Aliphatic Polyesters: Synthesis, Properties and Applications

Tetrazine-norbornene click reactions to functionalize degradable polymers derived from lactide

Post-polymerization modification of polymers derived from sustainable resources using the click reaction between tetrazines and norbornenes is shown to provide a mild and efficient route for the synthesis of functional degradable polymers. Norbornene chain-end functional poly(lactide) was synthesized using organocatalytic methods and functionalized by the addition of 3,6-di-2-pyridyl-1,2,4,5-tetrazine without degradation of the polymer backbone. The versatility of this reaction was demonstrated by the application of analogues bearing amine and poly(ethylene oxide) groups to realize amine-functional polymers and block copolymers. Poly(spiro[6-methyl-1,4-dioxane-2,5-dione-3,2'-bicyclo[2.2.1]hept[5]ene]) was prepared from lactide. The pendant norbornene group on the backbone of the resultant polymer was modified in a similar manner to produce functional degradable polymers and graft co-polymers.

Synthesis and biomedical applications of functional poly(α-hydroxy acids) via ring-opening polymerization of O-carboxyanhydrides

Poly(α-hydroxy acids) (PAHAs) are a class of biodegradable and biocompatible polymers that are widely used in numerous applications. One drawback of these conventional polymers, however, is their lack of side-chain functionalities, which makes it difficult to conjugate active moieties to PAHA or to fine-tune the physical and chemical properties of PAHA-derived materials through side-chain modifications. Thus, extensive efforts have been devoted to the development of methodology that allows facile preparation of PAHAs with controlled molecular weights and a variety of functionalities for widespread utilities. However, it is highly challenging to introduce functional groups into conventional PAHAs derived from ring-opening polymerization (ROP) of lactides and glycolides to yield functional PAHAs with favorable properties, such as tunable hydrophilicity/hydrophobicity, facile postpolymerization modification, and well-defined physicochemical properties. Amino acids are excellent resources for functional polymers because of their low cost, availability, and structural as well as stereochemical diversity. Nevertheless, the synthesis of functional PAHAs using amino acids as building blocks has been rarely reported because of the difficulty of preparing large-scale monomers and poor yields during the synthesis. The synthesis of functionalized PAHAs from O-carboxyanhydrides (OCAs), a class of five-membered cyclic anhydrides derived from amino acids, has proven to be one of the most promising strategies and has thus attracted tremendous interest recently. In this Account, we highlight the recent progress in our group on the synthesis of functional PAHAs via ROP of OCAs and their self-assembly and biomedical applications. New synthetic methodologies that allow the facile preparation of PAHAs with controlled molecular weights and various functionalities through ROP of OCAs are reviewed and evaluated. The in vivo stability, side-chain functionalities, and/or trigger responsiveness of several functional PAHAs are evaluated. Their biomedical applications in drug and gene delivery are also discussed. The ready availability of starting materials from renewable resources and the facile postmodification strategies such as azide-alkyne cycloaddition and the thiol-yne "click" reaction have enabled the production of a multitude of PAHAs with controlled molecular weights, narrow polydispersity, high terminal group fidelities, and structural diversities that are amenable for self-assembly and bioapplications. We anticipate that this new generation of PAHAs and their self-assembled nanosystems as biomaterials will open up exciting new opportunities and have widespread utilities for biological applications.

Rapid Synthesis of a Lipocationic Polyester Library via Ring-Opening Polymerization of Functional Valerolactones for Efficacious siRNA Delivery

The ability to control chemical functionality is an exciting feature of modern polymer science that enables precise design of drug delivery systems. Ring-opening polymerization of functional monomers has emerged as a versatile method to prepare clinically translatable degradable polyesters.1 A variety of functional groups have been introduced into lactones; however, the direct polymerization of tertiary amine functionalized cyclic esters has remained elusive. We report a strategy that enabled the rapid synthesis of >130 lipocationic polyesters directly from functional monomers without protecting groups. These polymers are highly effective for siRNA delivery at low doses in vitro and in vivo.

