Chain reaction terms Links

Another common feature of free-radical reactions is that they tend to be chain processes. Since any chemical reaction must exhibit conservation of spin, the reaction of a free radical widi a closed-shell (fully electron paired) molecule must result in the production of a new free-radical species which can participate in subsequent free-radical reactions. The series of free-radical reactions leading to product is often a cyclic process in which the initial free radical is produced once again in die last step of the cycle so that the reaction sequence starts over again. The process is termed a chain reaction because each step of the process is linked directly to die preceding step. 

TrifunctionaL. In a polymer-producing reaction, the term applied to a reactant possessing three functional groups in its structure, hence capable of propagating a chain and cross-linking it to other chains. Example ... 

Chain. A linear or branched macromolecule is often called a chain because the repeating units are joined together like links in a chain. Many polymers are polymerized by chain reactions, which arc characterized by a scries of successive reactions initiated by a single primary event. Here the term chain is used to designate a kinetic sequence of reaction events which results in the production of a molecular chain composed of linked repeating units. 

We will discuss the various polymerization mechanisms in greater detail in Chapter 2. The original classification of polymers as either condensation or addition polymers as proposed by Carothers does not permit a complete differentiation between the two classes or polymers, particularly in view of the new polymerization processes that have been developed in recent years. Consequently, this classification has been replaced by the terms step-reaction (condensation) and chain-reaction (addition) polymerization. These terms focus more on the manner in which the monomers are linked together during polymerization. 

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. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

Networks with tri- and tetra-functional cross-links produced by end-linking of short strands give moduli which are more in accord with the new theory if quantitative reaction can be assumed (3...13) However, the data on polydimethylsiloxane networks, may equally well be analyzed in terms of modulus contributions from chemical cross-links and chain entangling, both, if imperfect reaction is taken into account (J 4). Absence of a modulus contribution from chain entangling has therefore not been demonstrated by end-linked networks. 

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