Chain transfer, irreversible

If both addition and fragmentation arc irreversible the kinetics differ little from conventional chain transfer. In the more general case, the rate constant for chain transfer is defined in terms of the rate constant for addition and a partition coefficient which defines how the adduct is partitioned between products and starting materials (eq. 19). 

Since the dithiocarbatnyl end groups 8 are thermally stable but pholochemically labile at usual polymerization temperatures, only photo-initiated polymerizations have the potential to show living characteristics. However, various disulfides, for example, 9 and 10, have been used to prepare end-functional polymers37 and block copolymers38 by irreversible chain transfer in non-living thermally-initiated polymerization (Section 7.5.1). 

A number of interesting and non-obvious insights into molecular weight distributions can be gained from these simulations. For example, Fig. 3 demonstrates the effect of X on M IM as a function of C0 for irreversible chain transfer where C = 0. 

Fie. 3 Simulated M IM vs. conversion as a function of chain transfer constant for irreversible... 

Polymer with stable, polymerization-active sites formed by a chain polymerization in which irreversible chain transfer and chain termination are absent. 

While termination leads to the irreversible disappearance of an active center, chain transfer results in the growth of a second chain while the first one is terminated. Here, the active center is transferred to another molecule (solvent, initiator, monomer,...) where it is able to initiate further chain growth. The resulting dead polymer, on the other hand, can continue its growth only when activated in a subsequent transfer step. Because this re-activation in general does not occur at the terminal monomer unit but somewhere in the chain, branched or cross-linked products will result ... 

In this review the reactions terminating a growing chain are denoted as chain-terminating processes. We refer to the chain-terminating process with a reinitiation reaction as chain transfer, and to the process with an irreversible deactivation of propagating centers as chain termination. 

N is active only toward P, while P is active not only toward N and the monomer (propagation, Eq. (2.87)) but also toward P (irreversible bimolecular termination, Eq. (2.89)) and toward neutral molecules (chain transfer, Eq. (2.88)). When the last two reactions are unimportant compared with the first two, the system may be viewed as a living (controlled) polymerization. 

One of the most useful features of living polymerizations, which proceed in the absence of chain transfer to monomer and irreversible termination, is the ability to prepare block copolymers. Compared with living anionic polymerization the development of living cationic polymerization is rather re-... 

In each of the studies quoted in Table 23 increasing nAi/ Nd-ratios result in decreases of molar mass. Between these studies there is unanimous agreement that molar mass reduction is caused by chain transfer with the cocatalyst. Most of the studies quoted in Table 23 consider the transfer reaction as irreversible. Only Friebe et al. explain their results on the basis of a reversible transfer of living polybutadienyl chains between Nd and Al [178,179]. A comparison of chain transfer efficiencies between DIBAH and TIBA reveals that chain transfer is much less pronounced for TIBA (Sect. 4.5). For DIBAH chain transfer efficiency is 8-fold over that of TIBA and the substitution probability... 

Linear Increase ofMn with Monomer Conversion (no Irreversible Chain Transfer)... 

In summary, it has to be mentioned that in many studies intrinsic viscosity, inherent viscosity and dilute solution viscosity (DSV) were used in order to monitor the increase of molar mass on monomer conversion. Unfortunately, only a few studies use GPC rather than viscosity measurements. For a few Nd-carboxylate-based catalyst systems linear dependencies of Mn on monomer conversion were established and proof in favor of requirement No. 2 linear increase of Mn with monomer conversion (no irreversible chain transfer) was provided. A more detailed analysis of the data, however, reveals deviations from linearity particularly at low monomer conversions (< 20%). These deviations are particularly pronounced for polymerizations with induction periods. Also the extrapolation of the straight lines to zero monomer conversion reveals intercepts on the Mn-axis. 

If requirement No. 2 linear increase of Mn with monomer conversion (no irreversible chain transfer) is substituted by the more stringent requirement linear dependence of Mn on monomer conversion which passes through the origin it has to be concluded that the compliance of Nd-catalyzed polymerizations with requirement No. 2 is rather an exception than a general rule. 

