Decomposition of initiator

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

Since the quantum chemical calculations used to parameterize equations 6 and 7 are relatively crude semiempirical methods, these equations should not be used to prove or disprove differences in mechanisms of decomposition within a family of initiators. The assumption made in the present study has been that the mechanism of decomposition of initiators does not change within a particular family of initiators (reactions 1-4). It is generally accepted that trow5-symmetric bisalkyl diazenes (1) decompose entirely by a concerted, synchronous mechanism and that trans-phenyl, alkyl diazenes (2) decompose by a stepwise mechanism, with an intermediate phenyldiazenyl radical (37). For R groups with equal or larger pi-... 

Refers to decomposition of initiator Refers to initiation process Refers to propagation reaction Refers to termination reaction... 

Unimolecular Decomposition of Initiator with One Bond Splitting... 

Different mechanisms of free radical formation as a result of the decomposition of initiators are known. 

Due to the elongation of the dissociating bond (for example, O—O in peroxide), the volume of the transition state V is greater than the volume of reactant V. As a result, the difference in the volumes Al/ V — V is positive. The study of the decomposition of initiators with one bond dissociation under high pressure gives evidence that AF is positive [2,7]. 

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

Probability (e) of Free Radical Escaping from the Cage into Bulk Volume in the Decomposition of Initiators in Polymer Matrix... 

The compounds generally used as initiators are thermally unstable compounds which decompose to produce the free radicals. The decomposition of initiators can be brought about by supplying them energy either in the form of heat or light. The molecule then cleaves homolytically producing free radicals, e.g. 

It has been found that the rate of decomposition of initiators depends on their chemical nature, temperature and solvent. Some initiators can be decomposed by UV light, e.g. 

The initiation of polymerisation takes place in two steps The decomposition of initiator to form the free radical and the addition of the radical to the monomer with the generation of another radical. If the radical produced by decomposition of the initiator, I, is designated as R, then the process of initiation could be shown as under ... 

The decomposition of initiator can be followed by usual analytical methods and k can be determined. The efficiency factor/can be obtained by comparing the amount of initiator [I] decomposed with the number of polymer chain formed. The rate of polymerization can be determined by monitoring the change in a physical or chemical property of the system. Generally, dilatometry technique is used for determination of the rate of polymerization. Let the extent of polymerization be small and concentration of initiator be constant. Let r0, rt and r be the readings on dilatometer initially, at time t and at the completion of reaction, respectively. If reaction is first order in [M],... 

The rate of decomposition of initiators usually follows first-order kinetics and is dependent on the solvent present and the temperature of polymerization. The rate is usually expressed as a half-life time (h/2), where ti/2 = In 2/A d = 0.693/fcd- The rate constant ( d) changes with temperature in accordance with the Arrhenius equation as shown below ... 

When a material balance is performed on the amount of initiator that is decomposed during a polymerization and compared with that which initiates polymerization, it is apparent that the initiator is inefficiently used. There is wastage of initiator due to two reactions. One is the induced decomposition of initiator by the attack of propagating radicals on the initiator, for example... 

This reaction is termed chain transfer to initiator and is considered further in Sec. 3-6b. The induced decomposition of initiator does not change the radical concentration during the polymerization, since the newly formed radical (polymer chain. However, the reaction does result in a wastage of initiator. A molecule of initiator is decomposed without an increase in the number of propagating radicals or the amount of monomer being converted to polymer. 

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