Random copolymers, conformation

We have designed PBUILD, a new CHEMLAB module, for easy construction of random copolymers. A library of monomers has been developed from which the chemists can select a particular sequence to generate a polymeric model. PBUILD takes care of all the atom numbering, three dimensional coordinates, and knows about stereochemistry (tacticity) as well as positional isomerism (head to tail versus head to head attachment). The result is a model of the selected polymer (or more likely a polymer fragment) in an all trans conformation, inserted into the CHEMLAB molecular workspace in literally a few minutes. 

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

The use of simultaneous equations with at least three sets of experimental data can, however, be applied to Eq. (101) or (102) to yield M, MA and MB. This is especially useful, if the specific co-ordinates of the whole parabola cannot be realised experimentally as is often the case. The data159-1 in Table 14 relate to a random copolymer of styrene (A)/di-n-butyl itaconate (B) in which WA = 0.169. Solution of simultaneous equations [Eq. (101)] yields M = 67300, MA = 56800 and MB =64700. The compositional heterogeneity is such that the data conform to the required theoretical relationship149 ... 

In a dilute solution, when the polymer is in a coil state (Fig. 6a), the diffusion of hydrophobic particles into the coil is normally faster than the chemical reaction [53]. In this case, the local concentration of particles H inside the coil is practically the same as in the bulk. Therefore, we expect that at the initial stage, the reaction will lead to a random copolymer some of the P monomeric units will attach to H reagent and thereby they will acquire amphiphilic (A) properties P + H —A (Fig. 6b). As long as the number of modified A units is not too large, the chain remains in a swollen coillike conformation (Fig. 6b). However, when this number becomes sufficiently large, the hydrophobically modified polymer segments would tend to form... 

The second question is related to the existence of an additional transition at 29.4 °C. As expected, hydrophobic styrene stickers tend to associate in water. At lower temperatures, water is such a good solvent for PNIPAM that the copolymer chain adopts a random coil conformation. The movements of the stickers are also random and not correlated to each other because the PNIPAM segments randomly fluctuate in solution. As the temperature increases in the range 25-30.6 °C, the solvency of water for PNIPAM gradually decreases and the hydrophobic stickers tend to gather towards the chain center and to move in a more correlated fashion. Since water is not a poor solvent yet, the PNIPAM chain backbone is still in its swollen and coiled state, reflected in the fact that the decrease of (R) in Fig. 24 is only 30%. In order to distinguish such a chain conformation from a normal random coil, it was named an ordered coil . 

Calculations of end-to-end distances of random coil conformations have also been carried out for polypeptide copolymers (Miller et al., 1967). The values of (rzyQlnplz were found to vary markedly with composition and amino acid sequence in the copolymers. For example, the introduction of glycine residues randomly into poly-L-alanine led to a monotonic decrease 6 ... 

Fig. 2. Conformation of AB copolymers in solution (a) Random copolymer (b) diblock copolymer (c) triblock copolymer.

Mesomorphic behavior has been observed in a number of random copolymers consisting of two kinds of units, one rigid and rodlike and the other flexible , in the sense of offering a variety of conformations and, hence, capable of disrupting perpetuation of the axis of the rigid unit. The p-hydroxybenzoate (PHB) unit is prototypal of a rigid unit it has often been incorporated with ethylene terephthaiate (ET) as the flexible unit present as the minor constituent (0.3-0.4 mole fraction). 

Fig.43. Schematic of the conformation of A-r-B random copolymer chains at the interface between A and B homopolymers, a A single chain b at low coverage (low 2) and c at high coverage (high 1)...

Partial crystallization, small crystal sizes, impurity and copolymer content, and straining of molecules from the random coil conformation are the major effects to be considered. The changes of heats of transition and temperatures of transition and their use to characterize the non-equilibrium state have been given an initial critical survey (2) ... 

PolyCACN) has a rigid chain structure yet can form excimers with alternate units along the chain (8), or by stacking in a helical conformation. Excimer formation has been reported for alternate copolymers of ACN with styrene (9) and for ACN with maleic anhydride CIO). The situation is different for 2-vinylnaphthalene since alternating copolymers of 2VN with methyl methacrylate or methacrylic acid did not form excimers, yet random copolymers of the same systems showed excimer fluorescence Cll). Only random copolymers of ACN were prepared in this work. 

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