Step-growth polymers: chains

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Synthetic polymers can be classified as either chain-growth polymers or step-growth polymers. Chain-growth polymers are prepared by chain-atactic, 1267 reaction polymerization of vinyl monomers in the presence of a radical, an... 

In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

The alkene and diene polymers discussed in Sections 7.10 and 14.6 are called chain-growth polymers because they are produced by chain reactions. An initiator adds to a C=C bond to give a reactive intermediate, which adds to a second alkene molecule to produce a new1 intermediate, which adds to a third molecule, and so on. By contrast, polyamides and polyesters are called step-growth polymers because each bond in the polymer is formed independently of the others. A large number of different step-growth polymers have been made some of the more important ones are shown in Table 21.2. 

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. 

Identify the montuner units from which each of the following polymers is made, and tell whether each is a chain-growth or a step-growth polymer. 

Polymerization reactions are classified as being either chain growth or step growth. In chain-growth polymerization, a small molecule reacts with a growing polymer chain to form an incrementally longer chain ... 

Polymers made with two or more monomers are copolymers. Most step growth polymers are copolymers. Chain growth copolymers are made to achieve specific properties. Chain growth copolymers may be ... 

Comparison of Chain Growth and Step Growth Polymers... 

A summary of properties of chain growth and step growth polymers can be found in Table 2.4. 

The chemical nature of the main-chain linkages of step-growth polymers makes this class of polymers particularly reactive to a wide variety of chemical species. Solvolysis reactions break the C-X bond at the polymer linkage bonds. These types of reactions are often pH-dependent, so the stability of the polymer is highly dependent on the acidity or basicity of the prodegradant. 

The addition and condensation polymers are nowadays also referred as chain growth polymers and step growth polymers depending on the type of polymerisation mechanism they undergo during their formation. 

At the beginning of this chapter we described step growth polymerization chiefly to contrast it with chain polymerization. We now consider this subject in more detail and discuss various types of step growth polymers. 

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. 

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