Radical-chain polymerization

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 

Chain-growth polymerization, as well as all other typical chain reactions, are fast reactions typified by three normally distinguishable processes, viz., (i) initiation of the chain, (ii) propagation or growth of the chain, and (iii) termination of the chain. (A fourth process, chain transfer, may also be involved.) 

The initiation is usually a direct consequence of generation of a highly active species R by dissociation or degradation of some monomer molecules (M) under the influence of such physical agencies as heat, light, radiation etc., or as a consequence of dissociation or decomposition of some chemical additives commonly known as initiators (I)  

The reactive species R may be a free radical, cation, or anion, which adds itself, if conditions are favorable, to the monomer molecule by opening the 7T bond to form a fresh reactive center (radical, cation, or anion center), depending on the nature of R. The new reactive center adds to another 

The chain growth is terminated at some stage by annihilation of the reactive center by one or more convenient and appropriate mechanism which depends largely on the type of reactive center (radical, cation or anion), nature of the monomer M, and the overall chemical environment and condition of reaction. 

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Very approximately the S2O content of a gas mixture can be estimated from the color of the condensate at -196 °C in a glass trap (provided that all other components are colorless at this temperature). Due to the formation of highly colored decomposition products the condensate is yellow at <2 mol% S2O, orange-yellow at 5-10%, orange at 20-30%, cherry-red at 40-70%, and dark-red at >85% [10]. These colors [14] are caused by small sulfur molecules like S3 and 84 [15, 16] as well as by sulfur radicals formed in the radical-chain polymerization of S2O to polysulfuroxides (S 0)x and SO2 [10, 17] ... 

In this section, the important concepts related to the formation of hydrogels by free radical copolymerization/cross-linking are examined. Greater depth beyond the scope of this chapter can be obtained from textbooks on polymer chemistry and the papers cited herein. As stated earlier, almost all gels produced from monomers for pharmaceutical applications are synthesized by free radical chain polymerizations. 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

Commonly used monomers for UV curing include acrylates (7), styrene/unsaturated polyesters (8,9), and thiol-ene compositions (10-12). Currently, acrylate-functional systems constitute a major share of the UV curable polymers market, mainly due to their rapid curing via free radical chain polymerization. 

Free Radical Chain Polymerization (Addition Polymerization) 173... 

Energies of Activation for Propagation (fp) and Termination (ft) in Free Radical Chain Polymerization... 

Chain Transfer Constants of Solvent to Styrene in Free Radical Chain Polymerization at 60°C... 

Name three widely used thermoplastics produced by free radical chain polymerization ... 

The most widely used graft copolymer is the styrene-unsaturated polyester copolymer (Equation 7.35). This copolymer, which is usually reinforced by fibrous glass, is prepared by the free radical chain polymerization of a styrene solution of unsaturated polyester. 

Thus, almost all substituents are able to stabilize the propagating radical by delocalization of the radical over two or more atoms. The remainder of this chapter will be concerned with the detailed characteristics of radical chain polymerization. Ionic chain polymerizations will be... 

Radical chain polymerization is a chain reaction consisting of a sequence of three steps— initiation, propagation, and termination. The initiation step is considered to involve two... 

The steady-state assumption is not unique to polymerization kinetics. It is often used in developing the kinetics of many small-molecule reactions that involve highly reactive intermediates present at very low concentrations—conditions that are present in radical chain polymerizations. The theoretical validity of the steady-state assumption has been discussed [Kondratiev, 1969] and its experimental validity shown in many polymerizations. Typical polymerizations achieve a steady-state after a period, which may be at most a minute. 

It is appropriate at this point to briefly discuss the experimental procedures used to determine polymerization rates for both step and radical chain polymerizations. Rp can be experimentally followed by measuring the change in any property that differs for the monomer(s) and polymer, for example, solubility, density, refractive index, and spectral absorption [Collins et al., 1973 Giz et al., 2001 McCaffery, 1970 Stickler, 1987 Yamazoe et al., 2001]. Some techniques are equally useful for step and chain polymerizations, while others are more appropriate for only one or the other. Techniques useful for radical chain polymerizations are generally applicable to ionic chain polymerizations. The utility of any particular technique also depends on its precision and accuracy at low, medium, and high percentages of conversion. Some of the techniques have the inherent advantage of not needing to stop the polymerization to determine the percent conversion, that is, conversion can be followed versus time on the same reaction sample. 

Equation 3-32 describes the most common case of radical chain polymerization. It shows the polymerization rate to be dependent on the square root of the initiator concentration. This... 

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