Effect of Chain Transfer

Case Relative Rate Constants for Transfer, Propagation, and Reinitiation Type of Effect Effect on Rp Effect on X  

4 kp kfr ka kp Degradative chain transfer Large decrease Large decrease 

The first term in the denominator denotes termination by a combination of coupling and disproportionation, and the other terms denote chain transfer by monomer, chain-transfer agent, and initiator, respectively. 

Consider now the simpler case when disproportionation does not occur (a = 1). Equation 3-105 becomes 

A chain-transfer constant C for a substance is defined as the ratio of the rate constant klr for the chain transfer of a propagating radical with that substance to the rate constant kp for propagation of the radical. The chain-transfer constants for monomer, chain-transfer agent, and initiator are then given by 

In many polymerization systems the polymer molecular weight is observed to he lower than predicted on the basis of the experimentally observed extents of temtination by coupling and disproportionation. This effect is due to the premature temtination of a growing polymer by the transfer of a hydrogen or other atom or species to it from some compound present in the system—the monomer, initiator, or solvent, as the case may be. These radical displacement reactions are termed chain-transfer reactions and may be depicted as 

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It is apparent from these reactions how chain transfer lowers the molecular weight of a chain-growth polymer. The effect of chain transfer on the rate of polymerization depends on the rate at which the new radicals reinitiate polymerization ... 

Figure 6.8 Effect of chain transfer to solvent according to Eq. (6.89) for polystyrene at 100°C. Solvents used were ethyl benzene ( ), isopropylbenzene (o), toluene (- ), and benzene (°). [Data from R. A. Gregg and F. R. Mayo, Discuss. Faraday Soc. 2 328 (1947).]...

The compound R X is a chain-transfer agent, with X usually H or Cl. The net effect of chain transfer is to kill a growing chain and start a new one in its place, thus shortening the chains. Mercaptan chain-transfer agents ate often used to limit molecular weight, but under appropriate conditions, almost anything in the reaction mass (solvent, dead polymer, initiator) can act as a chain-transfer agent to a certain extent. 

Figure 2.19. Effect of chain transfer solvents on the degree of polymerisation of polystyrene. (After...

Fig. 3-5 Dependence of the degree of polymerization of styrene on the polymerization rate. The effect of chain transfer to initiator is shown for t-butyl hydroperoxide (o), cumyl hydroperoxide ( ). benzoyl peroxide ( ), and azobisisobutyronitrile ( ) at 60°C. After Baysal and Tobolsky [1952] (by permission of Wiley-Interscience, New York).

Transfer to polymer cannot, however, be neglected for the practical situation where polymerization is carried to complete or high conversion. The effect of chain transfer to polymer plays a very significant role in determining the physical properties and the ultimate applications of a polymer [Small, 1975], As indicated in Chap. 1, branching drastically decreases the crystallinity of a polymer. 

In the product, there should be a ladder-type blocks linked by segments composed of p-cresyl methacrylate units. This type of structure was confirmed by IR and NMR spectrometry. However, by preparation of such copolymers with labeled end-groups (using radioactive AIBN), and by fractionating and radiometric analysis, it was shown that copolymers obtained are slightly branched. There is slightly more branch points than in the case of copolymers with styrene. It could be an effect of chain transfer reaction. 

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

We can tentatively conclude, therefore, that the effect of chain transfer is still making itself felt in the polymerization of vinyl caproate in spite of its low water solubility. Except at the lowest particle concentrations, chain transfer is important. The polymerization in these regions is midway betwen Case I and Case II. When variables are considered separately, there is some dependence of polymerization rate on particle concentration, and also some dependence on initiator concentration. In addition, at constant organic volume, while the rate of polymerization increases as the particle concentration increases (Rp oc 2V- ), the rate per particle decreases as the particles get smaller. This shows that transferred radicals are mainly trapped in the particles, but some diffuse out and can undergo termination with other growing radicals. 

Table I. Effect of Chain Transfer Agent on the Reduced Viscosity of the Copolymer0...

