One-Pot Synthesis of Polyglycidol-Containing Block Copolymers with Alkyllithium Initiators Using the Phosphazene Base t-BuP4

Polyglycerols as Multi-Functional Platforms: Synthesis and Biomedical Applications

The remarkable and unique characteristics of polyglycerols (PG) have made them an attractive candidate for many applications in the biomedical and pharmaceutical fields. The presence of multiple hydroxy groups on the flexible polyether backbone not only enables the further modification of the PG structure but also makes the polymer highly water-soluble and results in excellent biocompatibility. In this review, the polymerization routes leading to PG with different architectures are discussed. Moreover, we discuss the role of these polymers in different biomedical applications such as drug delivery systems, protein conjugation, and surface modification.

Polyethers Based on Short-Chain Alkyl Glycidyl Ethers: Thermoresponsive and Highly Biocompatible Materials

The polymerization of short-chain alkyl glycidyl ethers (SCAGEs) enables the synthesis of biocompatible polyethers with finely tunable hydrophilicity. Aliphatic polyethers, most prominently poly(ethylene glycol) (PEG), are utilized in manifold biomedical applications due to their excellent biocompatibility and aqueous solubility. By incorporation of short hydrophobic side-chains at linear polyglycerol, control of aqueous solubility and the respective lower critical solution temperature (LCST) in aqueous solution is feasible. Concurrently, the chemically inert character in analogy to PEG is maintained, as no further functional groups are introduced at the polyether structure. Adjustment of the hydrophilicity and the thermoresponsive behavior of the resulting poly(glycidyl ether)s in a broad temperature range is achieved either by the combination of the different SCAGEs or with PEG as a hydrophilic block. Homopolymers of methyl and ethyl glycidyl ether (PGME, PEGE) are soluble in aqueous solution at room temperature. In contrast, n-propyl glycidyl ether and iso-propyl glycidyl ether lead to hydrophobic polyethers. The use of a variety of ring-opening polymerization techniques allows for controlled polymerization, while simultaneously determining the resulting microstructures. Atactic as well as isotactic polymers are accessible by utilization of the respective racemic or enantiomerically pure monomers. Polymer architectures varying from statistical copolymers, di- and triblock structures to star-shaped architectures, in combination with PEG, have been applied in various thermoresponsive hydrogel formulations or polymeric surface coatings for cell sheet engineering. Materials responding to stimuli are of increasing importance for "smart" biomedical systems, making thermoresponsive polyethers with short-alkyl ether side chains promising candidates for future biomaterials.

Organocatalytic Copolymerization of Cyclic Lysine Derivative and ε-Caprolactam toward Antibacterial Nylon-6 Polymers

Functional polymers of nylon-6, particularly those with sustained antibacterial functions, have many practical applications. However, the development of functional ε-caprolactam monomers for the subsequent ring-opening copolymerization (ROCOP) formation of these materials remains a challenge. Here we report a t-BuP₄-mediated ROCOP of dimethyl-protected cyclic lysine with ε-caprolactam, followed by quaternization, affording antibacterial nylon-6 polymers bearing quaternary ammonium functionality with high molecular weight (up to 77.4 kDa). The antibacterial nylon-6 polymers exhibited good physical and mechanical properties and strong antimicrobial activities. At 25 mol % quaternary ammonium group incorporation, the nylon-6 polymer demonstrated complete killing of Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). The results from this study may provide a strategy for the facile preparation of antibacterial nylon-6 polymers to addressing the public health and safety challenges.

Deepening the insight into poly(butylene oxide)-block-poly(glycidol) synthesis and self-assemblies: micelles, worms and vesicles

Aqueous self-assembly of amphiphilic block copolymers is studied extensively for biomedical applications like drug delivery and nanoreactors. The commonly used hydrophilic block poly(ethylene oxide) (PEO), however, suffers from several drawbacks. As a potent alternative, poly(glycidol) (PG) has gained increasing interest, benefiting from its easy synthesis, high biocompatibility and flexibility as well as enhanced functionality compared to PEO. In this study, we present a quick and well-controlled synthesis of poly(butylene oxide)-block-poly(glycidol) (PBO-b-PG) amphiphilic diblock copolymers together with a straight-forward self-assembly protocol. Depending on the hydrophilic mass fraction of the copolymer, nanoscopic micelles, worms and polymersomes were formed as well as microscopic giant unilamellar vesicles. The particles were analysed regarding their size and shape, using dynamic and static light scattering, TEM and Cryo-TEM imaging as well as confocal laser scanning microscopy. We have discovered a strong dependence of the formed morphology on the self-assembly method and show that only solvent exchange leads to the formation of homogenous phases. Thus, a variety of different structures can be obtained from a highly flexible copolymer, justifying a potential use in biomedical applications.

Improved synthesis and well controlled self-assembly of PBO-b-PG amphiphilic diblock copolymers led to homogenous phases of micelles, worms and vesicles.

Polyglycidol, Its Derivatives, and Polyglycidol-Containing Copolymers-Synthesis and Medical Applications

Polyglycidol (or polyglycerol) is a biocompatible polymer with a main chain structure similar to that of poly(ethylene oxide) but with a ⁻CH₂OH reactive side group in every structural unit. The hydroxyl groups in polyglycidol not only increase the hydrophilicity of this polymer but also allow for its modification, leading to polymers with carboxyl, amine, and vinyl groups, as well as to polymers with bonded aliphatic chains, sugar moieties, and covalently immobilized bioactive compounds in particular proteins. The paper describes the current state of knowledge on the synthesis of polyglycidols with various topology (linear, branched, and star-like) and with various molar masses. We provide information on polyglycidol-rich surfaces with protein-repelling properties. We also describe methods for the synthesis of polyglycidol-containing copolymers and the preparation of nano- and microparticles that could be derived from these copolymers. The paper summarizes recent advances in the application of polyglycidol and polyglycidol-containing polymers as drug carriers, reagents for diagnostic systems, and elements of biosensors.

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation

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.