Different polymer architectures achieved by step polymerization

2 Different polymer architectures achieved by step polymerization 

In Section 1.2 it was noted that the linear homopolymer was only one possible chain conformation and it is appropriate to examine the types of different molecular architecmre that have been demonstrated to be formed through simple step-polymerization reactions. 

Cyclizahon may occur in a number of types of polymerization reachon where there is a finite probability that the reactive chain end may react with the other end of the same chain rather than with a different molecule. The result is a polymer with no chain ends and with a set of conformahon-dependent properties different from those of the linear precursor. For 

In addition to the above thermodynamic conditions for ring formation, the kinetics of the reactions must be considered. Thus, for a reaction to take place the two ends of the polymer chain must be in the correct conformation for sufficient time for the new bond to form. The kinetic factor for cyclization is proportional to Rg, so the net effect of the thermodynamic and kinetic factors is that rings are not favoured between n = 8 and k = 11. Suter (1989) has considered the theoretical approaches of Jacobsen and Stockmayer and compared theoretical and experimental values for macrocyclization equilibrium constants. This has also been performed for Monte Carlo as well as rotational-isomeric-state calculations for the statistical conformations of cyclic esters (decamethylene fiimarates and maleates) and agreement with experimental molar cyclization equilibrium constants found (Heath et al, 2000). 

The earlier example of the formation of a poly(urethane-co-urea) showed the change in properties made possible by including a comonomer (in that case a difunctional amine) together with the usual diol for reaction with a di-isocyanate. This can be extended to a wide range of step polymerizations where an additional reactant is added. Examples could be the use of two AB-type monomers (e.g. amino acids) or two AA (e.g. diacids) to react with one BB (a diamine) to form co-polyamides. Seveml features of step polymerization help in understanding the resultant copolymer. For example, since high-molar-mass polymer is formed only late in the reaction, the composition of the copolymer will be that of the feed ratio of the monomers. 

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