Step-growth polymerization. See

Unsaturated Polyester Systems. Coatings in the furniture industry rely heavily upon resin formulations containing unsaturated polyesters, styrene, and photoinitiator (3,4,13,43,44). The imsaturated polyesters are synthesized using step-growth polymerization (see Polyesters, Unsaturated). Upon illumination, the carbon-carbon double bond in the imsaturated polyester and st5Tene copolymerize to form a cross-linked network (eq. 6). Equation 6 shows a generalized reaction scheme for an imsaturated polyester system. 

Another example of the step-growth polymerization is the synthesis of polyurethanes. Here, linear polyurethanes are produced by the reaction of bifunctional alcohols, HO — R — OH, with bifunctional isocyanates, OCN — R — NCO, to produce a polyurethane (see Figure 3.21). 

One polymer that isoften used in these products is poly ethylene oxide) (PEO), a polymer most often prepared by the ring-opening polymerization of ethylene oxide (see Table 5-3). Alternatively, it is sometimes prepared by a step-growth polymerization of ethylene glycol, in which case it is called polyethylene glycol) (PEG). PEO/PEG is a water-soluble polymer that can be synthesized to a molar mass in the millions. 

Another important aspect of step-growth polymerizations is that, in addition to driving the reaction to high degree of conversions, it is also necessary to have precisely equivalent amounts of the monomers (you will see why when you study the statistics of these reactions). This is not always easily accom-... 

Fig. 17. Extent of reaction at vitrification vs. reaction temperature for nonlinear step-growth polymerization (A + 2B2). All kinetic orders have the same p at vitrification. For mpdel parameters and system, see Fig. 16 caption. [Aronhime,...

We can see from this simple example why step-growth polymerizations require very high conversions of functional groups compared to syntheses of the same linkages in conventional organic reactions. 

We first consider the polymerization where each kinetic chain yields one polymer molecule. This happens when the growth of microradical chains is terminated by disproportionation and/or chain transfer (i.e., ktc = 0). The situation here is completely analogous to that for linear, reversible step-growth polymerization described in Chapter 5. If we select an initiator fragment at the end of a macromolecule, the probability that the monomer molecule picked up by this initiator radical has added another monomer molecule is P. Continuing in this way the probability that x monomer molecules have been added one after another is (see p. 347). Since... 

What amounts to invoking the shortsightedness principle has become standard practice in polymerization kinetics where, for most purposes, the rate coefficients of step or chain growth and termination are taken to be independent of the lengths of the polymer chains (see Sections 11.2.1, 11.3.1, 11.4.1,and 11.5.1). In fact, the principle is the basis of Carother s approach to step-growth polymerization kinetics leading to his equations 11.6 and 11.7. 

In any reaction resulting in the formation of a chain or network of high molar mass, the functionality (see Section 1.2) of the monomer is of prime importance. In step-growth polymerization, a linear chain of monomer residues is obtained by the stepwise intermolecular condensation or addition of the reactive groups in bifunctional monomers. These reactions are analogous to simple reactions involving monofunctional units as typified by a polyesterification reaction involving a diol and a diacid. 

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