Step- and Chain-Growth Polymerizations

Our primary purpose in this section is to point out some of the similarities and differences between step-growth and chain-growth polymerizations. In so doing we shall also have the opportunity to indicate some of the different types of chain-growth polymerization systems. 

We encounter homogeneous catalysts in both step-growth and chain-growth polymerization processes. We saw several examples of these types of reactions in Chapter 2. For example, the acid catalyzed polymerization of polyesters occurs via a homogeneous process as do some metallocene catalyzed polymerization of polyolefins. 

A Comparison of Step-Growth and Chain-Growth Polymerizations Step-Growth Chain-Growth... 

Some 50 years ago, Paul Flory chose the terms step-growth and chain-growth polymerization to describe the processes by which many monomers are converted to polymer (Flory 1953). Although not perfect, the terms are still commonly used and can help us understand the major mechanisms of polymerizations. A mechanism for a reaction describes the processes and pathways by which that reaction proceeds. Mechanisms are important because they help us understand the details of a chemical reaction as well as help us predict the outcome of new reactions. 

The polymerization of some monomers does not fall neatly into either of the mechanisms discussed above. We will take up a few of them (e.g., anionic and coordination polymerizations) after we further develop step-growth and chain-growth polymerizations. Some polymerizations can proceed by either mechanism, depending upon the specific monomer or the reaction conditions. The most notable examples, ring-opening polymerization and some of the newer chemistries, are presented as separate categories toward the end of the chapter. 

There are some fundamental differences in the engineering of step-growth and chain-growth polymerizations because of basic distinctions in the mechanisms of these reactions. A propagation reaction (Section 6.3.2) in a kinetic chain sequence must be fast or the series of monomer additions will not be long enough to produce... 

Macosko and Miller (1976) and Scranton and Peppas (1990) also developed a recursive statistical theory of network formation whereby polymer structures evolve through the probability of bond formation between monomer units this theory includes substitution effects of adjacent monomer groups. These statistical models have been used successfully in step-growth polymerizations of amine-cured epoxies (Dusek, 1986a) and urethanes (Dusek et al, 1990). This method enables calculation of the molar mass and mechanical properties, but appears to predict heterogeneous and chain-growth polymerization poorly. 

In the previous chapter, the synthesis of polymers by step polymerization and the kinetics of the process were considered. We turn our attention now to chain-growth polymerizations. The reader should recall that the features.that distinguish step-growth and chain-growth polymerizations are summarized in Table 5.1. A large number of different class of unsaturated monomers, such as ethylene (CH2=CH2, the simplest olefin), a-olefins (CH2=CHR, where R is an alkyl group), vinyl compounds (CH2=CHX, where X = Cl, Br, I, alkoxy, CN, COOH, COOR, CeHs, etc., atoms or groups), and... 

Step-growth and chain-growth polymerizations are defined by virtue of this classification, [3],... 

The description of the variety of chemistries that are used to produce thermosetting polymers can be the subject of a whole book and is beyond the scope of this chapter. A description of chemistries involved in the synthesis of several families of thermosets can be found elsewhere [2]. In this section, we focus on some aspects of the chemistry of epoxy polymers because it provides examples of both step-growth and chain-growth polymerizations employed in the synthesis of polymer networks. 

Several epoxy formulations are cured by both step-growth and chain-growth polymerizations occurring sequentially or in parallel. For example, BF3 complexes or tertiary amines may be added as catalysts of an amine-epoxy reaction, leading to different reaction mechanisms taking place whose relative significance depends on the cure temperature (or thermal cycle) and the initial stoichiometry. The structure and properties of the resulting polymer networks depend on the relative contribution of both mechanisms. 

In Sect. 3.1.1 it was established that polymerization reactions must have high precision in repetition in order to yield macromolecules. The two most common reactions for such high-yield polymerizations are step-growth and chain-growth polymerizations. 

To better understand the fundamental and practical differences between step-growth polymerization and chain-growth polymerization (see Table 1.4), consider the industrial chain-growth polymerization of ethylene (by either coordination polymerization or high-pressure free-radical polymerization) to produce polyethylene. 

For polymerization reactions two different mechanisms can be distinguished step-growth polymerization and chain-growth polymerization. 

The Kansas City symposium on New Monomers and Polymers consisted of twenty-six papers, reasonably representative of the current interest in, direction of, and opportunities for research on new monomers and polymers. The papers clearly showed that great opportunities exist for new work in both step-growth and chain-growth polymerization and in the introduction of new monomers. For a variety of reasons, not all the papers from that symposium are Included in this book, even though each author was Invited to contribute. However, additional papers were solicited and/or accepted to broaden the scope of the book and give readers a better picture of the many opportunities and challenges that exist for preparing new monomers and polymers. 

An IPN may be defined as a combination of two polymers in network form, at least one of which was polymerized and/or cross-linked in the immediate presence of the other. Both sequential IPN syntheses (2-8) and simultaneous interpenetrating network, SIN, syntheses were undertaken (9-13). The latter involving simultaneous but independent pol3mierizations via step and chain-growth mechanisms yielded the more practical of the two routes. 

The polymerization process can in simple terms be divided into step-growth and chain-growth polymerization. 

It would be tempting to consider the step-growth and chain-growth polymerization reactions as if they were independent and one could have the choice of either in any particular situation. The truth is that there are aspects of both types of polymerization in the cure of almost every epoxy structural adhesive. Such multiple-cure reactions often make it difficult to calculate the stoichiometry of an epoxy adhesive formulation. One type might predominate, depending on the formulation and cure conditions, but the effects of the other could not be completely discounted. The significance of... 

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