Unsaturated monomers, addition polymerization

Because no molecule is spHt out, the molecular weight of the repeating unit is identical to that of the monomer. Vinyl monomers, H2C=CHR (Table 2) undergo addition polymerization to form many important and familiar polymers. Diene (two double bonds) monomers also undergo addition polymerization. Normally, one double bond remains, leaving an unsaturated polymer, with one double bond per repeating unit. These double bonds provide sites for subsequent reaction, eg, vulcanization. 

Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by basic (anionic) catalysts. The chain-carrying step is conjugate nucleophilic addition of an anion to the unsaturated monomer (Section 19.13). 

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... 

Until now the discussion has centered on the addition polymers obtained fiom unsaturated monomers by reaction of the C=C or C=0 double bond. However, polymers obtained by other methods (ring-opening polymerization, polycondensation, etc.) offer interesting stereochemical phenomena also. As a rule, in these classes of macromolecular compounds the monomer units are clearly defined, the direction of the chain is often distinguishable and the stereo-isomeric elements present in the chain already preexist in the monomer. There are, however, numerous exceptions and further clarification is called for. 

The first stereochemical analysis of the polymerization mechanism of unsaturated monomers was proposed by Arcus 30 years ago (58, 269). The diaste-reomeric relationships within a single chain may be connected to monomer configuration by two factors the type of addition to the double bond (cis or trans) and the way in which successive monomer molecules approach the growing chain. For a CHA=CHB olefin the relationship between the stmcture of the monomer and that of the adding monomeric unit is illustrated in Scheme 22 (31), where cis or trans addition indicates that the new bonds are formed in the same half-space or in the two opposite half-spaces, provided there is no rotation aroimd the central C—C single bond. 

Acyclic an open-chained compound Addition Polymerization bonding of monomers without the elimination of atoms, formation of polymer by the bonding of unsaturated monomers Adhesion attraction between the surface of two different bodies... 

ESBR and SSBR are made from two different addition polymerization techniques one ladical and one ionic, ESBR polymerization is based on free radicals that attack the unsaturation of the monomers, causing addition of monomer units to the end of the polymer chain, whereas the basis for SSBR is by use of ionic initiators. 

In the formation of block copolymers by sequential addition of monomers it generally does not matter which monomer is polymerized first, and diblock or multiblock copolymers of narrow MWD and of any desired sequence length are readily prepared. Termination is usually effected by reaction of the living ends with aldehydes ketones can be used for terminating titanacyclobutane ends, while unsaturated ethers are used for terminating ruthenium carbene complexes. 

Vinylidene chloride polymerizes by both ionic and free-radical reactions. Processes based on the latter are far more common (23). Vinylidene chloride is of average reactivity when compared with other unsaturated monomers. The chlorine substituents stabilize radicals in the intermediate state of an addition reaction. Because they are also strongly electron-withdrawing, they polarize the double bond, making it susceptible to anionic attack. For the same reason, a carbonium ion intermediate is not favored. 

In addition (chain) polymerization, monomers containing an unsaturated (vinyl) bond polymerize in the presence of an initiator, which generates an active site at the end of the chain. Several chemical reactions take place simultaneously in the course of the polymerization. First, an initiation reaction via photo- or heat-decomposition of the initiator occurs to form the active species, which are either peroxides or azo compounds. The active species react with a monomer to generate the active site (i.e., initiation). 

Ring-opening polymerization is different from the addition and condensation polymerizations described so far. It does not produce byproducts (e.g., water) as polycondensation does, and there is no unsaturated double bond in the monomers to lead to additional polymerization. However, some similarities do exist. Ringopening polymerization is initiated by the opening of a cyclic structure in the monomers and followed by polyaddition. As a result, a linear polymer with a chemical composition identical to that of the monomer is obtained. 

The reiteration of the insertion step of unsaturated compounds into metal-alkyl bonds leads to polymerizations, dealt with in Chapter 7. However a series of important commodities, mainly used as monomers for polymerization, are manufactured by catalytic processes based on early termination steps of sequential olefin additions. 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

Addition polymerization In addition polymerization, all of the atoms present in the monomers are retained in the polymer product. When the monomer is ethene, an addition polymerization results in the polymer polyethylene, as shown in Table 23-3. Unsaturated bonds are broken in addition polymerization just as they are in addition reactions. The difference is that... 

A combination of variables controls the outcome of the copolymerization of two or more unsaturated monomers by CCT free-radical polymerization.382 Of course, all of the features that control the outcome of a normal free-radical polymerization come into effect.40 426 429 These include the molar ratio of monomers, their relative reactivity ratios and their normal chain-transfer constants, the polymerization temperature, and the conversion. In the presence of a CCT catalyst, the important variables also include their relative CCT chain-transfer constants and the concentration of the Co chain-transfer agent. The combination of all of these features controls the molecular weight of the polymer and the nature of the vinyl end group. In addition, they can also control the degree of branching of the product. 

There are two general routes to the synthesis of conjugated polymers addition polymerization of unsaturated monomers and condensation polymerization or stepwise coupling of monomers with difunctional groups. 

Classical emulsion polymerization is divided into three kinetic stages. At the start of the process, the unsaturated monomers are dispersed into small droplets, stabilized with surfactants. Additional surfactant aggregates into micelles. These micelles are very small ( 10nm) relative to monomer droplets ( 1-10 pm). During stage 1 the initial formation of polymer... 

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