The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.

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A protected catechol-containing epoxide monomer, catechol acetonide glycidyl ether (CAGE), is introduced. CAGE is conveniently obtained in three steps and enables the incorporation of surface-active catechol moieties into a broad variety of hydrophilic and biocompatible polyether architectures by copolymerization. Via acidic cleavage of the acetal protecting groups, the polymer-attached catechol functionalities are liberated and available for surface attachment or metal complexation. CAGE has been copolymerized with ethylene oxide and glycidol to obtain both linear poly(ethylene glycol) and hyperbranched polyglycerol copolymers, respectively, with multiple surface-adhesive catechol moieties. The CAGE content in the copolymers was varied from 1 to 16%, and all polymers exhibit moderate polydispersity (linear: M w /M n = 1.05−1.33; hyperbranched: M w /M n = 1.44−1.86). In situ kinetic studies of the simultaneous copolymerization of EO and CAGE via NMR spectroscopy have been performed to determine the microstructure of the linear poly(ethylene oxide-co-catechol acetonide glycidyl ether), P(EO-co-CAGE), copolymers. EO shows slightly higher reactivity than CAGE (r EO = 1.14, r CAGE = 0.88), leading to an almost ideally random copolymerization. Because of the catechol units, the copolymers form pH-induced cross-linked networks through metal−ligand interactions. ABA triblock copolymers of the type PCAGE-b-PEG-b-PCAGE formed highly swellable hydrogels upon addition of FeCl 3 . Furthermore, static water contact angle measurements demonstrate an increase in the hydrophilicity of iron, PTFE, and PVC surfaces after coating with catechol-functional mf-PEGs.

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In this study, injectable hydrogels were prepared by the horseradish peroxidase mediated co‐crosslinking of dextran‐tyramine (Dex‐TA) and hyaluronic acid‐tyramine (HA‐TA) conjugates intended for cartilage tissue engineering. In general the gelation times of 10 wt% polysaccharide solutions are < 20 seconds and their storage moduli can be adjusted by varying the composition between Dex‐TA and HA‐TA. Dex‐TA/HA‐TA (50/50) hydrogels were fully degradable in the presence of hyaluronidase. Chondrocytes incorporated in 10 wt% Dex‐TA and Dex‐TA/HA‐TA (50/50) gels showed good viability after 28 days. These results indicate that Dex‐TA/HA‐TA (50/50) hydrogels are promising injectable and biodegradable hydrogels for cartilage repair.