Polymerization-chain transfer process

Hexenylsilane was introduced as a co-monomer into the organotitanium-mediated polymerization of ethylene to produce silane-terminated ethylene-5-hexenylsilane copolymers. High activities and narrow polydispersities were observed in the polymerization-chain transfer process. Ethylene-5-hexenylsilane copolymer molecular weights... 

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]. 

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... 

Minor (by amount) functionality is introduced into polymers as a consequence of the initiation, termination and chain transfer processes (Chapters 3, 5 and 6 respectively). These groups may either be at the chain ends (as a result of initiation, disproportionation, or chain transfer,) or they may be part of the backbone (as a consequence of termination by combination or the copolymerization of byproducts or impurities). In Section 8.2 wc consider three polymers (PS, PMMA and PVC) and discuss the types of defect structure that may be present, their origin and influence on polymer properties, and the prospects for controlling these properties through appropriate selection of polymerization conditions. 

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. 

Ethylene pressure studies have revealed a first-order dependence on ethylene for both the rate of chain propagation and the rate of chain transfer. This polymerization behavior together with X-ray analyses and DFT calculations has provided strong support for (1-11 transfer to an incoming monomer, which is responsible for the production of vinyl-terminated PEs. The calculations thus suggest that the catalysts disfavor (I-11 transfer to the Zr metal because of the extreme instability of the Zr hydride species that is produced in such a chain transfer process. 

The reasons behind this accelerated rate behavior have been attributed to a decrease in chain transfer processes (28,29) and a decreased termination rate (24,25) indicated by molecular weight measurements (26). Recently, direct evidence of decreases in the termination rate have been shown (27) and in these studies both the termination and propagation kinetic constants were determined for polymerizations exhibiting enhanced rates in a smectic phase. The propagation constant, kp, decreases slightly in the ordered phase from the isotropic polymerization. Such a decrease would be expected because of the lower temperature in the smectic phase. The termination kinetic constant, kt, however, decreases almost two orders of magnitude for the ordered polymerization, indicating a dramatically suppressed termination rate. 

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. 

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. 

In immortal polymerization, a reversible chain transfer reaction takes place much more rapidly than a propagation reaction, thereby uniformity of the molecular weight of polymer is realized. The successful high-speed immortal polymerization assisted by a Lewis acid, mentioned above, suggests that not only the propagation step (Scheme 7) but also the chain transfer process (Scheme 8) is accelerated by a Lewis acid. 

Determinations of aluminum have been carried out on fractions of polymeric product containing isotactic chains deriving from the polymerization. These measurements were performed in an attempt to establish whether the chain transfer process, depending on the alkylaluminum concentration, leads to the formation of macromolecules which remain bound to the aluminum. The polymer has been therefore purified by physical methods from the unreacted ethylaluminum and from the heterogeneous catalyst. 

The above-recorded chain transfer processes and the processes of exchange between alkylaluminum in solution and alkylaluminum bound to the catal34 ic complex could also be effected by soluble polymeric alkylaluminum of the type (C2Hb)A1P2 or (C2H6)2A1P (where P = polymeric chain). This... 

In any case, such chain transfer processes lead to a continuous increase of the molecular weight of the alkylaluminum compounds during the polymerization. It is, however, possible that alkylaluminum molecules having a low molecular weight are regenerated by a mechanism similar to those reported in the study of the kinetic behavior of ethylene polymerization, in the presence of trialkylaluminum 36) or through a dissociation to a hydride ... 

If we assume a priori, as will be proved in a further paragraph, that each simple chain transfer process whose rate depends on the catalyst concentration is followed by the insertion in the polymeric chains of —CjHs groups (deriving from triethylaluminum), the rate of the chain transfer process, depending on the amount of Ti, then would be... 

The dependence of the rate of the considered chain transfer process on the olefin pressure may be justified if we assume that such a process requires an activation state of the solid catalyst. Such an activation state would correspond to the activated intermediate complex which is formed during the polymerization process at the particular stage in which a monomeric unit bounds itself to the catalytic complex. As a result, the rate of the transfer process would depend on the rate of the chain-growing process, since the two processes (growing and chain transfer) would be considered as parallel and deriving from the same activated complex. 

The chain transfer process whose rate depends only upon the alkylaluminum concentration (or upon the zinc-diethyl concentration) is, on the contrary, independent of the partial pressure of the olefin and may occur even if the polymerization stops for lack of monomer. 

Operating at low temperature (<80°), ki can be neglected with respect to k Pc st Therefore, the relationship (29) is equal to the ratio between the number of polymeric chains interrupted by the chain transfer processes, depending on the catalyst concentration, and the number of polymeric chains interrupted by the chain transfer with monomer. 

Fig. 33. Values of the ratio between the rate of the chain-transfer processes depending on the catalyst concentration and the rate of the chain-transfer process with the monomer (plus the rate of the spontaneous termination process) (I = 70°, pc,ut = 950 mm. Hg, ground a-TiCU sample A). The values were calculated assuming for the isotactic polymeric fraction x = Kih] = KiC. ...

From the above summarized data, it follows that, during the polymerization, in the presence of triethylaluminum, there is a consumption of A1 atoms and ethyl groups bound to the aluminum, because the triethylaluminum is involved in chain transfer processes without being regenerated. Therefore, the polymerization process is now thoroughly catalytic only with... 

This is the same reaction discussed in the termination section (equation 22) but becomes a transfer and not a termination reaction when the MtXn formed is capable of generating new chains by itself. Such a chain transfer process has not yet been clearly demonstrated but might be possible in the case of SbClJ or PF gegenions since SbCl5 and PFS initiate polymerization without added promotor. 

When chain transfer occurs, xn << vn, and the molecular weight of the resulting polymer will be lowered by the chain transfer process. If the rate of the chain transfer addition process, ka, is less than the propagation rate kp then the overall polymerization rate Rp will decrease as a result of chain transfer. This is sometimes called degradative chain transfer. If ka << kp, then the degradation can become so severe as to result in inhibition (Fig. 2). 

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. 

The process of chain transfer has received very little quantitative study insofar as the anionic systems are concerned. The first study of an anionic chain transfer process was that of Robertson and Marion 274) on the polymerization of 1,3-butadiene by sodium in toluene. The reaction of toluene with the sodium active center led to the formation of benzyl sodium. This work was the first to demonstrate the important role of solvent in transfer reactions involving anionic active centers ... 

Gatzke 278) has investigated the chain transfer process involving toluene and poly(styryl)lithium at 60 °C. A relationship between the number-average degree of polymerization and the transfer constant was derived ... 

Low molecular weight polybutadienes of various mixed microstructures are prepared281 commercially via an anionic chain transfer process. These polymerizations use toluene as the solvent and transfer agent and lithium as the counter ion. The transfer reactions is promoted by the use of diamines, e.g., tetramethylethylene-diamine, or potassium t-butoxide. The preparation, modification, and applications of these materials has been described by Luxton281). 

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). 

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