For an adiabatic reactor, we may be able to control temperature and conversion using the initiator feed flow (Fig. 4.38). Incomplete conversion introduces recycle streams for the monomer. Because of the effect of chain transfer agents, we often must be able to measure the feed compositions to the reactor. Further, we must know what the chain transfer agents do if they are dominant variables to have any chance of controlling the molecular weight. So our control will only be as good as our correlations or models. Hence in polymer reactors we often have to use what is basically steady-state control on the setpoints of the dominant variables to achieve many of the control objectives that de-... 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

In Section 7.1 the effects of chain-transfer on polymerization rate were not considered. There is, however, evidence from the dependence of rate on monomer concentration to show that initiation reactions in olefin polymerization are slower than propagation. If the rate equation... 

The effects of chain-transfer agents such as oc-tanethiol on copper-catalyzed polymerizations are similar to those on conventional radical polymerizations.295 These may also mean little difference among the growing species generated from the carbon—halo-... 

Since chain transfer stops growing chain, it always results in a lower molecular weight than would occur in its absence. The effect of chain transfer on the rate of polymerization varies, however, and depends on the relative rates of the transfer [Eq. (6.135)] and reinitiation [Eq. (6.136)] compared to that of the normal propagation reaction [Eq. (6.6d)]. Several possible situations that may be encountered are sumniarized in Table 6.8. These are all instances of chain transfer, but they are usually given different names, as shown, depending on the net effects on polymerization rate and molecular weight. 

Table 6.8 Effect of Chain Transfer on Polymerization Rate and Polymer Molecular Weight...

Fig. 9. Random chain scission initiation, initial most probable distribution [191. The effect of chain transfer on weight remaining against time. The relative weight remaining, Mx /M°, is plotted logarithmically for three ratios of zip length to initial d.p., (l/y)/x°. At each zip length ratio, curves for three values of the fraction of the initial weight loss rate due to chain transfer, F = kjRKkjR + ks), are plotted.

Fig. 11. End-group initiation, initial most probable distribution [19]. The effect of chain transfer on (a) the relative rate of weight loss, (dAf( ldt)l(dMt ldt)0, against conversion, and on (b) the relative d.p., x/x°, against conversion. Curves are plotted atone value of the ratio of initial zip length to initial d.p., (1/7° )/x° = 0.01, for several values of the initial transfer parameter times initial d.p., O0x° = (k R/ke)x0.

This irreversible process affects the polymerization only if the rate of ionization of RX (i.e., ion-generation) is slow relative to that of propagation if initiation is instantaneous or very rapid the effect of chain transfer to initiator is negligible [1, 24, 45]. If the rate of ionization of HX is high relative to that of propagation, chain transfer to HX may be neglected. Since HM X and RMnX (see Scheme la) can be reactivated by the quasiliving equilibrium (see Sect. 2.1, and Scheme 1), chain transfer to initiator is by no means a termination reaction [see also 43]. 

The second column, Presence or Absence of Events that Determine the Number of Polymer Chains (N), comprises 6 sub-columns and shows the 28 scenarios selected for examination. Among the very large number of theoretically possible scenarios we have selected for analysis only those which seemed to be closest to real-life situations. Specifically, we have examined the effects on N of fast and slow initiation (and in the case of slow initiation, the respective effects of slow ion-generation and slow cationation), the effect of initiation by adventitious protic impurity (i.e., HX = H20 ), and the effects of chain transfer to monomer, specifically both monomolecular or zero-order-in-monomer chain transfer and bimolecular or first-order-in-monomer chain transfer. The presence or absence of these effects is indicated by the symbols, 1 or 0 in column two. The organization of the 28 scenarios is as follows ... 

For example, the rate of initiation may be slow relative to that of propagation, in which case the MWD will tend to become broader. If ion-generation is slow, the effects of chain transfer to initiator and slow initiation may overlap and may not even be distinguishable. Chain transfer to monomer, initiation by protic impurity ( H20 ), etc., may be present which will further complicate the synthesis of uniform predetermined molecular weight polymers. 

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