Order with respect to initiator

It was found that conventional equation can be applied for description of the process. The process is the first order with respect to monomer and the order with respect to initiator is 0.25. The energy of activation was estimated at 43 kJ/mol. Moreover, it was found that without initiator, photopolymerization also occurs when the system is illuminated by UV light. 

With diphenylhexyllithium 121) (the product of addition of butyl-lithium to 1,1-diphenylethylene) kinetic results are the same as found for fluorenyllithium initiation in the presence of moderate amounts of ether. Even in pure toluene, the rates are first order with respect to initiator concentration and monomer concentration. This simple behaviour is caused by a constant fraction of the initiator forming low molecular weight polymer. If butyllithium is used as initiator, the kinetic behaviour is too complex for analysis. 

In aliphatic solvents, centres are formed more slowly than in benzene (see Fig. 3). Evidently the deaggregation mechanism is different from that in the aromatics. The dependence of the initiation rate on the type of initiating alkyl is preserved. The apparent orders with respect to initiator are higher, around 1. In this case, initiation may be the result of direct contact between monomer and aggregate, yielding a mixed associate [152]... 

When the assumptions under which relations (23), (28), (31) and (35) have been derived are not fulfilled, these relations are no longer valid. This is manifested by a change in the reaction orders with respect to initiator and to monomer with changing concentration of these components. One of the most general causes of deviations from ideality is termination by primary radicals. 

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

The rate of initiation has no effect on rT which is considered a constant equal to 1/2. Thus, if the particle number is held constant and the rate of initiation is varied, one finds that rate is zero order with respect to initiator. 

Figure 9. Dependence of t, the order with respect to initiator, on surfactant concentration, S. Recipe butadiene 500 g and CO (III) (acac) 1-10 g per 1000 g aqueous solution of 25-60 g/l. surfactant, 10 g/l.

The more polar the solvent the faster the polymerization rate. In benzene the polymerization order with respect to initiator was one. The order with respect to monomer rose from two at -5°C to above three at +40°C. Hie following three step kinetic scheme was proposed to account for this. 

By plotting log d[M]/dt( vs. log[I]o it could be shown that the order with respect to initiator is indeed one. However, the order with respect to monomer depends on the polymerization solvent being above two in solvents with dielectric constant below the monomers and only equal to two when dielectric constants of solvent and monomer are equal. 

Machacek, Mejzlik, and Pac3g), who carried out the first kinetic experiments with dibutylamine, assume that only the hydroxide ion formed by hydrolysis of the amine adds to the double bond. In diethyl ether at -60°C eonversion/time curves displayed a high internal order with respect to monomer and external first order with respect to initiator. 

It was found by Burnett and Melville36 in 1947 that the radical polymerization of vinyl acetate was retarded in aromatic solvents. This retardation effect was confirmed by several researchers37-42. It is characterized by three features all of which cannot be simultaneously explained by the conventional kinetic scheme involving degradative chain transfer to solvent. (1) The rate of polymerization is markedly reduced in comparison with that in many aliphatic solvents. (2) The order with respect to initiator remains close to one-half over a wide range of initiator concentration. 

Polymerization in solution follows conventional kinetics except for certain solvent-specific side reactions. At monomer concentrations above 2-2.5 M, the reaction order with respect to monomer and initiator has been found to be 1.0 and 0.5, respectively [35]. In DMF, however, a monomer reaction order greater than expected was explained by chain transfer to solvent followed by slow reinitiating by the DMF radical [43]. At higher monomer concentrations, however, the monomer has the effect of adding a nonsolvent to the reaction mixture. Under these conditions, the reaction orders with respect to initiator and monomer can deviate from the expected values. Vidotto et al. [35] found that the reaction became heterogeneous at... 

According to these assumptions, the overall rate should be first order with respect to monomer and 0.5 order with respect to initiator. Thus, a plot of the overall rate versus the square root of the initiator concentration for constant monomer concentrations (low yields) should give a straight line according to equation (20-59). However, it is frequently observed that the overall rate increases more slowly than with the root of the initiator concentration for increasing initiator concentrations. This behavior is explained by additional termination by initiator free radicals ... 

Thus, the rate of polymerization is internally first order in monomer, externally first order with respect to initiator and activator, Cu(I), and negative first order with respect to deactivator, XCu However, the kinetics may be more complex because of the formation of XCu species via the persistent radical effect. The actual kinetics depend on many factors, including the solubility of activator and deactivator, their possible interactions, and variations of their structures and reactivities with concentrations and composition of the reaction medium. It should also be noted that the contribution of persistent radical effect at the initial stages might be affected by the mixing method, solubility of the metal compoimd and ligand, etc (239-242). 

As a result of kinetic investigations at initial conversions, it was determined, that independently of medium (water/DMSO) a usual for radical polymerization reaction half order with respect to initiator is observed, indicating bimolecular mechanism of the growing chain failure, as well as deficiency of degradative chain transfer to the monomer, intrinsical to allyl monomers. 

Peggion et al. [76] have reported that the rate of emulsion polymerization of vinyl chloride is proportional to the 0.5th power of the initiator concentration (with anionic emulsifier (SDS) above its CMC). The reaction order thus differs slightly from that proposed for the conventional emulsion polymerization of vinyl monomers. These data were discussed in terms of the bimolecular termination of growing radicals. The number of particles was found to be independent of the initiator concentration. The data were supposed to be affected by the restricted penetration of radicals and monomers into the polymer phase and separation of the growing radicals from the monomer phase and chain transfer events. The trapping of radicals increases with conversion and so the reaction order with respect to [initiator] increases [77]. The authors [76] suggested that the emulsion polymerization of VC is similar to a heterogeneous polymerization [56] in which the formation of occluded radicals increases the order with respect to [initiator]. 

Giskenhaug [83] has reported that the emulsion polymerization of VC is influenced by the water-phase polymerization. The half order with respect to initiator, as well as the slight effect of the particle concentration on the rate of polymerization was discussed in terms of the bimolecular termination of growing radicals. The following expression for the rate of the water-polymerization... 

In the water-phase polymerization (at low conversion) of VC, the reaction order with respect to the initiator concentration is around 0.5. This is interpreted in terms of the high water solubility of VC and bimolecular termination of growing radicals. The independence of the rate of polymerization from the particle concentration is taken as an evidence of water - phase polymerization. In systems with high initiator concentration, the primary radical concentration decreased the reaction order below 0.5 or 0.4. In most cases, however, the reaction order with respect to [initiator] (at low or medium initiator concentration) was found to be above 0.5. This behavior was ascribed to the restricted bimolecular termination of growing radicals and increased contribution of the monomolecular termination events. The long lifetime and large fraction of... 

Figire 8 Reaction order with respect to initiator concentration for the polymerization of MMA with TMEDA-chelated Li counterion in THF at -20 °C ( ) with TMEDA and ( ) without TMEDA. Reprinted with permission from Wiley-VCH. 

Candau et al. [41] later studied the polymerization of AOT-stabiUzed acrylamide inverse microemulsion by a thermal process using either oil soluble AIBN or water soluble K2S2O8 as the initiator and found the rate of polymerization to be first order with respect to initial monomer concentration in the presence of AIBN but 1.5 order in the presence of K2S2O8. An inverse relationship was found between polymer molecular weight and the surfactant concentration which suggested participation of the surfactant in the initiation reaction. This was further confirmed by the observed independence of the polymer molecular weight on the concentration of the initiator. 

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