Polymer growth mechanisms

A comprehensive classification of both linear and crosslinked polymers may be based on the mechanism of the polymerization process. From the point of view of the polymer growth mechanism, two entirely different processes, step and chain polymerization, are distinguishable. 

As a last point, let us consider how copolymerization relates to the polymer growth mechanism. Eirst, most step-growth polymerizations (e.g., the production of nylon 6/6 by the reaction of hexamethylene diamine with adipic acid) use two monomers to produce the final polymer. One can say that these are inherently copolymerizations. Considering... 

The keys of these technologies have been the development of "Ziegler-Natta" catalyst support systems and the control of the polymer growth mechanism. 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

The step-growth polymer nylon 6 is prepared from caprolactam. The reaction involves initial reaction of caprolactam with water to give an intermediate open-chain amino acid, followed by heating to form the polymer. Propose mechanisms for both steps, and show the structure of nylon 6. 

The importance of twinned crystals in demonstrating that nucleation is the relevant growth mechanism has been realized since 1949 [64, 99]6. They were first investigated extensively in polymer crystals by Blundell and Keller [82] and they have recently received increased attention as a means of establishing, or otherwise, the nucleation postulate for lamellar growth [90, 91, 95,100-102]. The diversity of opinion in the literature shows that it is very difficult to draw definite conclusions from the experimental evidence, and the calculations are often founded upon implicit assumptions which may or may not be justified. We therefore restrict our discussion to an introduction to the problem, the complicating features which make any a priori assumptions difficult, and the remaining information which may be fairly confidently deduced. 

Although the mechanisms discussed above are still topics of debate, it is now firmly established that the electrodeposition of conducting polymers proceeds via some kind of nucleation and phase-growth mechanism, akin to the electrodeposition of metals.56,72-74 Both cyclic voltammetry and potential step techniques have been widely used to investigate these processes, and the electrochemical observations have been supported by various types of spectroscopy62,75-78 and microscopy.78-80... 

The generic condensation polymerization begins with monomers AMA and BMB and produces molecules of the forms A A, B B, and A B. Each step of the reaction generates a longer polymer by the step-growth mechanism of Equation (13.2) and produces 1 mol of condensation product AB. 

After the discovery of the photopolymerization of 2,5-DSP crystals, several types of photoproducts were found, not only the linear polymers, but some other derivatives, e.g. the V-shaped dimer or cyclophane (Hasegawa and Hashimoto, 1992). The photopolymerization occurs in a step-growth mechanism by cyclobutane formation between the excited olefin and the olefin in the ground state. 

To explain the formation of non-crosslinked polymers from the diallyl quaternary ammonium system, Butler and Angelo proposed a chain growth mechanism which involved a series of intra- and inter-molecular propagation steps (15). This type of polymerization was subsequently shown to occur in a wide variety of symmetrical diene systems which cyclize to form five or six-membered ring structures. This mode of propagation of a non-conjugated diene with subsequent ring formation was later called cyclopolymerization. 

The choice of one polymerization method over another is defined by the type of monomer and the desired properties of the polymer. Table 2.1 lists advantages and disadvantages of the different chain growth mechanisms. Table 2.2 summarizes some well known addition polymers and the methods by which they can be polymerized. 

The addition of heat shifts the equilibrium concentrations away from the products and back towards the reactants, the monomers. This is one reason why processing these types of polymers is often more difficult than processing products of chain growth mechanisms. The thermal degradation process can be dramatically accelerated by the presence of the low molecular weight condensation products such as water. Polyester, as an example, can depolymerize rapidly if processed in the presence of absorbed or entrained water. 

As can be seen from Figures 2.63(a) to (c), A, and intensity all lag behind the current response. More particularly, A and 4 lag more than the intensity and this has important implications for the growth mechanism since A and 4 are only effected by the film on the electrode surface, whereas the intensity responds to species both in solution and at the surface. In addition, when the growth was carried out for only 0.4 s, the responses of A, 4 and the intensity during the growth were identical to those shown in Figures 2.63(a) to (c). However, on stepping the potential down to 0 V, where consumption of the monomer ceases, no evidence for a polymer film is observed. 

It should be emphasised here that very different growth mechanisms are found according to the conditions of the experiment and that the study of the growth of conducting polymers such as polypyrrole represents a major area of investigation in its own right. The work of Christensen and Hamnett (1991) on the growth of polypyrrole should not, therefore, be taken as representative, it is specific to the conditions they used. 

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

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