Random coiled Copolymerization

The nature of the helical conformation of poly(phenylacetylene) has been studied in detail (74). The stabihty of the helical conformation of poly(phenylacetylenes) was estimated by the chiroptical properties of the copolymers from chiral and achiral phenylacetylenes. When the monomer possesses sterically less bulky ring substitnents, a clear cooperative nature on the copolymerization is not observed. A chiral amplification phenomenon is attainable only when the monomers have bulky ring snbstitnents. This result coincides with the poor chiroptical property of poly(42) (73) and also with the very intense CD effects of poly(44) having bulky chiral silyl groups (256). Computational simulations verified that, unlike polyisocyanates which have a long persistence length of helical structure because of their stiff main chain, the main chain of poly(phenylacetylene) is quite fiexible and that, unless bulky substituents are incorporated, poly(phenylacetylene) exists in essentially randomly coiled conformation or in a helical conformation with very short persistence length. 

The comonomer distribution can be alternated by controlling the synthesis conditions, such as the copolymerization at different reaction temperatures at which the thermally sensitive chain backbone has different conformations (extended coil or collapsed globule). In this way, hydrophilic comonomers can be incorporated into the thermally sensitive chain backbone in a more random or more segmented (protein-like) fashion. On the other hand, short segments made of hydrophobic comonomers can be inserted into a hydrophilic chain backbone by micelle polymerization. One of the most convenient ways to control and alternate the degree of amphiphilicity of a copolymer chain, i.e., the solubility difference of different comonomers in a selective solvent, is to use a thermally sensitive polymer as the chain backbone, such as poly(N-isopropylacrylamidc) (PNIPAM) and Poly(N,N-diethylacrylamide) (PDEA). In this way, the incorporation of a hydrophilic or hydrophobic comonomer into a thermally sensitive chain backbone allows us to adjust the degree of amphiphilicity by a temperature variation. 

Triphenylmethyl methacrylate (TrMA) and azobenzene-modified methacrylates were randomly copolymerized in toluene at — 78°C with chiral catalysts to give optically active helical copolymers (19 in Fig. 6) [65]. The optical activity (optical rotation) of the copolymers decreased with the increasing content of the azobenzene-modified methacrylates in the copolymers. The single helical conformation of PTrMA is quite stable in solution, but the copolymers of TrMA with less bulky methacrylates cannot keep their helical structure and lose their optical activity during the polymerization or after the polymerization in solution, which is highly dependent on the bulkiness of the comonomers [22]. The copolymer (19 x = 2) containing 26 mol% azobenzene units, also lost its optical activity upon irradiation within 20 min. This change is due to the helix-to-coil transition of the copolymer and can occur in the dark. 

Typical examples are solvatochromism and thermochromism. The former chromism is observed upon dissolution of the polymers in solvent. This effect is caused by the localization of the electronic wave function as a result of the disorder introduced by the coiled (random) conformation [48]. These chromisms depend largely upon solvent quality. Choice of suitable side-groups on the polythiophene backbone brings about interesting chromism in the solid form as well. Copolymerization supplies further variations in the arrangement of the side-groups on the backbone and, hence, produees specific effects. 

Water-soluble polyampholytes have been synthesized by copolymerization of both anionic and cationic monomers ipto the polymer backbone (2-8) or by incorporating zwitterionic monomer units into the polymer (9-JO). In pure water most polyampholytes having randomly distributed (+) and (-) functional groups, in the 40/60 to 60/40 range of mol ratios are insoluble due to intrapolymer electrostatic attractions (6-8). The solubility and the solution viscosity increase upon adding salt due to brea p of the intrapolymer ion ag egates and expansion of the polymer coil. 

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