Novel synthesis and characterization of a collagen-based biopolymer initiated by hydroxyapatite nanoparticles

In this study, we developed a novel synthesis method to create a complex collagen-based biopolymer that promises to possess the necessary material properties for a bone graft substitute. The synthesis was carried out in several steps. In the first step, a ring-opening polymerization reaction initiated by hydroxyapatite nanoparticles was used to polymerize d,l-lactide and glycolide monomers to form poly(lactide-co-glycolide) co-polymer. In the second step, the polymerization product was coupled with succinic anhydride, and subsequently was reacted with N-hydroxysuccinimide in the presence of dicyclohexylcarbodiimide as the cross-linking agent, in order to activate the co-polymer for collagen attachment. In the third and final step, the activated co-polymer was attached to calf skin collagen type I, in hydrochloric acid/phosphate buffer solution and the precipitated co-polymer with attached collagen was isolated. The synthesis was monitored by proton nuclear magnetic resonance, infrared and Raman spectroscopies, and the products after each step were characterized by thermal and mechanical analysis. Calculations of the relative amounts of the various components, coupled with initial dynamic mechanical analysis testing of the resulting biopolymer, afforded a preliminary assessment of the structure of the complex biomaterial formed by this novel polymerization process.

Polymeric formulations for drug release prepared by hot melt extrusion: application and characterization

Over the past few decades hot melt extrusion (HME) has emerged as a powerful processing technology for the production of pharmaceutical solid dosage forms in which an active pharmaceutical ingredient (API) is dispersed into polymer matrices. It has been shown that formulations using HME can provide time-controlled, sustained and targeted drug delivery, and improved bioavailability of poorly soluble drugs. In this review, the basic principles of the HME process are described together with an overview of some of the most common biodegradable and nonbiodegradable polymers used for the preparation of different formulations using this method. Further, the applications of HME in drug delivery and analytical techniques employed to characterize HME products are addressed.

Cell microencapsulation with synthetic polymers

The encapsulation of cells into polymeric microspheres or microcapsules has permitted the transplantation of cells into human and animal subjects without the need for immunosuppressants. Cell-based therapies use donor cells to provide sustained release of a therapeutic product, such as insulin, and have shown promise in treating a variety of diseases. Immunoisolation of these cells via microencapsulation is a hotly investigated field, and the preferred material of choice has been alginate, a natural polymer derived from seaweed due to its gelling conditions. Although many natural polymers tend to gel in conditions favorable to mammalian cell encapsulation, there remain challenges such as batch to batch variability and residual components from the original source that can lead to an immune response when implanted into a recipient. Synthetic materials have the potential to avoid these issues; however, historically they have required harsh polymerization conditions that are not favorable to mammalian cells. As research into microencapsulation grows, more investigators are exploring methods to microencapsulate cells into synthetic polymers. This review describes a variety of synthetic polymers used to microencapsulate cells.

Ring-opening polymerization of prodrugs: a versatile approach to prepare well-defined drug-loaded nanoparticles

The synthesis of polymer-drug conjugates from prodrug monomers consisting of a cyclic polymerizable group that is appended to a drug through a cleavable linker is achieved by organocatalyzed ring-opening polymerization. The monomers polymerize into well-defined polymer prodrugs that are designed to self-assemble into nanoparticles and release the drug in response to a physiologically relevant stimulus. This method is compatible with structurally diverse drugs and allows different drugs to be copolymerized with quantitative conversion of the monomers. The drug loading can be controlled by adjusting the monomer(s)/initiator feed ratio and drug release can be encoded into the polymer by the choice of linker. Initiating these monomers from a poly(ethylene glycol) macroinitiator results in amphiphilic diblock copolymers that spontaneously self-assemble into micelles with a long plasma circulation, which is useful for systemic therapy.

Fabrication of nanostructured and microstructured chitin materials through gelation with suitable dispersion media

Regeneration from chitin gels with suitable dispersion media results in the efficient fabrication of nano- and microstructured materials.

Polymerization and degradation of aliphatic polyesters synthesized by atom transfer radical polyaddition

By tuning leaving group affinity, the aliphatic polyesters synthesized by ATRPA can avoid lactonization and obtain high molecular weights.

Chitosan-coated electrospun nanofibers with antibacterial activity

Charged poly(ε-caprolactone)-based nanofibers electrospun in the presence of a methacrylic random or block copolymer were layer-coated with chitosan providing efficient bactericidal membranes.

Recent developments in micellar drug carriers featuring substituted poly(ε-caprolactone)s

Synthetic modification of caprolactone monomers and polymers provides a route to self-assembling block copolymers for use in drug carrier applications.

Entropically-driven ring-opening metathesis polymerization (ED-ROMP) of macrocyclic olefins prepared from deoxycholic acid to give functionalized polymers

The macrocycles shown were prepared, then polymerized and copolymerized by ROMP. This gave polymers with free OH or free CO₂R groups. Treatment of the polymers having R =t-Bu with trifluoroacetic acid gave polymers with free CO₂H groups.

