Chain termination by disproportionation

The mechanisms of chain termination by disproportionation of secondary and tertiary peroxyl radicals are sufficiently different (see Chapter 2). Secondary R02 disproportionate by reaction [4-6]... 

The rate constants of chain termination by disproportionation of two acylperoxyl radicals are collected in Table 8.4. 

The similarity between polymers and homologous low molecular weight compounds may be sometimes disturbed on account of the presence of some highly reactive sites, e.g. unsaturated end groups due to chain termination by disproportionation (a) or to chain transfer with monomer (6) ... 

The value of X IXn for the cumulative polymermsy, of course, be much higher, depending on the changes in the value ofp with increasing conversion. It should be noted, however, that this is valid only where the growing chains terminate by disproportionation or transfer, not by combination. It can be shown in the latter case (Flory, 1953f) that the increment distribution is much narrower, i.e.. 

Two growing chains can combine to form one long polymer molecule (termination by combination) or a chain radical abstracts a proton from a neighboring group in another chain end, resulting in two nongrowing chains (termination by disproportionation). A third mechanism is chain transfer, where an active chain transfers its radical to a monomer molecule. The chain stops growing and the monomer molecule acts as the start of a new chain. 

Reaction with unsaturated polymer chains Termination by disproportionation, transfer to monomer, and chain scission (discussed later in this section) create polymer chains with terminal unsaturation (denoted in Eq. (30) by D"). These reactive chains, sometimes called macromonomers, can add to a growing radical to form a long-chain branch. 

There are different modes of termination combination and disproportionation. Active chains terminated by disproportionation will have the same molecular weight, where one of the chains will have an unsaturation and the other will be fully saturated. When chains are terminated by combination, because two propagating chains combine, the number of chains decreases by one, and the resultant molecular weight is the sum of the two macroradicals, thereby increasing the final molecular weight distribution. 

Chain termination by disproportionation. In this case or with chain transfer (not to polymer ) the probability of an i-mer = the mole fraction of i-mer is given by the Flory distribution ... 

Termination of free-radical chain polymerization may also take place by disproportionation. The description for chain termination by disproportionation is given in Eq. (9). The kinetic chain length (v) is the number of monomer molecules consumed by each primary radical and is equal to the rate of propagation divided by the rate of initiation for termination by disproportionation. The kinetic equation for the termination by disproportionation is... 

The degree of polymerization in Eq. (6.41) can be replaced with the kinetic chain length, and the resulting expression simplified. To proceed, however, we must choose between the possibilities described by Eqs. (6.34) and (6.35). Assuming termination by disproportionation, we replace n, by v, using Eq. (6.37) ... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

Chains terminate by either of two mechanisms combination or disproportionation. Two chain radicals may combine to form a single bond between... 

The minimum polydispersity index from a free-radical polymerization is 1.5 if termination is by combination, or 2.0 if chains ate terminated by disproportionation and/or transfer. Changes in concentrations and temperature during the reaction can lead to much greater polydispersities, however. These concepts of polymerization reaction engineering have been introduced in more detail elsewhere (6). 

Which mechanism of termination will be preferably applied depends largely on the monomer used. Thus, methyl methacrylate chains terminate to a large extent by disproportionation, whereas styrene chains tend to termination by combination. The ratios of termination rate constants 8 = ktJkic (for disproportionation, td, combination,, c) are 5 == 0 and 5 = 2 for styrene [95] and methyl methacrylate [96], respectively. In the case of styrene, however, the values of 8 reported in the literature are at variance. Berger and Meyerhoff [97] found 8 = 0.2, at 52°C. Therefore, it is possible that a fraction of styrene terminates by disproportionation. 

Chains with uttdesired functionality from termination by combination or disproportionation cannot be totally avoided. Tn attempts to prepare a monofunctional polymer, any termination by combination will give rise to a difunctional impurity. Similarly, when a difunctional polymer is required, termination by disproportionation will yield a monofunctional impurity. The amount of termination by radical-radical reactions can be minimized by using the lowest practical rate of initiation (and of polymerization). Computer modeling has been used as a means of predicting the sources of chain ends during polymerization and examining their dependence on reaction conditions (Section 7.5.612 0 J The main limitations on accuracy are the precision of rate constants which characterize the polymerization. 

Note, however, that chain ends 4 and 5 may give different chemistry to those formed in termination by disproportionation (2, see Scheme 8.5) or the processes under (a) above. Chain scission (3 to the double bond will not lead to a MMA propagating species. It is not established whether the presence of these ends will give impaired thermal stability. 

Figure 9.2 Calculated (a) number and (b) GPC distributions for three polymers each with =100. The number distributions of chains formed by conventional radical polymerization with termination by disproportionation or chain transfer...

In the absence of any transfer, only the type of termination reaction has to be taken into account to obtain the number average degree of polymerisation (or chain length) Xn. Indeed, if the reaction terminates by addition, two radicals give one chain, while if the reaction terminates by disproportionation, one radical generates one chain. Using 0.5 < x < 1, one obtains... 

Rate Constants of Chain Termination by the Disproportionation of Tertiary Peroxyl Radicals in Hydrocarbon Solutions (Experimental Data)... 

When the termination involves only combination, the polymerization gives a polymer with two initiator fragments at its chain ends. Because termination in the bulk polymerization of St with AIBN at a moderate temperature occurs by combination, the polymer obtained has two initiator fragments at both chain ends. In the radical polymerization of most monomers, however, termination by disproportionation and chain transfer reactions occur it is therefore impossible to control these termination reactions, i.e., the chain-end structure. Therefore, the number of initiator fragments per one molecule is always less than two. 

The kinetic chain length for termination by disproportionation is DP = v (compared with the relationship for coupling of DP = 2v. The extent of the two types of termination is experimentally found by determining the number of head-to-head sites [coupling] and unsaturated end groups [disproportionation]). 

For termination by disproportionation (Eq. 3-16b) the kinetic chain length is synonymous with the number-average degree of polymerization... 

Consider the situation where one polymer molecule is produced from each kinetic chain. This is the case for termination by disproportionation or chain transfer or a combination of the two, but without combination. The molecular weight distributions are derived in this case in exactly the same manner as for linear step polymerization (Sec. 2-7). Equations 2-86, 2-88, 2-89, 2-27, 2-96, and 2-97 describe the number-fraction, number, and weight-fraction... 

---