Radical chain polymerization initiator efficiency

The determination of the various rate constants (ki, kp, kt, kts, ktr) for cationic chain polymerization is much more difficult than in radical chain polymerization (or in anionic chain polymerization). It is convenient to use Rp data from experiments under steady-state conditions, since the concentration of propagating species is not required. The Rp data from non-steady-state conditions can be used, but only when the concentration of the propagating species is known. For example, the value of kp is obtained directly from Eq. (8.143) from a determination of the polymerization rate when [M J is known. The literature contains too many instances where [M" "] is taken equal to the concentration of the initiator, [IB], in order to determine kp from measured Rp. (For two-component initiator-coinitiator systems, [M" ] is taken to be the initiator concentration [IB] when the coinitiator is in excess or the coinitiator concentration [L] when the initiator is in excess.) Such an assumption holds only if Ri > Rp and the initiator is active, i.e., efficiency is 100%. Using this assumption without experimental verification may thus lead to erroneous results. 

Kinetic studies revealed that the polymerization by Cp 2Sm(THF)2 is zero-order in [yMMBL] and second-order in [catalyst], as a result of two samarium centers working in tandem to produce one polymer chain. This result, coupled with the polymerization initiator efficiency result, which also pointed to the bimetallic nature of the propagation, conforms to the proposed MMA polymerization mechanism by the same divalent catalyst involving a redox-then-radical-coupling initiation process, with the true active species being the two trivalent samarocene centers attached to the single growing polymer chain. 

It has been suggested that the amine radical cation (46) is not directly involved in initiating chains and that most polymerization is initiated by benzoyloxy radicals.179 However, Sato et a ." employed spin trapping (3.5.2.1) to demonstrate that the anilinomethyl radical (45) was formed from the radical cation (46) by loss of a proton and proposed that the radical 45 also initiates polymerization. Overall efficiencies for initiation by amine-peroxide redox... 

In ATRP, the initiator (RX) determines the number of growing chains. Ideally, the degree of polymerization is given by eq. 7 and the molecular weight by cq. 8. Note the appearance of the initiator efficiency (/ ) in the numerator of these expressions. In practice, the molecular weight is ofien higher than anticipated because the initiator efficiency is decreased by side reactions. In some cases, these take the form of heterolytic decomposition or elimination reactions. Further redox chemistry of the initially formed radicals is also known. The initiator efficiencies are dependent on the particular catalyst employed. 

Although benzoyl peroxide will initiate the polymerization (by a radical chain reaction) of either styrene or acrylonitrile, -methoxy- -nitrobenzoyl peroxide will not initiate polymerization efficiently in the latter monomer because it is too rapidly destroyed by the polar decomposition. Acrylonitrile, but not styrene, causes the polar decomposition to predominate, and the intermediates of the polar decomposition are not catalysts for the polymerization of acrylonitrile. 

The rate constant /c, contains a factor that allows for the efficiency of initiation not all the radicals generated by the initiator are capable of starting polymer chains, some are lost by combination or other reactions. The initiator efficiency is defined as the ratio of the number of initiator molecules that start polymer chains to the number of initiator molecules decomposed under the given conditions of the polymerization. With most radical initiators the efficiency lies between 0.6 and 0.9 it also depends on the nature of the monomer. 

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

Current views on polymerization of acrylonitrile in homogeneous solution are illustrated by a description of the reaction in N,N-dimethyl-formamide (DMF) as initiated by azobisisobutyronitrile (AIBN) at about SO to 60°. Primary radicals from the decomposition of AIBN react with monomer to start a growing chain. About one-half of the primary radicals are effective, the others being lost in side reactions not leading to polymer. Bevington and Eaves (32) estimated initiator efficiency by use of AIBN labelled with C-14, whereas Bamford, Jenkins and Johnson (13) used the FeCls termination technique. Both of these methods require that the rate of AIBN decomposition be known, and the numerical value of this rate has undergone a number of revisions that require recalculation of efficiency results. From recently proposed rate expressions for AIBN decomposition at 60° (22, 136) one calculates an efficiency of about 40% by the tracer technique and 60—65% by the FeCl3 method. 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

In contrast, recent kinetic investigation of the polymerization of spacerless G2 dendron-substi-tuted styrene and methylmethacrylate, respectively, in solution lead to the unexpected conclusion that above a certain critical monomer concentration a strong increase in the rate of the free radical polymerization is observed [21]. The results can be explained by self-organization of the growing polymer chain to a spherical or columnar superstructure in solution, depending on the degree of polymerization (DP, Fig. 2). The rate constants and low initiator efficiency lead one to conclude that the self-assembled... 

If equal concentrations of acrylonitrile and methyl methacrylate were each polymerized at 60°C with equal concentrations of the same initiator, which polymer would have the higher DP and by how much Assume polyacrylonitrile undergoes termination only by radical combination and poly(methyl methacrylate) by disproportionation, that no chain transfer occurs, and that initiator efficiencies are the same in both reactions. (Use Table 4-1 below.)... 

The initiation efficiency depends on the relationship between the rate of macroradical formation and that of initiation. In Table 1 are compiled results on certain redox systems employed for the initiation of cellulose grafting either by direct oxidation of cellulose (or its derivative) or chain transfer from active low molecular weight radicals. Table 1 indicates that in systems where the matrix acts as a reducing agent (1st group), the initiation efficiency does not exceed 15%, i.e. only a minor portion of macroradicals formed at the first stage of oxidation initiates graft polymerization while the rest is oxidized to stable... 

Moreover, the polymers based on benzoin moieties are less active than the corresponding benzoin methyl ether derivatives. The greater reactivity of the polymeric initiators has been attributed by the authors to their role as chain terminators. In fact, the polymer chain may well shield the polymer-bound substituted benzyl radicals, thereby diminishing their termination efficiency... 

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