About: Copolymer is a(n) research topic. Over the lifetime, 84093 publication(s) have been published within this topic receiving 1296738 citation(s).
TL;DR: Block copolymers are macromolecules composed of sequences, or blocks, of chemically distinct repeat units that make possible the sequential addition of monomers to various carbanion-ter minated ("living") linear polymer chains.
Abstract: Block copolymers are macromolecules composed of sequences, or blocks, of chemically distinct repeat units. The development of this field originated with the discovery of termination-free anionic polymerization, which made possible the sequential addition of monomers to various carbanion-ter minated ("living") linear polymer chains. Polymerization of just two dis tinct monomer types (e.g. styrene and isoprene) leads to a class of materials referred to as AB block copolymers. Within this class, a variety of molec ular architectures is possible. For example, the simplest combination, obtained by the two-step anionic polymerization of A and B monomers, is an (A-B) dioblock copolymer. A three-step reaction provides for the preparation of (ABA) or (BAB) triblock copolymer. Alternatively, "living" diblock copolymers can be reacted with an n-functional coupling agent to produce (A-B)n star-block copolymers, where n = 2 constitutes a triblock copolymer. Several representative (A-B)n block copolymer architectures
Abstract: Current studies on novel, metallocenebased catalysts for the polymerization of α-olefins have far-reaching implications for the development of new materials as well as for the understanding of basic reaction mechanisms responsible for the growth of a polymer chain at a catalyst center and the control of its stereoregularity. In contrast to heterogeneous Ziegler–Natta catalysts, polymerization by a homogeneous, metallocene-based catalyst occurs principally at a single type of metal center with a defined coordination environment. This makes it possible to correlate metallocene structures with polymer properties such as molecular weight, stereochemical microstructure, crystallization behavior, and mechanical properties. Homogeneous catalyst systems now afford efficient control of regio- and stereoregularities, molecular weights and molecular weight distributions, and comonomer incorporation. By providing a means for the homo- and copolymerization of cyclic olefins, the cyclopolymerization of dienes, and access even to functionalized polyolefins, these catalysts greatly expand the range and versatility of technically feasible types of polyolefin materials. For corrigendum see DOI:10.1002/anie.199513681
Gérard Riess1•Institutions (1)
Abstract: This article deals with recent progress including the author’s work in the field of block copolymer self-assembly in solution and on solid surfaces. The synthesis methods for producing block copolymers with well-defined structures, molecular weights and composition are outlined with emphasis on ionic and controlled free radical polymerization techniques. A general overview of the preparation, characterization and theories of block copolymer micellar systems is presented. Selected examples of micelle formation in aqueous and organic medium are given for di- and triblock copolymers, as well as for block copolymers with more complex architectures. Current and potential application possibilities of block copolymer colloidal assemblies as stabilizers, flocculants, nanoreservoir in, among others, controlled delivery of bioactive agents, catalysis, latex agglomeration and stabilization of non-aqueous emulsion are also discussed. q 2003 Elsevier Science Ltd. All rights reserved.
Abstract: The association properties of poly(ethylene oxide)-block-poly(propyleneoxide)-block-poly(ethylene oxide) (PEOPPOPEO) copolymers (commercially available as Poloxamers and Pluronics) in aqueous solutions, and the adsorption of these copolymers at interfaces are reviewed. At low temperatures and/or concentrations the PEOPPOPEO copolymers exist in solution as individual coils (unimers). Thermodynamically stable micelles are formed with increasing copolymer concentration and/or solution temperature, as revealed by surface tension, light scattering, and dye solubilization experiments. The unimer-to-micelle transition is not sharp, but spans a concentration decade or 10 K. The critical micellization concentration (CMC) and temperature (CMT) decrease with an increase in the copolymer PPO content or molecular weight. The dependence of CMC on temperature, together with differential scanning calorimetry experiments, indicates that the micellization process of PEOPPOPEO copolymers in water is endothermic and driven by a decrease in the polarity of ethylene oxide (EO) and propylene oxide (PO) segments as the temperature increases, and by the entropy gain in water when unimers aggregate to form micelles (hydrophobic effect). The free energy and enthalpy of micellization can be correlated to the total number of EO and PO segments in the copolymer and its molecular weight. The micelles have hydrodynamic radii of approximately 10 nm and aggregation numbers in the order of 50. The aggregation number is thought to be independent of the copolymer concentration and to increase with temperature. Phenomenological and mean-field lattice models for the formation of micelles can describe qualitatively the trends observed experimentally. In addition, the lattice models can provide information on the distribution of the EO and PO segments in the micelle. The PEOPPOPEO copolymers adsorb on both airwater and solidwater interfaces; the PPO block is located at the interface while the PEO block extends into the solution, when copolymers are adsorbed at hydrophobic interfaces. Gels are formed by certain PEOPPOPEO block copolymers at high concentrations, with the micelles remaining apparently intact in the form of a “crystal”. The gelation onset temperature and the thermal stability range of the gel increase with increasing PEO block length. A comparison of PEOPPO copolymers with PEOPBO and PEO PS block copolymers and CiEj surfactants is made, and selected applications of PEOPPOPEO block copolymer solutions (such as solubilization of organics, protection of microorganisms, and biomedical uses of micelles and gels) are presented.