Polymerization, chain

1 Chain polymerization (originally called addition polymerization) 

This occurs by initiation followed by monomer molecules adding in rapid succession, i.e. propagation (at the rate of several thousand units per second) and then termination. Thus, at any stage, the reaction mixture consists almost entirely of fully-formed polymer and monomer units. 

In these equations, the exact nature of the initiating and chain-carrying species can vary from essentially covalent for transition-metal organometallic species in 

Because this type of polymerization is a completely random process, with all molecules having equal probability of reacting, the distribution of molecular weights corresponds to the most probable, or binomial, distribution, which is related to the extent of polymerization as follows (Flory, 1953d)  

Hence the weight/number ratio of chain lengths in these systems undergoes a steady increase with extent of reaction, approaching an ultimate value of 2. 

we see that polyaddition and polycondensation are characterized by the following features  

The polymerization rates are essentially described by the concentrations and reactivity of the functional groups. 

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Chain Polymerization The growth process of a polymer postulates a three-step mechanism ... 

Thus, the process of chain polymerization is first-order with respecl to monomer and half-order with respecl to initiator. 

The chain polymerization of formaldehyde CH2O was the first example of a chemical conversion for which the low-temperature limit of the rate constant was discovered (see reviews by Goldanskii [1976, 1979]). As found by Mansueto et al. [1989] and Mansueto and Wight [1989], the chain growth is driven by proton transfer at each step of adding a new link... 

Mercaptan-modified polychloroprenes only contain sulphur at the ends of the polymer chain. Polymerization is carried out in the presence of a mercaptan to act... 

In contrast to ionic chain polymerizations, free radical polymerizations offer a facile route to copolymers ([9] p. 459). The ability of monomers to undergo copolymerization is described by the reactivity ratios, which have been tabulated for many monomer systems for a tabulation of reactivity ratios, see Section 11/154 in Brandrup and Immergut [14]. These tabulations must be used with care, however, as reactivity ratios are not always calculated in an optimum manner [15]. Systems in which one reactivity ratio is much greater than one (1) and the other is much less than one indicate poor copolymerization. Such systems form a mixture of homopolymers rather than a copolymer. Uncontrolled phase separation may take place, and mechanical properties can suffer. An important ramification of the ease of forming copolymers will be discussed in Section 3.1. 

Figure 15.27 Structure of the monomeric, trimeric and chain-polymeric forms of sulfur trioxide.

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

B. Formation of MAIs by Cationic Chain Polymerization—Cation Radical Transfer... 

With respect to the initiation of cationic chain polymerizations, the reaction of chlorine-terminated azo compounds with various silver salts has been thoroughly studied. ACPC, a compound often used in condensation type reactions discussed previously, was reacted with Ag X , X, being BF4 [10,61] or SbFa [11,62]. This reaction resulted in two oxocarbenium cations, being very suitable initiating sites for cationic polymerization. Thus, poly(tetrahydrofuran) with Mn between 3 x 10 and 4 x lO containing exactly one central azo group per molecule was synthesized [62a]. Furthermore, N-... 

Instinctively it would seem that Step C would be rate controlling and the slowest. In the case of the Fischer-Tropsch reaction, one would postulate that the surface would offer more methylene groups for chain polymerization. This mechanism differs from that of Vlasenko and Uzefo-vich (4) essentially in the concept that the whole molecule interacts with the surface. Furthermore, the HCOH intermediate is wholly horizontal to the surface rather than perpendicular. 

From an industrial stand-point, a major virtue of radical polymerizations is that they can often be carried out under relatively undemanding conditions. In marked contrast to ionic or coordination polymerizations, they exhibit a tolerance of trace impurities, A consequence of this is that high molecular weight polymers can often be produced without removal of the stabilizers present in commercial monomers, in the presence of trace amounts of oxygen, or in solvents that have not been rigorously dried or purified, Indeed, radical polymerizations are remarkable amongst chain polymerization processes in that they can be conveniently-conducted in aqueous media. 

Step-growth polymerizations have widely been developed in industrial applications whereas knowledge of their mechanisms and of their kinetics has remained far below that of chain polymerization reactions. 

Fig. 27 a and b. Schematic representation of the molecular structure of a side chain polymeric liquid crystals b polymer model membranes studied by 2H NMR... 

At this point it is appropriate to discuss the mechanism for ADMET, because ADMET polymerization is more involved than its chain polymerization counterpart— ROMP. Figure 8.6 illustrates the accepted mechanistic pathway which leads to productive metathesis polymerization, as first described by Wagener et al.14a A general model reaction between an a,o>-diene with a metal alkylidene... 

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] ... 

Chain polymerization involves three steps. To start the reaction, a catalyst that can generate an active site, such as a free radical (R ), is used. In the initiation step, the radical adds to the double bond, and the radical site is moved to the end carbon. This new radical reacts with another molecule to give a larger radical, and the propagation reaction is imderway. Usually, the number of monomers in the chain is greater than 1000. In the above formulae. 

FIGURE 3 Schematic representation of a pseudopoly (amino acid) derived from the side chain polymerization of a dipeptide carrying protecting groups X and Y. The wavy line symbolizes a nonamide bond. In this polymer, the amino acid side chains are an integral part of the polymer backbone while the termini have become pendant chains. In the backbone, amide and nonamide bonds strictly alternate. 

Ring-to-chain polymerizations which proceed according to the above... 

Ring Formation vs. Chain Polymerization from Six- and Seven-membered Units... 

Both polyalkylene glycols [45] and side chain polymeric alcohols such as polyvinyl alcohol have been suggested. These substances are comparatively environmentally safe [1420,1553]. 

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