Environmentally benign synthesis of saturated and unsaturated aliphatic polyesters via enzymatic polymerization of biobased monomers derived from renewable resources

Biobased saturated aliphatic polyesters and photo-curable unsaturated aliphatic polyesters are enzymatically polymerized, and their structure–property relationships are systematically studied.

hMSCs bridging across micro-patterned grooves

hMSCs spanned across a groove with 100 μm width.

Polyester-based (bio)degradable polymers as environmentally friendly materials for sustainable development

This review focuses on the polyesters such as polylactide and polyhydroxyalkonoates, as well as polyamides produced from renewable resources, which are currently among the most promising (bio)degradable polymers. Synthetic pathways, favourable properties and utilisation (most important applications) of these attractive polymer families are outlined. Environmental impact and in particular (bio)degradation of aliphatic polyesters, polyamides and related copolymer structures are described in view of the potential applications in various fields.

Polysaccharide-coated PCL nanofibers for wound dressing applications

Polysaccharide-based nanofibers with a multilayered structure are prepared by combining electrospinning (ESP) and layer-by-layer (LBL) deposition techniques. Charged nanofibers are firstly prepared by electrospinning poly(ε-caprolactone) (PCL) with a block-copolymer bearing carboxylic acid functions. After deprotonation of the acid groups, the layer-by-layer deposition of polyelectrolyte polysaccharides, notably chitosan and hyaluronic acid, is used to coat the electrospun fibers. A multilayered structure is achieved by alternating the deposition of the positively charged chitosan with the deposition of a negatively charged polyelectrolyte. The construction of this multilayered structure is followed by Zeta potential measurements, and confirmed by observation of hollow nanofibers resulting from the dissolution of the PCL core in a selective solvent. These novel polysaccharide-coated PCL fiber mats remarkably combine the mechanical resistance typical of the core material (PCL)-particularly in the hydrated state-with the surface properties of chitosan. The control of the nanofiber structure offered by the electrospinning technology, makes the developed process very promising to precisely design biomaterials for tissue engineering. Preliminary cell culture tests corroborate the potential use of such system in wound healing applications.

Organocatalytic ring-opening polymerization of morpholinones: new strategies to functionalized polyesters

The oxidative lactonization of N-substituted diethanolamines with the Pd catalyst [LPd(OAc)]2(2+)[OTf(-)]2 generates N-substituted morpholin-2-ones. The organocatalytic ring-opening polymerization of N-acyl morpholin-2-ones occurs readily to generate functionalized poly(aminoesters) with N-acylated amines in the polyester backbone. The thermodynamics of the ring-opening polymerization depends sensitively on the hybridization of the nitrogen of the heterocyclic lactone. N-Acyl morpholin-2-ones polymerize readily to generate polymorpholinones, but the N-aryl or N-alkyl substituted morpholin-2-ones do not polymerize. Experimental and theoretical studies reveal that the thermodynamics of ring opening correlates to the degree of pyramidalization of the endocyclic N-atom. Deprotection of the poly(N-Boc-morpholin-2-one) yields a water-soluble, cationic polymorpholinone.

Influence of a resorcin[4]arene core structure on the spatial directionality of multi-arm poly(ε-caprolactone)s

The spatial directionality of polymer chains in multi-arm polymers can be used to manipulate their thermal and physical properties. Synthesis of directional poly(ε-caprolactone), based on a rigid and flexible resorcin[4]arene initiator core, was accomplished via ring-opening polymerization catalyzed by Sn(Oct)₂ in bulk at 120 °C.

Tunable thermo-responsive P(NIPAAm-co-DMAAm)-b-PLLA-b-P(NIPAAm-co-DMAAm) triblock copolymer micelles as drug carriers

Thermo-responsive P(NIPAAm-co-DMAAm)-b-PLLA-b-P(NIPAAm-co-DMAAm) triblock copolymers are synthesized via combination of ring-opening polymerization and atom transfer radical polymerization.

Synthesis and biomedical applications of functional poly(α-hydroxyl acid)s

This review highlights the recent progress in the synthesis and biomedical applications of poly(α-hydroxyl acid)s with pendent functional groups.

