of poly(ε-caprolactone) derivatives and poly(L-lactide) show room temperature processability under hydraulic pressure of 34.5 MPa without polymer degradation. Mechanism of the pressure-induced flow is investigated by small-angle X-ray scattering. A scattering associated with a lamellae structure observed at ambient conditions decreases with elevating hydrostatic pressures, indicating pressure-induced phase mixing. Traces of the pressure-induced phase transition are studied by differential scanning calorimetry and X-ray diffraction. Tensile test of the block copolymers reveals that the mechanical properties can be readily controlled by changing composition, molecular weight, and chemical structure of the blocks. Among them, the hard segment PLLA fraction is the key factor to characterize the properties. Young's modulus of the block copolyesters is similar to that of polyethylene.

Facile Synthesis of Functionalized Lactones and Organocatalytic Ring-Opening Polymerization

A facile one-step synthesis of functionalized valerolactones was carried out by the conjugate addition of thiols to the α,β-unsaturated valerolactone 5,6-dihydro-2H-pyran-2-one. The resultant 3-mercaptovalerolactones undergo ring-opening polymerization in solution or in the melt to generate polyesters functionalized either with benzyl mercaptans or oligoethylene glycol pendant groups. The copolymerization of the 3-mercaptovalerolactones with ε-caprolactone generates random copolymers.

PEGylated polymers for medicine: from conjugation to self-assembled systems

Synthetic polymers have transformed society in many areas of science and technology, including recent breakthroughs in medicine. Synthetic polymers now offer unique and versatile platforms for drug delivery, as they can be "bio-tailored" for applications as implants, medical devices, and injectable polymer-drug conjugates. However, while several currently used therapeutic proteins and small molecule drugs have benefited from synthetic polymers, the full potential of polymer-based drug delivery platforms has not yet been realized. This review examines both general advantages and specific cases of synthetic polymers in drug delivery, focusing on PEGylation in the context of polymer architecture, self-assembly, and conjugation techniques that show considerable effectiveness and/or potential in therapeutics.

Microfluidic etching and oxime-based tailoring of biodegradable polyketoesters

A straightforward, flexible, and inexpensive method to etch biodegradable poly(1,2,6-hexanetriol alpha-ketoglutarate) films is reported. Microfluidic delivery of the etchant, a solution of NaOH, can create micron-scale channels through local hydrolysis of the polyester film. In addition, the presence of a ketone in the repeat unit allows for prior or post chemoselective modifications, enabling the design of functionalized microchannels. Delivery of oxyamine tethered ligands react with ketone groups on the polyketoester to generate covalent oxime linkages. By thermally sealing an etched film to a second flat surface, poly(1,2,6-hexanetriol alpha-ketoglutarate) can be used to create biodegradable microfluidic devices. In order to determine the versatility of the microfluidic etch technique, poly(epsilon-caprolactone) was etched with acetone. This strategy provides a facile method for the direct patterning of biodegradable materials, both through etching and chemoselective ligand immobilization.

Cross-linked and functionalized polyester materials constructed using ketoxime ether linkages

A modular and simple approach to the graft functionalization and cross-linking of ketone-containing poly(ε-caprolactone)s has been investigated for the preparation of novel gel and nanoparticulate materials. Poly(ε-caprolactone--2-oxepane-1,5-dione) (P(CL--OPD)), was grafted by reaction with 1-aminooxydodecane and cross-linked by reaction with 1,6-bis(aminooxy)hexane, each at room temperature in tetrahydrofuran at 1 and 10 wt% polymer in the absence of an acid catalyst, and at 1, 5 and 10 wt% polymer in the presence of -toluenesulfonic acid. The grafting process served as a model system for the cross-linking reactions, affording products that were fully characterizable and retained the physical properties of (P(CL--OPD)), with a slight increase in measured molecular weight and characteristic spectroscopic signatures for the dodecyl chains and the newly introduced ketoxime functionalities. Early stages of the intermolecular cross-linking were followed by gel permeation chromatography and atomic force microscopy. Ultimately, insoluble, but tetrahydrofuran-swollen gelled products were obtained, which were characterized by infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. These materials exhibited interesting melting transition profiles, undergoing melting at lower temperatures and broader temperature ranges than observed for their polymer precursors. This study represents an advance in the development of rapid and efficient chemistry for the preparation of functional and robust hydrolytically-degradable polymer materials with degradable linkages.

Synthesis of functionalized biodegradable polyesters

This tutorial review summarizes recent developments in the syntheses of functionalized aliphatic polyesters. These polymers are attracting attention as sustainable alternatives to petrochemicals and for applications in medicine. Two main syntheses are described: step polymerization using mild chemo/enzymatic catalysis and ring opening polymerization, which is usually initiated by metal complexes. The methods are compared and their utility illustrated with reference to interesting new materials.

Coupling Hydrophilic Amine-Containing Molecules to the Backbone of Poly(ε-Caprolactone)

A poly(ϵ-caprolactone) (PCL) based biodegradable polymer containing robust, amine-reactive side chains has been successfully synthesized. The specific reactivity of the side chains allows for the coupling of unmodified amine-containing molecules such as poly(l-lysine) (PLL) to PCL to occur in the presence of other unprotected functional groups. The reactivity of this polymer has been demonstrated through successful coupling of both benzylamine (a model compound) and PLL. This novel amine-reactive polymer could have numerous applications in biomedical fields such as tissue engineering and drug delivery.