As discussed in Section II.C, in any system in cationic ring-opening polymerization, a reaction of active species with polymer repeating unit may lead to chain transfer to polymer or termination (if the resulting branched or cyclic onium ions are not active), whereas recombination with counterion leads to termination in the case of irreversible reaction. The later reaction may be avoided by the proper choice of counterion. As the onium ions are generally inherently stable there is no other termination reaction, provided that impurities that may act as terminating agents are eliminated. 

As discussed already, termination of ring-opening polymerization may proceed by (a) irreversible recombination with counterion and (b) irreversible chain transfer to polymer. Other sources of termination are also possible, depending on the system (c) reaction with other components of the system, solvent or impurities and (d) different reactions of more reactive species existing in equilibrium with stable onium species. 

Because the chain transfer to polymer is fast as compared with reformation of active species of propagation [Eq. (128)] and there is a reaction pathway, which due to the formation of isomerized products is irreversible [reaction (129)], continuous degradation of the already formed polythiirane chains occurs if the reaction system is kept unterminated [159]. Also isolated polymers, treated with cationic initiators degrade to low molecular weight, predominantly cyclic oligomers. Consequently, cationic polymerization of thiiranes is very strongly affected by chain transfer to polymer processes. 

The branched sulfonium ions are not reactive, thus Reaction (130) is an irreversible termination. The measured ratios of rate constants kp/k, [(rate constant of propagation to rate constant of termination according to Eq. (130)] reflect the general phenomenon that by increasing the number and size of the substituents the contribution of chain transfer to polymer may be considerably reduced [161] (Table 12). 

All limitations described above do not apply for the systems, which seem to be of considerable practical interest, namely oxazolines. Polymerization of these monomers proceeds practically irreversibly on long-living active species and, as discussed in Section III.F., chain transfer to polymer does not interfere with propagation. On the other hand, due to the possibility of obtaining oxazolines (or 6-membered analogs) with different substituents R ... 

In many systems, however, the analysis of the cationic copolymerization of heterocyclic monomers is complicated by two factors (1) at least some of the homo- and cross-propagation reactions may be reversible (2) redistribution of the sequences of comonomers within the chain may occur as a result of chain transfer to polymer. Therefore, the conventional treatment involving four irreversible propagation steps is rarely applicable in cationic ring-opening copolymerization. Instead, the diad model should involve four reversible reactions, i.e., eight rate constants... 

Both reactions, namely tte intermolecular process (131) and the intramolecular one (132) can be either reversible or irreversible (termination). In the case of reversible reactions true chain transfer takes place vdien the rate constant of the backward reaction (kj ) becomes comparable with the rate constant of M-opaptun. This applies to the polymerization of cyclic acetals where the product of chain traiKfer is equally active in propaption. 

Reaction 1 appears to result solely in termination. In hydrogenolysis experiments with various chelates we have observed precipitation of lithium hydride in all cases at room temperature. Attempts to generate chelated LiH in situ by adding hydrogen during ethylene polymerization also caused a rapid, irreversible loss of activity. Since there is no evidence that lithium hydride can add to ethylene under moderate polymerization conditions, it is unlikely that any significant chain transfer occurs via this mechanism. Potassium alkyls readily eliminate olefin with the formation of metal hydride, and sodium alkyls do so at elevated temperatures (56). It was noted earlier that chelation of lithium alkyls makes them more like sodium or potassium compounds, so it is quite probable that some termination occurs by eliminating LiH. It is conceivable that this could be a chain transfer mechanism with more reactive monomers than ethylene because addition to lithium hydride would be more favorable. 

In conventional addition polymerizations the growing chains are formed by some initiation processes and destroyed by some virtually irreversible terminations. The conversion of monomer into polymer eventually could be quantitative, provided that the initiation continues throughout the process. In the absence of termination or chain transfer the growing polymers remain living and then the polymerizing system ultimately has to attain a state in which the living polymers are in equilibrium with their monomer. The equilibrium concentration of the monomer, Me, provides valuable information leading to the determination of the appropriate Kp. To clarify this point, let us consider the equilibria... 

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