Evaluation of a mPEG-polyester-based hydrogel as cell carrier for chondrocytes

Temperature-sensitive hydrogels are attractive alternatives to porous cell-seeded scaffolds and is minimally invasive through simple injection and in situ gelling. In this study, we compared the performance of two types of temperature-sensitive hydrogels on chondrocytes encapsulation for the use of tissue engineering of cartilage. The two hydrogels are composed of methoxy poly(ethylene glycol)- poly(lactic-co-valerolactone) (mPEG-PVLA), and methoxy poly(ethylene glycol)-poly(lactic- co-glycolide) (mPEG-PLGA). Osmolarity and pH were optimized through the manipulation of polymer concentration and dispersion medium. Chondrocytes proliferation in mPEG-PVLA hydrogels was observed as well as accumulation of GAGs and collagen. On the other hand, chondrocytes encapsulated in mPEG-PLGA hydrogels showed low viability and chondrogenesis. Also, mPEG-PVLA hydrogel, which is more hydrophobic, retained physical integrity after 14 days while mPEG-PLGA hydrogel underwent full degradation due to faster hydrolysis rate and more pronounced acidic self-catalyzed degradation. The mPEG-PVLA hydrogel can be furthered tuned by manipulation of molecular weights to obtain hydrogels with different swelling and degradation characteristics, which may be useful as producing a selection of hydrogels compatible with different cell types. Taken together, these results demonstrate that mPEG-PVLA hydrogels are promising to serve as three-dimensional cell carriers for chondrocytes and potentially applicable in cartilage tissue engineering.

Structural and kinetic studies of the polymerization reactions of ε-caprolactone catalyzed by (pyrazol-1-ylmethyl)pyridine Cu(II) and Zn(II) complexes

The structural and kinetic studies of polymerization reactions of ε-caprolactone (ε-CL) using (pyrazolylmethyl)pyridine Cu(II) and Zn(II) complexes as initiators is described. Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2) with Zn(Ac)2·2H2O or Cu(Ac)2·2H2O produced the corresponding complexes [Zn(Ac)2(L1)] (1), [Cu(Ac)2(L1)] (2), [Zn(Ac)2(L2)] (3) and [Cu2(Ac)4(L2)2] (4) respectively. Solid state structures of 1 and 4 confirmed that complexes 1 and 4 are monomeric and dimeric respectively and that L1 is bidentate in 1 while L2 is monodentate in 4. X-band EPR spectra of 2 and 4 revealed that complex 2 is monomeric both in solid and solution state, while the paddle-wheel structure of 4 is retained in solution. Complexes 1-4 formed active initiators in the ring opening polymerization of ε-CL. Zn(II) complexes 1 and 3 exhibited higher rate constants of 0.044 h(-1) and 0.096 h(-1) respectively compared to rate constants of 0.017 h(-1) and 0.031 h(-1) observed for the corresponding Cu(II) complexes 2 and 4 respectively at 110 °C. All the polymerization reactions follow pseudo first-order kinetic with respect to ε-CL monomer. Initiator 1 showed first-order dependency on the polymerization reactions and utilizes only one active site as the initiating group. The molecular weights of the polymers produced range from 1982 g mol(-1) to 14,568 g mol(-1) and exhibited relatively broad molecular weight distributions associated with transesterification reactions.

Force interactions of nonagglomerating polylactide particles obtained through covalent surface grafting with hydrophilic polymers

Nonagglomerating polylactide (PLA) particles with various interaction forces were designed by covalent photografting. PLA particles were surface grafted with hydrophilic poly(acrylic acid) (PAA) or poly(acrylamide) (PAAm), and force interactions were determined using colloidal probe atomic force microscopy. Long-range repulsive interactions were detected in the hydrophilic/hydrophilic systems and in the hydrophobic/hydrophilic PLA/PLA-g-PAAm system. In contrast, attractive interactions were observed in the hydrophobic PLA/PLA and in the hydrophobic/hydrophilic PLA/PLA-g-PAA systems. AFM was also used in the tapping mode to determine the surface roughness of both neat and surface-grafted PLA film substrates. The imaging was performed in the dry state as well as in salt solutions of different concentrations. Differences in surface roughness were identified as conformational changes induced by the altered Debye screening length. To understand the origin of the repulsive force, the AFM force profiles were compared to the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory and the Alexander de Gennes (AdG) model. The steric repulsion provided by the different grafted hydrophilic polymers is a useful tool to inhibit agglomeration of polymeric particles. This is a key aspect in many applications of polymer particles, for example in drug delivery.