Radical chain transfer process

A radical chain transfer process for grafting has an initiating step ... 

The success of using the PDMS macroinitiator approach for the synthesis of polydimethyIsiloxane-vinyl monomer block polymers is dependent on two additional considerations. The first of these involves the control of free radical chain transfer processes. If substantial chain transfer to polymer occurs during the free radical polymerization, the block polymers which are formed may be branched rather than linear. Similarly, chain transfer to monomer or to solvents would result in the formation of homopolymers. We have sought, therefore, to minimize these possible sources of chain transfer by selection of those particular monomers and solvents which have low chain transfer constants and by careful purification of all the reagents which were used in the polymerization. A second factor which must be considered is the effect of the mode of termination on the microstructure of the final block polymer. Scheme 2 displays these effects schematically. 

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

In this section, the reactions undergone by radicals generated in the initiation or chain transfer processes are detailed. Emphasis is placed on the specificity of radical-monomer reactions and other processes likely to take place in polymerization media under typical polymerization conditions. The various factors important in determining the rate and selectivity of radicals in addition and... 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

Other chain transfer processes may occur. For example, the radical may abstract an atom from along the backbone of a previously formed polymer molecule, and thus initiate the growth of a branch to the main chain. There can also be chain transfer to monomer, which in the nature of the polymerisation process must be a relatively rare phenomenon. However, it can occur infrequently and give rise to a restriction in the size of the polymer molecules without ceasing the overall radical chain reaction. 

The reaction of a chain radical with a unit of a previously formed polymer represents an additional possible chain transfer process not previously considered in Chapter IV. The point of attack might again be located in the substituent X, or it might involve removal of the tertiary hydrogen on the substituted chain carbon. The following sequence of reactions, in which the latter alternative has arbitrarily been assumed, would then lead to a branched polymer molecule as indicated. ... 

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

Carbon dioxide has also proven to be an exemplary medium for the polymerization of TFE with perfluorinated alkylvinyl ether monomers containing sulfonyl fluoride such as CF2=CF0CF2CF(CF3)0CF2CF2S02F (PSEVPE). As seen in Table 13.2, the dramatic difference in the number of acid end groups between the commercial sample and those made in C02 indicates that chain-transfer processes stemming from vinyl ether radical arrangement are not nearly as prevalent in C02 as in conventional systems. 

Initiation of graft copolymerization by radical mechanisms can occur by (a) a redox process on the substrate or (bl a chain transfer process to the substrate. In addition to grafting, formation of homopolymer may occur in both cases. 

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 chain fragments formed by the recombination of free radicals can be reconverted into radicals by a variety of reinitiation processes, some of which are listed in Table 1. Such reactions can occur in the gas phase via electron collision and on the polymer surface by impact of charged particles or photon absorption. Reinitiation may also be induced in both the gas phase and on the polymer surface by hydrogen transfer reactions. These last processes are similar to the chain transfer processes which occur during homogeneous polymerization. Expressions for the rates of reinitiation are given by Eqns. 20 through 23. 

A termination frequently encountered in many polymerizations results from a chain transfer process. In a radical polymerization such a reaction involves usually a transfer of a hydrogen atom and yields a radical which may or may not initiate further polymerization. The first alternative may be referred to as a proper chain transfer reaction, and such a transferring agent is known as a polymerization modifier. The second alternative is known as an inhibition or retardation of polymerization, the inhibitor or retarder being a substance which forms a stable radical, not sufficiently reactive in respect to the monomer, and therefore unable to initiate further polymerization. 

Another reported way to facilitate the chain transfer process is addition of the propagating radicals to allylic stannanes [286]. Because of their efficiency, such processes as these will undoubtedly be used more frequently by synthetic chemists in the future. 

Thus, for the first time, quantum-mechanical investigations have outlined the possibility of two-electron transfer in one stage by elementary reaction (5.12), which has expanded our knowledge of the properties of the H02 free radical, still relatively poorly studied, in radical-chain oxidation processes. 

If we apply the same reasoning to the second pressure limit as we did to the first one, we see that, since the explosion is quenched at constant composition and temperature by raising the total pressure, we must postulate a new chain-termination process which is of the same (i.e., first) kinetic order in radical concentration as the branching chain reaction and is at the same time higher than first order in total pressure. Such a reaction cannot be generalized. In the II2 + O2 system it seems to be satisfied by the chain-transfer process... 

In many free-radical polymerizations, the molecular weight of the polymer produced is lower than that predicted from Eq. (6-64). This is because the growth of macroradicals in these systems was terminated by transfer of an atom to the macroradical from some other species in the reaction mixture. The donor species itself becomes a radical in the process, and the kinetic chain is not terminated if this new radical can add monomer. Although the rate of monomer consumption may not be altered by this change of radical site, the initial macroradical will have ceased to grow and its size is less than it would have been in the absence of the atom transfer process. These reactions are called chain transfer processes. They can be classified as varieties of propagation reactions (Section 6.3.2). 

The ideal free-radical kinetics without chain transfer culminate in Eiqs. (6-64) and (6-65) in which termination of the growth of polymeric radicals is accounted for only by mutual reaction of two such radicals. Chain transfer can also end the physical growth of macroradicals, and the polymerization model will now be amended to include the latter process. This can be easily done by changing Eq. (6-62) to include transfer reactions in the rate of polymer production, <7[polymer]/[Pg.209]

The major product ethane is seen to be formed by abstraction by C2H5, and Polymer I, C H2 -2, by the chain-transfer process (6), the C H2 i radical having been built up by successive additions of radicals to C2H4 molecules. 

The methylene unit attached to the ethereal oxygen is particularly reactive towards radical attack and thus chain-transfer processes would be expected to substantially reduce the molecular weight of the final polymer. Some reduction is noticed but both this solvent and toluene can be used for free-radical polymerization (and note the successful polymerization of benzyl acrylate in Protocol 1). 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

These data indicate that the reactivity of the radical from styrene was rather weak and that, unlike vinyl chloride, the ENB content affected the chain transfer processes of this radical only slightly. This agrees with the low values of the transfer constant as measured in the styrene polymerization in the presence of a highly unsaturated polymer such as natural rubber (27). The magnitude of the transfer constants was not high, and this explains the rather low grafting yields obtained when the reaction occurred in the presence of excess monomer. 

The grafting reaction is determined primarily by chain transfer processes these decrease until they become negligible as the polymerization of excess vinyl monomer proceeds. Formation of the vinyl polymer causes separation of the elastomer from the initial system and a gradual decrease in the amount that participates in the radical activity of the reacting mass. 

It can be seen from this scheme that in chain transfer, radicals are neither created nor destroyed. Consequently, the overall polymerization rate is unaffected by chain-transfer processes. However, as we shall see presently, it does limit the obtainable molecular weight. 

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