Consequences of Chain Transfer

The primary effect is a decrease in the polymer chain length, but other less obvious occurrences can be detected. If is much larger than p, then a very small polymer is formed with x between 2 and 5. This is known as telomerization. The chain reinitiation process can also be slower than the propagation reaction, and a decrease in Vp is observed. However, the influence on3C is most important, and it can be estimated by considering all the transfer processes in a form known as the Mayo equation  

This is a simplified form in which the main assumption is that solvent transfer predominates and all other terms are included in l/xj. The chain transfer constant Cs is then (Klkj,). 

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The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

Polyethylenes obtained by free-radical polymerization have highly branched structures as a consequence of chain-transfer reactions (see eq. 3.42 and the structure below it). Ziegler-Natta polyethylene is mainly linear (CH2CH2)/7- It has a higher degree of crystallinity and a higher density than the polyethylene obtained by the free-radical process. 

The commercial production of LDPE is by free-radical polymerization. Supercritical bulk ethylene is fed into a tubular reactor operated at steady state. The polymers experience both short-and long-chain branching. The short-chain branches are a consequence of backbiting and the long-chain branches are a consequence of chain transfer to polymers. The low density of the product is a consequence of short-chain branching. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

CD and negative-Cotton CD intensities were greatly increased after thermal annealing. The enhancement in the CD signal intensities is a consequence of chirality transfer to 32 from the chiral side chain of 10 and/or helical main chain itself and is drastically amplified after annealing. Contrarily, minimal... 

One normatty assumes that systems such as styrene and methyl methacrylate, where transfer to monomer is not prominent, follow Case 2 kinetics when latex particles are small and termination in polymer particles is instantaneous. It has recently been shown that at low initiation rates radical desorption can be significant relative to radical absorption, and as a consequence w values appreciably smaller than 0.5 were found (Gilbert et al., 1980). At higher initiation rates n = 0-5 was approached. The use of chain-transfer agents would of course increase the desorption rate and lower n. 

In the absence of chain transfer, the monomer concentration, Af, can be determined from Equation (7-43) and concentration of initiator, from Equation (7-40). Consequently we have 1 as a function of time. 

Hence a plot of jDP vs. 1/[M] for polymer samples prepared with constant [A] should be a straight line having ktr,i/i/kp as the intercept and kt A [A]/fcj> + k,/kp) as the slope. Similarly when 1/ >P is plotted vs. [A] for polymer samples prepared with constant [M], a straight line should be obtained having (A r>l/ + ks/kp [M]) as the intercept and kt A/kp [M] as the slope. Consequently all chain transfer constants can be evaluated as ratios of chain transfer rate constants... 

Exactly what drives the response to H2 is not clear. It may be associated with the presence of aluminum. The response to H2 seems to be different from that to the other two chain-transfer agents represented in Figure 163, which are clearly associated with phosphate in the catalyst. This difference may be a consequence of the different mechanism of chain transfer with H2, that is, hydrogenolysis versus p-hydride elimination. 

Impairment of mitochondrial ATP regeneration, as a consequence of electron transfer chain inhibition or of a decrease in activities of enzymes of energy metabolism, could enhance (O2)" production in mitochondria. This can also impair clearance of ROS due to less effective detoxification systems (loss of GSH), leading to increased influx of Ca + and peroxidation of lipids and proteins (Fig. 5). 

In the parallel investigation of systems where metal-ligand charge-transfer (MLCT) occurs, studies into transition metal complexes of the redox-active quinone ligand have unravelled crossover behaviour that accompanies electron transfer. This reversible process, known as valence tautomerism, has been observed with bistability in the solid state. ° Exotic photomechanical behaviours, such as the bending of crystals of ID chain materials with IR irradiation,have been attributed to the unique structural consequences of electron transfer within the solid. 

Chlorine radicals that are kinetically free are only fonned to a veiy limited extent Although a detailed discussion of the merits of the various mechanisms of chain transfer are outside the scope of this ctuqiter, the question of whether the radical resulting from chain transfer is a chlorine radical or the mudi less wato -soluble radical (4) is important for the discussion of the desorption of monomer radicals from particles to the aqueous phase. Another important consequence of the pronounced chain transfer is that the same molar mass is obtained with both emulsion and suspension routes of polymerization when carried out at the same temperature. 

Our results suggest that a very slight degree of chain transfer, and consequent loss from the surface, occurs after longer irradiation times, since rinsing with fresh monomer... 

Chain transfer reactions have higher activation energies than monomer insertions consequently, a change in temperature strongly affects the relative rate of chain transfer as compared to propagation. 

In solution polymerization, the monomer is dissolved in a solvent prior to polymerization. This technique is commonly employed for the ionic polymerization of gaseous vinyl monomers. The solvent facilitates contact of monomer and initiator (which may or may not be soluble in the solvent) and assists dissipation of exothermic heat of reaction. A limitation of this technique is the possibility of chain transfer to the solvent with consequent formation of low molecular... 

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