Deactivation dead” chains

Chain transfer to the aluminium alkyl was also deserved. Using the method of moments the authors obtained an equation for the first three moments of active, temporarily deactivated, and dead chains. As a result of a computerized search for the values of constants, based on the model and on the experimental data obtained in a batch reactor (volume = 131), some of the values were found to differ considerably from those published in the literature. 

Abbreviations C, catalyst precursor C, active site C j, deactivated site Al, cocatalyst M, monomer P,., living chain of length r, D,., dead chain of length r Hj, hydrogen 1, catalyst poison. 

In the group of Fischer, a considerable amount of work was dedicated to the kinetic description of NMP. They introduced the concept of the so-called persistent radical effect (PRE) (49,50). The PRE relies on the limited buildup of the concentration of deactivator, which in the case of NMP is the nitroxide, the persistent radical. On the basis of the PRE, it can be predicted which conditions will lead to narrow MMDs, and to a small fraction of irreversibly terminated dead chains. 

Olefin polymerization with metallocene catalysts involves initiation, chain propagation, formation of a dead chain with a saturated chain end through the chain transfer agent, p-hydride elimination to form a dead chain with a vinyl terminal double bond, insertion of a macromer with a vinyl end group, and catalyst deactivation. Assuming that all the reactions associated with each step are first order, the reaction processes can be expressed as... 

On account of the activation and deactivation terms involved in the radical balances, eqn [104] is not a simple function as eqn [55] that gives the radical chain length distribution equation [58] by applying the SSH. We therefore resort to the use of the method of moments for average polymer chain properties. In addition to the radical and dead-chain moments defined by eqns [73] and [74], the dormant chain moments are... 

A growing chain is deactivated when it reacts with another chain to form a dead macromolecule. The recombination of two growing monoradicals is an example of this. Termination reactions destroy active centers both the rate of polymerization and the degree of polymerization are lowered. The deactivation through reaction of two free radicals in one of the reasons why ionic polymerizations are faster than free radical polymerizations. The deactivation reaction between two free radicals has a small activation energy, and therefore occurs very rapidly. Thus, the concentration of growing free radicals is very low in the stationary state... 

Principle of controlled radical polymerization (CRP) exemplified via nitroxide-mediated polymerization (NMP). X, nitroxide R,-, living polymer molecule P,+y, dead polymer molecule R (, dormant polymer molecule /, chain length RqX, NMP initiator activation kj a, deactivation /c2, propagation /ctc, termination by recombination (Fig. 10.5A) (Malmstrom and Hawker, 1998) for simplicity the activation/deactivation rate coefficients of the initiator species are assumed the same as... 

In NMP, living macrospecies can be temporarily trapped by a nitroxide species X resulting in the formation of dormant macrospecies (RjX), which are the targeted polymer molecules for CRP, in contrast with the typical dead polymer product P in other chain-growth polymerizations. For a sufficiently fast deactivation (k eact, Scheme 10.2), this dormant state is favored and the contribution of dead polymer molecules is minimized. This favoritism is enhanced as X does not undergo self-termination. 

Table 5 shows the effects of temperature and time on the reaction of the poly-(NMAAm) radicals with MA. Although the M A conversion increased with increasing reaction time at 50 °C, this system tended toward dead-end polymerization as shown in Fig. 26. Furthermore, the conversion of MA at 40 °C (run MA-9) was higher than that at SO C. These results show that living propagating radicals were deactivated, probably by chain transfer and bimolecular termination, at the higher temperature. Block copolymer constituted 75-83 % of the total resulting polymer at 40-50 °C. 

The rate constant denotes the activation frequency per chain, that is, the number of aaivation events occurring on a chain per unit time, which, in the quasi-equilibrium state, is approximately equal to the deactivation frequency per chain. This frequency determines the polydispersity of the product. In the ideal case with constant concentrations of monomer and all other components along with negligible fractions of dead and conventionally initiated chains, the PDI of the LRP product maybe given by (see Section 3.05.4.1.1)... 

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