Helical growth/polymerization

The authors discuss the various mechanisms which can lead to a constant value of R in (52). They conclude 1. There are many reasonable models giving equations with constant R in the case of catalyst-controlled polymerization mechanisms. 2. Under certain conditions, an end-controlled mechanism also leads to constant R. 3. Helical growth can be included in some of the models without changing the mathematical expressions. It should be noted that control by helical growth is kinetically a special case of end-control. 

Fig. 2 Growth mechanisms. Schematic theoretical variation of DP (or length) with unimer concentration for linear supramolecular polymerization according to (a) multistage open association (b) helical growth (c) growth-coupled-to-orientation. C and C are eritieal eoneentrations coiresponding to (b) and (c). Occurrenee of liquid-eiystalline phases (N. H, L) is indicated. From Ref. [1]. Copyright 2002 Wiley-VCH.

The theory of reversible helical polymerization of proteins has been fully described by Oosawa and Asakura (1975). The following equation describes polymer growth ... 

A method for obtaining optically active polyiminomethylenes from achiral monomers was recently devised by Nolte, Drenth and co-workers (420). It consists in the copolymerization of an achiral monomer (e.g., phenyl isocyanide) with an optically active isocyanide endowed with a low tendency to polymerize. The chiral monomer is incorporated in one of the two helices and, due to its low reactivity, stops or slows down its growth. The other helix is unaffected by this phenomenon and continues to grow, permitting the almost complete conversion of the achiral monomer into an optically active polymer. 

At high concentrations, the strands aggregate into large polymeric entities, initially via filament formation, followed by lateral, tree like growth. Figure 10.82a -c shows electron microscope images of the various mixtures under these conditions. Note, especially, the opposite handedness of the L- and D-triple helices (right- and left-handed helices, respectively). 

It was demonstrated that a stereocenter positioned far away from the reactive isocyanide group as in monomer 82b (Chart 14) can still induce chirality in the main chain of polyisocyanides, resulting in the formation of an excess of one particular helix.224-226 Kinetic control over the helix sense in the polymerization of 82b was confirmed by the noncooperative transfer of chirality from the monomer to the macromolecule e.g., a linear relation was found between the ee present in the isocyanide and the optical activity in the polymers formed.227 The kinetic inhibition of the growth of one particular handedness using (5)-2-isocyanovaleric acid184 (vide supra) is used to force 82b and 82c into a macromolecular helicity with a screw sense opposite that of the one preferred... 

Optically active polymers can be diff erentiated into systems with stereogenic centers within the polymer chain (generated from prostereogenic centers of the monomer, e.g., via addition) and those w ith preformed stereogenic centers in a side chain. Furthermore, chiral helical structures of the chain can be induced during or after chain growth. Accordingly, various types of asymmetric polymerization can be differentiated ... 

Recently, the greatest growth has been achieved in the structmed determination of natural polysaccharides, and hence this has increased our understanding in relation to structural features as well as functionahties of polysaccharides [1]. The basic knowledge of the structural feature of polysaccharides is essential toward the application as fundamental nanomaterials. For example. X-ray diffraction patterns of various natural polysaccharides have revealed that some of them adopt well-defined helical nanoarchitectures such as polynucleotides, which have never been produced through an artificial polymerization reaction, encouraging us to pursue the possibiUties of natural nano tubes. 

The fourth dassiiication scheme has been used by Cifetri, differentiating SPs on the basis of the physical stmcture assumed through polymerization. Example classes indude linear chains, helical chains, columnar assemblies, micdlar assemblies, planar assemblies, composite assemblies, and three-dimensional assemblies. Ciferri induded a breakdown and description of the linear and hdical chain assemblies as being controlled by different growth mechanisms however, these types are ultimatdy dassified by resulting stmcture. 

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