Chain transfer reaction measurements

Such an analysis is complicated by the fact that radicals can also terminate chains, in which case there would be two phenyl groups in such a polymer. This can be shown to be a minor error if the rate of initiation is as fast as the rate of propagation and the chain length is large. A more serious source of error arises from chain transfer reactions, which we shall discuss later. In principle these can be measured and allowed for. 

To give a specific example, the advantages of styrene as a substrate for peroxyl radical trapping antioxidants are well known" (i) Its rate constant, kp, for chain propagation is comparatively large (41 M s at 30 °C) so that oxidation occurs at a measurable, suppressed rate during the inhibition period and the inhibition relationship (equation 14) is applicable (ii) styrene contains no easily abstractable H-atom so it forms a polyper-oxyl radical instead of a hydroperoxide, so that the reverse reaction (equation 21), which complicates kinetic studies with many substrates, is avoided and (iii) the chain transfer reaction (pro-oxidant effect, equation 20) is not important with styrene since the mechanism is one involving radical addition of peroxyls to styrene. 

It is often observed that the measured molecular, weight of a polymer product made by free-radical chain polymerization is lower than the molecular weights predicted from Eq. (6.102) for termination by either coupling [Eq. (6.103)] or disproportionation [Eq. (6.104)]. Such an effect, when the mode of termination is known to be disproportionation, can be due to a growing polymer chain terminating prematurely by transfer of its radical center to other species, present in the reaction mixture. These are referred to as chain transfer reactions and may be generally written as... 

In view of the unusual mechanism of anionic polymerization, especially the absence of termination and chain transfer reactions, the kinetics of these systems can be treated quite differently than for the other mechanisms. Thus it is possible, by suitable experimental techniques, to examine separately the rates of the initiation and propagation reactions [172,173], since the stable organometallic chain ends are present in concentrations [10 -10 M] which are easily measured by ultraviolet-visible spectroscopy [174]. The propagation reaction is, of course, of considerable main interest and can be studied by making sure that initiation is complete. In this way, the kinetics of homogeneous anionic polymerization have been extensively elucidated with special reference to the nature of counterion and role of the solvent. 

The measured average molecular weights, eg, obtained from molecular weight distributions, of macromolecules generated by free radical polymerization processes are often lower than those predicted by accounting for initiation, propagation, and termination processes. This experimental observation can be attributed to chain stopping events via chain transfer reactions (163,164). The transfer reaction can be described via equations 22 and 23. 

The ideal chain transfer reaction does not alter the overall rate of polymerization, Rp. However, if the chain transfer reaction has a measurable effect on i p, this rate depends on the size of the propagation, reinitiation, and transfer rate coefficients. That is, the free radical which is newly formed by the transfer... 

As a numerical value for c, we assume value 53, derived in research [4] on the base of Eq. (12), at immediate experimental measurement of monomer and low-molecular RAFT-agent conversions. Since chain transfer reaction in RAFT-polym-erization is usually characterized by low value of activation energy, compared to activation energy of chain growth, it is supposed that constant C j doesn t depend or slightly depends on temperature. We will propose as an assumption that Ctt, doesn t depend on temperamre [ 10]. 

It was believed for a long time that head-to-head radical addition to monomers is a major route for formation of labile structures. Kinetic studies, in association with NMR measurements, reveal that formation of internal allylic and tertiary chlorine structures actually proceeds through an intramolecular or intermolecular chain-transfer reaction to polymer [Eqs. (31), (32) VC = vinyl chloride]. 

Content of prime - tertiary peroxide groups was measured by the quantity of products of complete decay, which were measured by chromatography. It is known that the main contents in products of the complete decay of Oct-MA-TBPMM samples are acetone and 2,2-dimethylpropanol, which arise in reactions of chain fragmentation of tert-butylperoxy radical or in reaction of chain transfer of this radical. In this case the sum of acetone and 2,2-dimethylpropanol molecules is equal to the quantity of peroxide groups in polymer. As an internal standard we used chloroform. 

The rate constants for chain transfer and propagation may well have a different dependence on temperature (i.e. the two reactions may have different activation parameters) and, as a consequence, transfer constants are temperature dependent. The temperature dependence of Clr has not been determined for most transfer agents. Care must therefore he taken when using literature values of Clr if the reaction conditions are different from those employed for the measurement of Ctr. For cases where the transfer constant is close to 1.0, it is sometimes possible to choose a reaction temperature such that the transfer constant is 1.0 and thus obtain ideal behavior. 3... 

Morton and Salatiello have deduced the ratio kpp/kp for radical polymerization of butadiene by applying the above described procedure, appropriately modified for the emulsion system they used. The primary molecular weight was controlled by a mercaptan acting as chain transfer agent, as in the experiments of Bardwell and Winkler cited above. Measurement of the mercaptan concentration over the course of the reaction provided the necessary information for calculating % at any stage of the process, and in particular at the critical conversion 6c for the initial appearance of gel. The velocity constant ratios which they obtained from their results through the use of Eq. 

Table 2. Intramolecular crosslinking of PVS [217], Reaction conditions PVS concentration = 0.975 mass % AIBN concentration = 1.65X10 3 M temperature = 70 °C n-butylmercaptan (chain transfer agent) concentration = 20 mL/L reaction time = 25 min. The Mw and Mn were measured by light scattering and membrane osmometry respectively.

In aprotic solvents, chain transfer occurs exclusively by fl-H elimination, unless a protic acid or water is present in the reaction mixture, in which case protonolysis may occur. Indirect evidence (for example, M, and M measurements) proves that P-H chain transfer in aprotic solvents is slower than methanolysis in protic solvents with comparable structures of the Pd" catalyst [5f, 17, 20, 21]. This effect and the possibility of using well-defined catalysts have remarkably favored the use of in situ NMR spectroscopy for the detection of intermediates during CO/copolymerisation in organic solvents. 

The PLP-SEC method, like the rotating sector method, involves a non-steady-state photopolymerization [Beuermann, 2002 Beuermann and Buback, 2002 Komherr et al., 2003 Nikitin et al., 2002], Under pulsed laser irradiation, primary radicals are formed in very short times ( 10 ns pulse width) compared to the cycle time ( 1 s). The laser pulse width is also very short compared to both the lifetimes of propagating radicals and the times for conversion of primary radicals to propagating radicals. The PLP-SEC method for measuring kp requires that reaction conditions be chosen so that no significant chain transfer is present. The first laser pulse generates an almost instantaneous burst of primary radicals at high... 

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