Sequence-controlled polymers synthesis

J.-F. Lutz, in Sequence-Controlled Polymers Synthesis, Self-Assembly, and Properties, American Chemical Society, 2014, vol. 1170, pp. 1-11. 

Z. Qiang, C. Jennifer, A. Athina, W. Russell, A. M. Daniel, C. R. Becer, W. Paul and M. H. David, in Sequence-Controlled Polymers Synthesis, Self-Assembly, and Properties, American Chemical Society, 2014, vol. 1170, pp. 327-348. 

Some examples of sequence-controlled polymers prepared by ring opening polymerization of cyclic esters have been reported. For instance, Thomas, Coates, and coworkers reported a very original method for the synthesis of alternating aliphatic polyesters.These authors used a stereoselective yttrium-based organometallic catalyst to control comonomer sequences. It had been previously shown that the polymerization of a racemic mixture of p-butyrolactone in the presence of this catalyst led to the formation of... 

Asides from chain-growth, step-growth, and multi-step growth strategies, sequence-defined polymers may also be prepared using polymerization concepts inspired by biological polymerizations such as replication, transcription, and translation. For instance, sequence-defined templates can be used for monomer sequence regulation in non-natural polymerizations. Alternatively, catalytic molecular machines inspired by biocatalysts such as enzymes and ribozymes have been tested for the synthesis of sequence-controlled polymers. These developments are summarized in this last section of the chapter. 

Protein polymers based on Lys-25 were prepared by recombinant DNA (rDNA) technology and bacterial protein expression. The main advantage of this approach is the ability to directly produce high molecular weight polypeptides of exact amino acid sequence with high fidelity as required for this investigation. In contrast to conventional polymer synthesis, protein biosynthesis proceeds with near-absolute control of macromolecular architecture, i.e., size, composition, sequence, topology, and stereochemistry. Biosynthetic polyfa-amino acids) can be considered as model uniform polymers and may possess unique structures and, hence, materials properties, as a consequence of their sequence specificity [11]. Protein biosynthesis affords an opportunity to completely specify the primary structure of the polypeptide repeat and analyze the effect of sequence and structural uniformity on the properties of the protein network. 

The phenomenal growth in commercial production of polymers by anionic polymerization can be attributed to the unprecedented control the process provides over the polymer properties. This control is most extensive in organolithium initiated polymerizations and includes polymer composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even monomer sequence distribution in copolymers. Furthermore, a judicious choice of process conditions affords termination and transfer free polymerization which leads to very efficient methods of block polymer synthesis. 

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

It is clear that polymerization of MMA via the unimolecular mechanism offers the greatest prospect for both control of polymer MWD, commoner sequence distributions, as well as tacticity. As chiral and prochiral group 4 metallocene complexes are more readily available than their group 3 or lanthanide analogs, we can expect continued effort in this area will eventually lead to a wide range of applications to polymer synthesis. 

A versatile, simple and inexpensive method has been recently proposed for the synthesis of sequence-controlled multiblock copolymers by one-pot polymerisation at ambient temperature. Aciylic block copolymerisation under UV irradiation 360 nm) was obtained in the absence of conventional photoredox catalysts and dye-sensitizers, by means of low concentrations of CuBra in synergy with MCe-Tren [MCe-Tren Tren = tris(2-aminoethyl)amine]. The potential of the method was demonstrated by alternating four different aciylate monomers in various combinations within the polymer composition. Quantitative conversion and narrow dispersity were achieved. " The procedure is versatile, as demonstrated by polymerisation of a number of (meth)aciylate monomers, including poly(ethylene glycol) methyl ether aciylate (PEGA480), te/t-butyl aciylate, methyl methaciylate, and styrene. Moreover, hydrojyl- and vic-diol-functional initiators are tolerated, forming a,co-heterofunctional poly(aciylates). Notably, temporal control is... 

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