Characteristics of Step-Growth Polymerization

It might be appropriate at this stage to summarize the main features of the step reactions. 

The monomer is almost all incorporated in a chain molecule in the early stages of the reaction, i.e., about 1% of monomer remains unreacted when x = 10. Hence, polymer yield is independent of the reaction time in the 

Initiation, propagation, and termination reaetions are essentially identical in rate and mechanism. 

The chain length increases steadily as the reaction proceeds. 

Long reaction times and high convCTsions are necessary for the production of a polymer with large x . 

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The main characteristics of step-growth polymerizations are well known and can be summarized as ... 

This is an unique characteristic of step-growth polymerizations and contrasts with chain-growth polymerizations (Chapter 9) that add only monomers (1-mers) to growing chains (Eq. 8.2Z)). 

The second general pattern of polymerization is step-growth or condensation polymerization. While terminal alkenes are the most common monomers in chain-growth polymerization, bifunctional molecules are the characteristic monomers for step-growth polymerization. The polyester-, polyamide-, and polyurethane-forming reactions shown below are examples of step-growth polymerizations ... 

The Stille reaction-based polymerization, by its polycondensation nature, falls into the major category of step-growth polymerization. Thus, characteristics concerning step-growth polymerization still exist and related fundamental principles apply, such as reaction kinetics, molecular-weight distribution and control, and end-group modifications. We will briefly discuss the latter two in the context of Stille polymerizations. 

There are two characteristic classes of polymerization process, those of step growth and of chain growth.73 In the former, monomers combine to form dimers, and dimers to form tetramers and all oligomers of intermediate size combine to form larger molecules at random. In this kind of process, typical of classical, equilibrium-condensation polymerization, products of high molecular weight are obtained at very high conversion only. 

As mentioned before, the main characteristic of the step-growth polymerization is that it proceeds stepwise, according to the reactivity of the two functionalities involved in the formation of the new linkage. The average... 

The mechanism for bacterial synthesis of PHA is not the simple dehydration reaction between alcohol and carboxyl groups. It is more complicated and involves the coenzyme A thioester derivative of the hydroxyalkanoic acid monomer (produced from the organic feedstock available to the bacteria) [Kamachi et al., 2001]. Growth involves an acyl transfer reaction catalyzed by the enzyme PHA synthase (also called a polymerase) [Blei and Odian, 2000]. The reaction is not a step polymerization but is a chain polymerization with the characteristics of a living polymerization [Su et al., 2000] (Secs. 3-15, 5-2g, Chap. 7). 

Despite these recent advances, step-growth polymerization does not offer many possibilities for the synthesis of complex star architectures, and the molecular characteristics cannot be controlled to the same degree as in the case of the living polymerization methods. 

The polymers are prepared from disubstitued dichlorosilanes by reacting them with alkali metal dispersions in a reductive coupling process. The polymerizations appear to have the characteristics of chain-growth rather than step-growth reactions [170] ... 

The main characteristic of chain-growth reactions is that the polymerization takes place in three distinct steps chain initiation, chain propagation, and chain termination. Macromolecules with high molar masses are formed at low conversion. Small molecules, such as H2O, are not eliminated in this process. Side reactions may occur, e.g. chain transfer to polymers, which result in branched macromolecules. 

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