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.

Backbone-thermoresponsive hyperbranched poly-(propylene oxide)-based polyether polyols have been synthesized by anionic ring-opening copolymerization of glycidol and propylene oxide. The number of functional hydroxyl end groups and the lower critical solution temperature (LCST) can be readily adjusted by varying the comonomer ratio. Molecular weights in the range of 1200−2000 g/mol were achieved. Hyperbranched polyether polyols with LCST values between 24 and 83 °C can be obtained in a convenient one-step reaction.

Hyperbranched poly(ethylene glycol) copolymers were synthesized by random anionic ring-opening multibranching copolymerization of ethylene oxide with glycidol as a branching agent, leading to poly(ethylene glycol) structure with glycerol branching points. Extending the available range of molar masses by novel synthesis strategies, a limited extent of control over the degree of polymerization was achieved by variation of the solvent in this copolymerization. Generally, absolute molar mass characterization of hyperbranched polymers still represents an unresolved challenge. A series of the hyperbranched poly(ethylene glycol)-co-(glycerol) copolymers (hbPEGs) of a wide range of molar masses (1400 < M < 1 700 000 g mol −1 ), degree of branching (DB) = 0.04−0.54, and moderate polydispersity (M w /M n ) ≈ 2.1 ± 0.2 were studied, in both water and dimethylformamide by the methods of molecular hydrodynamics. Analytical ultracentrifugation, intrinsic viscosity, translational diffusion measurements, and SEC were combined. Molar masses of hbPEGs were estimated from the comparison of the velocity sedimentation and translational diffusion coefficients, i.e., applying the Svedberg relationship. It was demonstrated that the use of linear PEG for the SEC calibration results in the significantly underestimated values of the molar masses of hbPEGs. The largest hbPEG samples exhibited a hydrodynamic radius of ≈14 nm in aqueous solution. The obtained Kuhn−Mark−Houwink− Sakurada scaling relations show linear trends in all range of molar masses. The detected scaling indexes virtually correspond to the homologous series characterized by a direct proportionality between the molar mass and the volume of the macromolecules that make up this series. The effect of branching on the molecular dimensions and on the hydrodynamic characteristics is discussed, and the corresponding contraction factors have been estimated.

Hyperbranched poly­(butylene oxide) polyols have been synthesized by multibranching anionic ring-opening copolymerization of 1,2-butylene oxide and glycidol. Systematic variation of the composition from 24 to 74% glycidol content resulted in a series of moderately distributed copolymers (Đ = 1.41–1.65, SEC), albeit with limited molecular weights in the solvent-free batch process in the range of 900–1300 g mol–1 (apparent M n determined by SEC with PEG standards). In situ monitoring of the copolymerization kinetics by 1H NMR showed a pronounced compositional drift with respect to the monomer feed, indicating a strongly tapered microstructure caused by the higher reactivity of glycidol. In the case of slow monomer addition considerably higher apparent molecular weights up to 8500 g mol–1 were obtained (SEC). By alteration of the comonomer ratio, aqueous solubility of the hyperbranched copolymers could be tailored, resulting in well-defined cloud points between 20 and 84 °C. Glass transition temperatures between −60 and −29 °C were observed for the resulting polyether polyols. High degrees of branching (DB) between 0.45 and 0.77 were calculated from inverse gated (IG) 13C NMR. Online viscosimetry and analytical ultracentrifugation (AUC) were employed to study hydrodynamic properties and to establish a universal calibration curve for the determination of absolute molecular weights. This resulted in M w values between 2100 and 35 000 g mol–1 that were generally 2–3 times higher than the apparent values determined by SEC with linear PEG standards.

Copolymerization of established epoxide monomers with glycidol (G) is a key reaction to prepare branched or hyperbranched polyethers. The kinetics of the multibranching anionic ring-opening copolymerization of glycidol (a cyclic latent AB2 monomer) with ethylene oxide (EO), propylene oxide (PO), and 1,2-butylene oxide (BO; cyclic latent AB monomers), respectively, in dimethyl sulfoxide was studied. Online 1H NMR spectroscopy was employed for in situ monitoring of the individual monomer consumption during the entire course of the statistical copolymerization. Varying the counterion, both the cesium alkoxide and potassium alkoxide initiated copolymerization were studied and compared. From the individual monomer consumption, reactivity ratios were calculated. The reactivity ratio of the alkylene oxides decreases from 0.44 to 0.11 with increasing alkyl chain length on going from EO to BO. Unexpectedly, glycidol was found to exhibit a higher reactivity ratio in each copolymerization, with reactivity ratios ranging from 2.34 (with EO) to 7.94 (copolymerization with BO). Different counterions had an impact on absolute reaction rates, however, relative monomer reactivities remained unchanged. The reactivity ratios determine both the molecular weight distribution and the topology as well as the degree of branching (DB) of the respective branched copolymers, implying a change from a hyperbranched random copolymer (glycidol/EO) to a multiarm star structure with increasing side chain length of the alkylene comonomer.