Chain transfer step

Competition between the various species present in the reaction mixture such as monomer, solvent, and backbone for the growing polymer radical, which means that there is competition between chain growth and various chain transferring steps. 

Thiol-ene polymerization was first reported in 1938.220 In this process, a polymer chain is built up by a sequence of thiyl radical addition and chain transfer steps (Scheme 7.17). The thiol-ene process is unique amongst radical polymerizations in that, while it is a radical chain process, the rate of molecular weight increase is more typical of a step-growth polymerization. Polymers ideally consist of alternating residues derived from the diene and the dithiol. However, when dienes with high kp and relatively low A-, monomers (e.g. acrylates) are used, short sequences of units derived from the diene are sometimes formed. 

The influence of selectivity in the initiation, termination or chain transfer steps on the distribution of monomer units within the copolymer chain is usually neglected. Galbraith et a .u provided the first detailed analysis of these factors. They applied Monte Carlo simulation to examine the influence of the initiation and termination steps on the compositional heterogeneity and molecular weight distribution of binary and ternary copolymers. Spurting et a/.250 extended this... 

Mechanisms of thermal degradation of PVC, the structure of PVC and the stabilization of PVC have been the subject of many reviews. Those by Starnes,44 Endo45 and Ivan46 are some of the more recent. Defect structures in PVC arise during the propagation and chain transfer steps. As with PMMA, PVC formed by... 

The reaction of benzyl bromide with vinyltrimethylsilane was used for studying a general kinetics of addition under conditions of metal complex initiation (ref. 26). One of the crucial questions in this case is how the chain transfer step proceeds by "purely" homolytic mechanism (via benzyl bromide) ... 

Free-radical polymerization ideally follows the preceding sequence of reaction steps. However, there are also slow but important steps that complicate this simple model. These involve chain transfer steps. We assumed that the only termination involves two radical species reacting with each other to form a stable dead molecule,... 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

Unfortunately, there is no direct evidence for this kind of chain transfer step, since all kinetic studies have been carried out under conditions where j5-elimination would dominate. However, the statement by Wilke [30] that the Aufbau process produces chains of at most =100 units agrees well with the predicted average degree of polymerization of =70 units [7]. 

The kinetics of the zinc diisopropyl dithiophosphate-in-hibited oxidation of cumene at 60°C. and Tetralin at 70°C. have been investigated. The results cannot be accounted for solely in terms of chain-breaking inhibition by a simple electrow-transfer mechanism. No complete explanation of the Tetralin kinetics has been found, but the cumene kinetics can be explained in terms of additional reactions involving radical-initiated oxidation of the zinc salt and a chain-transfer step. Proposed mechanisms by which zinc dialkyl dithiophosphates act as peroxide-decomposing antioxidants are discussed. 

Propagation steps are the heart of any chain and generally fall into two classes atom or group transfer reactions and addition reactions to tr-bonds (or the reverse elimination). The rate of the chain transfer step is especially important in synthetic planning because, by fixing the maximum lifetime that radicals can exist, it determines what reactions will (or will not) be permitted. Termination steps are generally undesirable but are naturally minimized during chain reactions because initiation events are relatively uncommon. 

Competition between monomer, solvent, and backbone for the growing polymer radicals. To obtain grafts with linear branches and to suppress homopolymerization, the chain transfer step to the backbone polymer must be the favored process. 

After the amines, acid anhydrides constitute the next most commonly used reagents for curing epoxy monomers. The epoxy-acid reaction proceeds through a stepwise mechanism (Sec. 2.2.4) while the reaction of epoxides with cyclic anhydrides, initiated by Lewis bases, proceeds through a chain-wise polymerization, comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Table 2.25 (Matejka et al., 1985b Mauri et al., 1997). 

The reaction is often described as a living polymerization (Matejka et al, 1983), but neither the distribution of molar masses nor the observed gel conversion corresponds to a pure living mechanism (Mauri et al., 1997). Experimental observations may be explained by assuming the presence of a chain transfer step that regenerates the active initiator. This step deter-... 

Thus dismutation does not seem to be a possible explanation of our results. There should be no isotope effect on the preferred pathway for dismutation, and the resulting radical is very active. The pathway is also inadequate to explain the kinetics. We are left therefore where we started. The major chain transfer step is probably hydrogen abstraction from the vinyl group with the production of a stable vinyl radical. 

The benzylic free radical produced by the addition of the carbamoyl radical to the ethyl cinnamate molecule is more stable than the alternative radical alpha to the ester group. With such an orientation of addition to the a,p-unsaturated ester, this reaction should lead to derivatives of malonic acid. However, it has been found that the intermediate radical, being a stable benzylic free radical, fails to perform the subsequent abstraction of a hydrogen atom from formamide, and thus no chain-transfer step takes place. Instead of performing this step it favours the combination with a semi-pinacol radical, which is present in solution, to yield the hydroxy ester which subsequently lactonizes to give the major product of the reaction (67). 

For example, at 60°C, kp = 2300 and kt = 2.9 x 107. An estimate of kinetic chain lifetime, ie, the time from initiation to termination by reaction with another radical, is 1—2 s at 50°C and 4% per hour rate of polymerization. If there are five chain-transfer steps in the course of the kinetic chain, then a PVAc molecule forms in 0.2—0.4 s. Faster rates of conversion give shorter kinetic chain lifetimes in inverse proportion, but an increased percentage of conversion leads to longer chain lifetimes. At 75% conversion and at 60°C, the radical lifetime is ca 10 s. 

Mixtures of alkyl halides and Lewis acids are well-known initiating systems for the polymerizations of alkenes, and the mechanism suggested for these reactions by Kennedy [55] appears to be generally accepted (Scheme 9), although the importance of the chain transfer step from initiator has been questioned [56]. 

Many examples of such eliminations have now been seen for the f-block and for d metals. This type of /3-aIkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis. When it occurs the polymer chain terminates in a C=C bond (equation 2) and in certain cases the aUcene chain end can undergo reinsertion and get back into the polymer growth... 

The specific reaction rates in chain transfer are all assumed to be independent of the chain length. We also note that while the radicals R produced in each of the chain transfer steps are different, they function in essentially the same manner as the radical R in the propagation step to form radical J 2. 

A later study [26] on just ECH using BF3. Et2 0 showed broadly similar results. The ether further complicated the picture. The reactions were characterized by slow initiation and an important chain transfer step. The use of Et3 0" BF4 was also briefly investigated. However, appreciable rates were observed only at temperatures > 35°C. 

Pericyclic electrochemical reactions are increasingly developed. They involve chain reactions with a radical cation as chain transferring step or the generation of reactive dienophiles (see Chapter 22, Sec. V). Transition metal complexes are increasingly applied in electrochemistry as electrocatalysts for reductive carboxylation [47], acylation or alkylation [41], or activation [51]. 

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