Other Addition Polymerization Mechanisms

The three classifications just described are essentially based on monomer structure. Condensation polymerizations arise when the two monomers are stable but have functionalities that can react with each other. Addition polymerizations require unsaturation in the monomer that is vulnerable to attack by radicals or ions, and ring-opening polymerizations require cyclic monomers. An alternative and more modern classification emphasizes the differing mechanisms of polymerization, producing two classes step-growth reactions and chain-growth reactions. Figure 13.13 contrasts the two mechanisms. 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

The process proceeds through the reaction of pairs of functional groups which combine to yield the urethane interunit linkage. From the standpoint of both the mechanism and the structure type produced, inclusion of this example with the condensation class clearly is desirable. Later in this chapter other examples will be cited of polymers formed by processes which must be regarded as addition polymerizations, but which possess within the polymer chain recurrent functional groups susceptible to hydrolysis. This situation arises most frequently where a cyclic compound consisting of one or more structural units may be converted to a polymer which is nominally identical with one obtained by intermolecular condensation of a bifunctional monomer e.g., lactide may be converted to a linear polymer... 

Addition polymerizations of unsaturated monomers leading to the formation of products of high molecular weight invariable proceed by chain reaction mechanisms. Primary activation of a monomer M (or a pair of monomers) is followed by the addition of other monomers in rapid succession... 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

A corresponding anionic mechanism in the presence of a strong base (or electron donor) is plausible. Other cyclic compounds may be susceptible to polymerization by similar ionic mechanisms. Inasmuch as the growth step must be extremely rapid, a chain reaction is indicated and classification with vinyl-type addition polymerizations should be appropriate in such cases. 

Given the vastness of the subject matter I have limited myself to dealing with the structural (or static) aspects of macromolecular stereochemistry. An adequate treatment of the stereochemistry of polymerization, with specific regard to the polymerization of olefins and conjugated diolefins, would have occupied so much space and called for such a variety of additional information as to make this article excessively long and complex. I trust that others will successfully dedicate themselves to this task. However, the connection between polymer structure and polymerization mechanism is so important that the fundamentals of dyruunic macromolecular stereochemistry cannot be completely ignored in this chapter. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

Polycyanurates and polyimides can be classified as thermally stable polymers. Other groups that react at high temperatures (higher than the melting temperature for crystalline monomers) can be used (Table 2.14). The reaction may be classified as an addition polymerization, but the mechanisms are very complex and not always well known. 

There are already several excellent detailed reviews on GTP [2-5]. In this chapter I will critically analyze the existing data that strongly support a dissociative (anionic) mechanism originally proposed by R. Quirk of Akron University [6]. I will also explain how GTP can operate at 80 °C when it is well known that the classical anionic polymerization of methacrylates does not proceed above ambient temperatures. In addition, GTP will be compared to other controlled polymerization methods. 

A microemulsion, Fig. 1, has a similar organization to that characteristic of a micelle but employs, rather than one, multiple surfactant components, allowing for introduction of other additives into the hydrophobic core [11], As with micelles, microemulsions are optically transparent and can be easily studied by standard spectroscopic methods. One important use of such microemulsions is in the photoinduced initiation of polymerization of monomers with low water solubility many such reactions involve a mechanism occurring through photoinduced interfacial electron transfer. 

Other polymerization mechanisms can produce distributions of molecular weights that are different from the most probable distribution. Much narrower distributions are obtained if a specified number of chains is initiated at the same time, and these chains grow exclusively by the addition of monomer to the reactive end. Then... 

Treatment of the monomer with an acidic catalyst leads initially to polymers of low molecular weight and ultimately to crosslinked, black, insoluble, heat-resistant resin (17). Despite their reportedly excellent properties, virtually no commercial use of such resins exists outside the Soviet Union. The structure and polymerization mechanism of these furfural-ketone polymers are described in a recent study (18). An excellent combustion-resistant resin has been reported (19) from the addition of dialkylphosphites to bis(2-furfurylidene) ketone (6). Furfural condensates with other aliphatic and aromatic ketones have been reported (20,21) to provide photo-crosslinkable resins and hypergol components. 

Complexes are frequently encountered in macromolecular chemistry. They always have the character of more or less labile compounds which can only rarely be isolated. Direct proof of their existence is difficult. By indirect proof only some of their properties are revealed, for example the ability to compete with other components during addition, a shift or change in the character of a reaction path, contribution to the overall thermodynamical parameters of the process, etc. In the author s opinion, the question of the existence of monomer complexes has not yet been fully appreciated. Work in this field should lead to important discoveries for elucidating polymerization mechanisms and kinetics, with corresponding consequences for industrial production. 

Thus far, we have considered addition polymerization routes - either catalyzed or uncatalyzed. Although this is sufficient to describe the synthesis of common packaging materials such as polyethylene, polypropylene, polystyrene, etc., other classes of polymers such as nylon, PETE, and polyacrylamide are generated through step-growth mechanisms. Although the synthetic pathway for these polymers is more straightforward than addition polymerization, there are many intricate considerations that affect overall polymer properties. 

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