Macroradicals, disproportionation

Destruction of macroradicals—scission of kinetic chains. A disproportionation reaction is most common at this stage ... 

The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

Transfer of the free radical to another molecule serves as one of the termination steps for general polymer growth. Thus, transfer of a hydrogen atom at one end of the chain to a free radical end of another chain is a chain transfer process we dealt with in Section 6.2 under termination via disproportionation. When abstraction occurs intramolecularly or intermolecularly by a hydrogen atom some distance away from the chain end, branching results. Each chain transfer process causes the termination of one macroradical and produces another macroradical. The new radical sites serve as branch points for chain extension or branching. As noted above, such chain transfer can occur within the same chain as shown below. 

The termination of the reaction can take place in two ways recombination (3) of two macroradicals Mn and Mp forming a macrochain Mll+p or disproportionation (4) yielding a double bond Mn = and a C-H bond at the chain terminus MpH. 

If the effect of co-agents on crosslinking efficiency is just the suppression of macroradical side reactions, such as chain scission and disproportionation, one should expect monofunctional co-agents to be as effective as their multi-functional analogues (if compared at the same molar level of unsaturation). This is definitely not the case, as will be demonstrated. 

FT-IR results also showed that one new (small) absorption at 1659 cm"1 appeared, which could not be attributed to peroxide decomposition products. This absorption also appeared when the peroxide-curing experiments were carried out using an amorphous EPM, indicating that the absorption did not relate to rearrangement of the third monomer moiety (ENB in this case). It is tentatively concluded that the absorption at 1659 cm 1 is related to EPDM main-chain modifications, resulting from disproportionation reactions of EPDM macroradicals with BHT radical fragments. 

The processes of macroradical decay have been studied in high vacuum and in oxygen. The main consequence of the mechanism of the process is that macroradicals are able to eliminate hydrogen atoms from the adjacent chains and thus transfer to them the radical state. The radical state is transmitted further on by a series of hydrogen elimination acts. Thus it migrates in the polymer until it collides with another radical state. This may result in the disappearance of both radicals by recombination or by disproportionation. 

Initiation is followed by the propagation step of the reaction, when monomer molecules are rapidly added, step by step. In a fraction of a second, the macroradical reaches 2000-10000 monomer units. On the addition of each monomer unit the active site moves to the end of the growing polymer chain. The last step, termination, can act as recombination of two growing polymer chains (macroradicals) or by disproportionation, when hydrogen is transferred from one chain to another. 

It is seen that free radical micromolecular or macromolecular initiators have been successfully employed for the synthesis of di-, tri- or multiblock copolymers. However, once again, the structure of these block copolymers depends upon the termination step of the polymerization, and especially on the recombination or disproportionation of macroradicals produced. Besides, such a method also generates homopolymers. Separation and purification of these different structures are usually very difficult or even impossible. Moreover, the copolymers obtained usually exhibit a broad polydispersity, a defect inherent in the classical radical process. 

The macroradicals C undergo termination, mostly by disproportionation. The polymeric chain renders the complex soluble in non-polar solvents (hexane). According to the authors, the active centres have the form [214]... 

Thus f, (m) is the (unnormalized) length distribution of inactive chains formed by disproportionation, particularly in systems where disproportionation represents an exclusive or predominating termination mechanism. f2(m) corresponds to the (unnormalized) length distribution of macroradicals. 

By means of the latter relations, kinetic schemes can be completely solved, mean degrees of polymerization can be derived, as well as the polydispersity coefficients of polymers terminated by disproportionation (= Sf +1 V p) and of macroradicals or inactive macroradicals after transfer (= S + 1 /S l). For the number, weight, and z average, k = 0, 1, and 2, respectively. 

For ideal radical polymerization to occur, three prerequisites must be fulfilled for both macro- and primary radicals, a stationary state must exist primary radicals have to be for initiation only and termination of macroradicals only occur by their mutual combination or disproportionation. The rate equation for an ideal polymerization is simple (see Chap. 8, Sect. 1.2) it reflects the simple course of this chain reaction. When the primary radicals are deactivated either mutually or with macroradicals, kinetic complications arise. Deviations from ideality are logically expected to be larger the higher the concentration of initiator and the lower the concentration of monomer. Today termination by primary radicals is an exclusively kinetic problem. Almost nothing has been published on the mechanism of radical liberation from the aggregation of other initiator fragments and from the cage of the... 

It is believed that most macroradicals terminate in free-radical polymerizations predominantly or entirely by combination. Experimental measurements of polymer systems are scanty, however. It can be expected that disproportionation will be... 

We first consider the polymerization where each kinetic chain yields one polymer molecule. This is the case for termination of the growth of macroradicals by disproportionation and/or chain transfer (A,c = 0). The situation is completely analogous to that for linear, reversible step-growth polymerization described in Section 5.4.3. If we randomly select an initiator residue at the end of a macromolecule, the probability that the monomer residue which was captured by this primary radical has added another monomer is S and the probability that this end is attached to a macromolecule which contains at least i monomers is S . The probability that this macromolecule contains exactly i monomers equals the product of 5 and the probability of a termination or transfer step. The latter probability must be equal to (I — S) since it is certain that the last monomer under consideration will undergo one of these three reactions. That is, the probability that a randomly selected molecule contains t monomer units is 5 (l — S). Since such probabilities are equal to the corresponding mole fraction of this size molecule, jc,, we have the expression... 

Thus, we must consider those reactions (of termination of macroradical with primary radical (Eq. (9)) which have not necessarily the same reactivity ratio, as the termination of two macroradicals. And this makes it often possible to obtain telechelic oligomers for monomers which give a non-negligible amount of disproportionation reactions in traditional polymerization. 

The main point of concern in this synthesis is the termination reaction. Only the combination reaction is allowed. Termination in polymerization is mostly the reaction of two macroradicals. In the formation of telechelics, the reaction with primary radicals cannot be ignored. The ratio of combination to disproportionation reactions may be different in both cases. Even if primary radicals react with each other by combination and macroradicals react by combination, the cross-reaction can be a complete disproportionation ( ). 

Termination, which takes place either in a bimolecular mode, involving the coupling of two primary radicals, or by disproportionation of the primary macroradicals. 

Termination typically occurs by coupling of two macroradicals (Equations 15 and 16) or through disproportionation (Equations 17 and 18)... 

Recombination (dimerization) reaction of macroradicals may occur parallel to disproportionation in which alkyl radicals decay via transfer of a hydrogen atom from one P-carbon of one of the two radicals to the carbon of the second radical caiying a radical site. 

Unsaturated bonds in polyethylene are not the product of the photofragmentation of macroradicals alone but also of their disproportionation. The disappearance of C=C bonds corresponds to addition reactions of radicals with multiple carbon-carbon bonds. Addition reactions increase the quantum yield of crosslinks when related to that of formation of free radicals. 

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