Step polymerization characteristics

The International Union of Pure and Applied Chemistry [IUPAC, 1994] suggested the term polycondensation instead of step polymerization, but polycondensation is a narrower term than step polymerization since it implies that the reactions are limited to condensations—reactions in which small molecules such as water are expelled during polymerization. The term step polymerization encompasses not only condensations but also polymerizations in which no small molecules are expelled. An example of the latter is the reaction of diols and diisocyantes to yield polyurethanes (Eq. 1-6). The formation of polyurethanes follows the same reaction characteristics as the formation of polyesters, polyamides, and other polymerizations in which small molecules are expelled. 

Many of the common condensation polymers are listed in Table 1-1. In all instances the polymerization reactions shown are those proceeding by the step polymerization mechanism. This chapter will consider the characteristics of step polymerization in detail. The synthesis of condensation polymers by ring-opening polymerization will be subsequently treated in Chap. 7. A number of different chemical reactions may be used to synthesize polymeric materials by step polymerization. These include esterification, amidation, the formation of urethanes, aromatic substitution, and others. Polymerization usually proceeds by the reactions between two different functional groups, for example, hydroxyl and carboxyl groups, or isocyanate and hydroxyl groups. 

The reaction mixture at any instance consists of various-sized diol, diacid, and hydroxy acid molecules. Any HO-containing molecule can react with any COOH-containing molecule. This is a general characteristic of step polymerization. 

Table 2-8 shows values of some kinetic and thermodynamic characteristics of typical step polymerizatiosn [Bekhli et al., 1967 Chelnokova et al., 1949 Fukumoto, 1956 Hamann et al., 1968 Malhotra and Avinash, 1975, 1976 Ravens and Ward, 1961 Saunders and Dobinson, 1976 Stevenson, 1969 Ueberreiter and Engel, 1977]. These data have implications on the temperature at which polymerization is carried out. Most step polymerizations... 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

One of the primary differences between CVD (or in the case of polymers, commonly referred to, as chemical vapor polymerization or CVP) and other conventional deposition techniques, particularly for polymer thin films, is that CVD is a dry process. There is no liquid intermediate between the gas phase reactants and the resultant solid thin film and thus, problems due to surface tension such as pulling away from the corners, sharp edges etc, are not present. Additionally, CVD enables deposition of uniform thin films in recesses, holes and other difficult three dimensional configurations unlike solution based techniques which are limited to planar substrates. In the terminology of thin film device fabrication, CVD films have excellent gap-filling and step coverage characteristics. 

Although the basic principles of polyketone formation are now reasonably well understood, further studies, both of polymerization characteristics and of the elementary steps underlying polyketone catalysis, will be needed to exploit fully the potential of these selective polymerizations. 

A. Knop, V. Bohmer and L. A. Pilato, in Comprehensive Polymer Science, Vol. 5, General Characteristics of Step Polymerization (Eds. G. Allen and J. Bevington), Chap. 35, Pergamon Press, Oxford, 1989, pp. 611-647. 

Most (but not all) of the step polymerization processes involve polycondensation (repeated condensation) reactions. Consequently, the terms "step polymerization" and "condensation polymerization" are often used synonymously. In a condensation reaction between two molecules, each molecule loses one atom or a group of atoms at the reacting end, which leads to the formation of a covalent bond between the two, while the eliminated atoms bond with each other to form small molecules such as water—hence the term condensation reactions. Consider, for example, the synthesis of a polyamide, i.e., a polymer with amide -(-CONH-)- as the characteristic... 

Thus, the polyesterification reaction mixture at any instance consists of various-sized diol, diacid, and hydroxyacid molecules. Any OH-containing molecule can react with any COOH-containing molecule. This is a general characteristic of step polymerization. A comparative account of the differences between step polymerization or condensation polymerization on the one hand and chain polymerization or addition polymerization (see Chapter 6) on the other hand is given in Table 5.1. 

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). 

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