Polymerization advanced polymer architecture

As indicated above, conventional free-radical poljnnerization is not very well suited for the synthesis of advanced polymer architectures. Even the synthesis of di- or triblock copolymers is hardly possible using this technique. The main reason is the continuous initiation and termination of chains. This results in the early generation of dead polymer chains that no longer participate in the polymerization process. The consecutive addition of monomers as employed in living anionic polymerization for the synthesis of eg poly(styrene-6Zoc -butadiene) will only lead to the synthesis of a mixture of two homopolymers. 

A wide variety of polymeric membranes with different barrier properties is already available, many of them in various formats and with various dedicated specifications. The ongoing development in the field is very dynamic and focused on further increasing barrier selectivities (if possible at maximum transmembrane fluxes) and/ or improving membrane stability in order to broaden the applicability. This tailoring of membrane performance is done via various routes controlled macro-molecular synthesis (with a focus on functional polymeric architectures), development of advanced polymer blends or mixed-matrix materials, preparation of novel composite membranes and selective surface modification are the most important trends. Advanced functional polymer membranes such as stimuli-responsive [54] or molecularly imprinted polymer (MIP) membranes [55] are examples of the development of another dimension in that field. On that basis, it is expected that polymeric membranes will play a major role in process intensification in many different fields. 

In the second chapter, we try to emphasize the possibilities of producing tailor-made polymers with predicted properties. By using different types of initiators and catalysts, ring-opening polymerization of lactones and lactides provides macromolecules with advanced molecular architecture - a careful selection of appropriate conditions is crucial. The purpose of this chapter is also to describe the mechanisms and typical kinetic features. 

Although the possibility of carrying out catalytic polymerizations in the presence of water had been known since the 1960s, significant advances in catalytic polymerizations in aqueous systems have only been achieved over the past decade. Today, (1) various different types of transition metal-catalyzed polymerizations can be carried out efficiently in aqueous systems. (2) A variety of polymers, ranging from hydrocarbons to water-soluble polymers, and a scope of polymer architectures are accessible. (3) Polymerization can be carried out in a controlled fashion. 

The combination of sohd phase peptide synthesis with polymer chemistry has proven to be a versatile method for the preparation of polymer-peptide hybrids. Introduction of native ligation methods even allows the synthesis of polymer modified polypeptides and proteins via an entire organic chemistry approach. In the field of polymer chemistry—besides the advances in NCA polymerization, which will be discussed by others and is therefore not part of the scope of this review—controlled radical polymerization has been shown to be a robust technique, capable of creating well-defined biofunctional polymer architectures. Through protein engineering, methods have been estabhshed that enable the construction of tailor-made proteins, which can be functionalized with synthetic polymer chains in a highly defined manner. 

A wide variety of chemical catalysts is nowadays available to polymerize monomers into well-defined polymers and polymer architectures that are applicable in advanced materials for example, as biomedical applications and nanotechnology. However, synthetic polymers rarely possess well-defined stereochemistries in their backbones. This sharply contrasts with the polymers made by nature where perfect stereocontrol is the norm. An interesting exception is poly-L-lactide, a polyester that is used in a variety of biomedical applications [1]. By simply playing with the stereochemistry of the backbone, properties ranging from a semicrystalline, high melting polymer (poly-L-lactide) to an amorphous high Tg polymer (poly-meso-lactide) have been achieved [2]. 

This chapter will serve to highlight recent advances in polymer science that have been aided by the use of click chemistry. The copper(l)-catalyzed azide-alkyne cycloaddition (CuAAC) and thiol-ene reactions will be discussed first, after which the utilization of these chemical transformations in the construction and fimction-ahzation of a multitude of different polymeric materials will be outlined. Particular attention will be focused on the preparation of highly complex polymer architectures, such as dendrimers and star polymers, which exempHfy the essential role that chck chemistry has assumed in the polymer science community. 

A significant outcome from the discovery of controlled living polymerization methods is the ability to prepare advanced macromolecular architectures beyond linear homopolymers including copolymers, brush/pendant polymers, branched polymers, and star polymers (Fig. 3.2) [29-36]. 

The last decade has seen significant new advances achieved in the field of living olefin polymerization. Many efficient and selective catalysts are now available for the hving polymerization of ethylene in addition to living and stereoselective polymerization of a-olefins, resulting in the creation of a variety of new polymer architectures, such as block copolymers and end-functionalized macromolecules. The ability to synthesize such polymers... 

Kwon, Y. and Faust, R. (2004) Synthesis of polyisobutylene-based block copolymers with precisely controlled architecture by living cationic polymerization. Advances in Polymer Science, 167,107-135. 

ROP reactions of cyclic metal-containing monomers have been utilized to prepare polymeric materials. This method has led to seminal advances in preparing high MW polymers with ferrocene in the mainchain. Ring-opening methods have led to enhanced control of polymer architecture. As with the other methods of polymerization, the metal can either be part of the cyclic structure or attached to an organic ligand bonded to the cychc system. 

Aliphatic polyesters are an attractive class of polymer that can be used in biomedical and pharmaceutical applications. One reason for the growing interest in this type of degradable polymer is that their physical and chemical properties can be varied over a wide range by, e.g., copolymerization and advanced macro-molecular architecture. The synthesis of novel polymer structures through ringopening polymerization has been studied for a number of years [1-5]. The development of macromolecules with strictly defined structures and properties, aimed at biomedical applications, leads to complex and advanced architecture and a diversification of the hydrolyzable polymers. 

Many recent advances in polymer synthesis have involved the development of new controlled polymerization systems proceeding via a variety of mechanisms. A number of architectures maybe produced as a result of the great versatility of the ROP of cyclic esters. Different strategies have been applied for the design of new polymeric materials. 

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