Chain-growth polymers. See Addition

Chain-growth polymer. See Addition polymer. Chair conformation (Section 6.5) The chairshaped conformation of cyclohexane that has no angle strain and has no torsional strain either be-... 

Chain-growth polymer (See Addition polymer and Special Topic... 

Chain-growth polymer (see Addition polymer and Special Topic B) Polymers (macromolecules with repeating units) formed by adding subunits (called monomers) repeatedly to form a chain. 

Typical acrylic resins are high MW polymers or copolymers of acrylate and/or methacrylate monomers prepared by radical-initiated chain-growth polymerization (see Figure 2.28). In addition to the (meth) acrylate monomers, other functional (meth)acrylate monomers as well as non-acrylate monomers (typically vinyl monomers) are frequently used in preparation of commercial acrylic copolymer resins to impart reactive functionality or special properties or for lower cost. Some examples of these monomers are shown in Figure 2.29. 

Chain growth continues at a rate dependent on the concentrations of monomer [M] and of active sites [MJ. Monomer exponents in the range 1.3 to 1.5 or higher had been observed (110, 123, 127) especially at low [M], but first order dependence has now been established over a broad range of [M] (21). A stationary level of [M ] is reached rapidly and is typically of the order of 10-8 molar. Chains grow rapidly by successive monomer additions until the polymer chain is terminated by transfer or by reaction with another radical. The rate constant for propagation (ft2) at 60° in DMF is 1960 m-1 Is-1 (16), which is a comparatively high value [see Table 1 and ref. (76)]. On the other hand it is only about one-tenth of that found for acrylonitrile in aqueous systems (Table 6)... 

In general, chain growth is associated with addition polymerization and step growth with condensation polymerization. It is not always so, however. WeTl see an example later in this chapter of an addition polymer in which step growth, not chain growth, characterizes macromolecule formation. 

The efficiency and randomness of the incorporation of a second monomer depend on the copolymerization parameters (see Section 3.1) and the type of chain growth method applied. For each monomer pair these factors have to be evaluated and considered for defining stmcture and functionality. In addition, basic polymer properties like solubility and thermal properties defined by the major monomer are strongly influenced by a second monomer of different functionality thus, in any random copolymerization process, it is usually not possible to introduce a second functionality without compromising the properties of the parent homopolymer, and this is enhanced with increasing the comonomer ratio. 

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents on the originally inducing center on the center portion must differ considerably in size. (3) The location... 

First-order Markov processes are therefore defined by two independent addition probabilities. Although the propagation steps shown above depict free radical polymerisation, the statistical models are equally applicable to other types of chain growth as found, for example, in ionic and Ziegler-Natta polymers (see section 2.3.4). 

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