Polymers propagator

Tlie formation of initiator radicals is not the only process that determines the concentration of free radicals in a polymerization system. Polymer propagation itself does not change the radical concentration it merely changes one radical to another. Termination steps also occur, however, and these remove radicals from the system. We shall discuss combination and disproportionation reactions as modes of termination. 

Polymer propagation steps do not change the total radical concentration, so we recognize that the two opposing processes, initiation and termination, will eventually reach a point of balance. This condition is called the stationary state and is characterized by a constant concentration of free radicals. Under stationary-state conditions (subscript s) the rate of initiation equals the rate of termination. Using Eq. (6.2) for the rate of initiation (that is, two radicals produced per initiator molecule) and Eq. (6.14) for termination, we write... 

Polymer propagation stops with the addition of a chain transfer agent. For example, carbon tetrachloride can serve as a chain transfer agent ... 

The resonance stability of the macroradical is an important factor in polymer propagation. Thus, for free radical polymerization, a conjugated monomer such as styrene is at least 30 times as apt to form a resonance-stabilized macroradical as VAc, resulting in a copolymer rich in styrene-derived units when these two are copolymerized. 

The fact that very high molecular weight materials form under these aqueous conditions indicates that if a termination reaction involving the hydrolysis of the carbon-metal bonds is occuring in either the metallacycle or metal carbene intermediates, it has a much slower rate (by several orders of magnitude) than the rate of polymer propagation. 

In a recent paper Saegusa and his co-workers (64) report that ethylene oxide is converted to dioxane in yields of up to 96% when the monomer is treated with a catalytic amount (generally 2—5 mol %) of a superacid such as trifluoromethanesul-fonic acid, or a derivative of a superacid such as ethylfluorosulfonate, in methylene chloride or nitromethane at temperatures between 10 and 40 °C. The authors propose that dioxane is formed by a simultaneous polymerization and degradation of the formed polymer. Propagation as well as degradation are assumed to occur via the ester species. 

Vanadium-containing coordination centres producing syndiotactic polypropylene at 195 K can be transformed to radical centres simply by raising the temperature to 298 K [252]. In this way, Japanese authors have prepared the copolymer poly(propene)-Wocfc-poly(methylmethacrylate). The radical end is probably formed by homolytic splitting of the C—V bond, and it can be stabilized by the V ion. The authors state that, in this way, two-component blocks of polypropylene with various polymers propagating by the radical mechanism can be prepared. 

A new rheo-photoacoustic Fourier transform infrared cell has been developed to perform stress-strain studies on polymeric materials. The rheo-photoacoustic measurements lead to the enhancement of the photoacoustic signal and allow one to monitor the effect of elongational forces on the molecular structure of polymers. Propagating acoustic waves are detected as a result of infrared reabsorption and the deformational and thermal property changes upon the applied stress. 

Cationic polymerizations are catalyzed by SnCU and other Lewis acids. Propagation is based upon the formation of a cationic species upon complexation with SnCU (Eq. 64) [96]. Radical pathways are also possible for polymer propagation [97]. 

Zaitoun, A., Kohler, N., 1987. The role of adsorption in polymer propagation through reservoir rock. Paper SPE 16274 presented at the SPE International Symposium on Oilfield Chemistry, San Antonio, 4-6 February. 

The most prominent suggestions as to the structure of the active site can be illustrated by reference to a bimetallic complex where polymer propagation could conceivably occur from (1) the transition metal center which is really a transition metal alkyl, (2) the reducing agent center, or (3) a bridge position between the two metals... 

Isomer B of [Cu(SCN)(dpt)j exhibits a one-dimensional ribbon polyiner structure constructed from dpt-decorated (CuSCN), stair-polymers (Fig. 3b). Each Cu(I) center adopts a N2S2 coordination sphere formed by three SCN anions and a single dpt ligand. In contrast to Isomer A, each dpt ligand is only monocoordinated and, therefore, is not involved in polymer propagation. 

Table 20-7. Activation Volume for Initiator Decomposition and Polymer Propagation in Free Radical Polymerizations...

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