Reversible deactivation distribution

During induction, catalyst activity and selectivities to aromatics and propene increase steadily. Improvement of catalyst performance is due to increase in Ga dispersion and formation of dispersed Ga species (Gao) which are efficient for the heterolytic recombinative release of hydrogen [18,191. The Ga/H-MFI catalyst then reaches its optimal aromatisation performance (stabilisation). Ci to C3 hydrocarbons productions are at their lowest. The gallium dispersion and the chemical distribution of Ga are optimum and balance the acid function of the zeolite. Reversible deactivation during induction and stabilisation of the catalyst is due to site coverage and limited pore blockage by coke deposition. 

Termination is formally an irreversible deactivation of growing species. That is, reversible termination is not a real termination process and would be more appropriately labeled reversible deactivation. If this reversible deactivation is sufficiently dynamic, the number of growing species remains constant throughout the polymerization and all chains have the same opportunity to grow, resulting in polymers with narrow molecular weight distributions. This will be discussed in detail in Chapter 4. 

During ATRP, alkyl halides function as initiators while transition metal complexes (ruthenium, osmium, iron, copper and so on) act as the catalyst. Metal complexes are used to generate radicals (such as peroxide) via a one electron transfer process and during this process the transition metal becomes oxidised. Thus, ATRP is a reversible-deactivation radical polymerisation and can be employed to prepare polymers with similar molecular weight (MW) and low MW distribution. Advantages of ATRP are the ease of preparation, use of commercially available and inexpensive catalysts and initiators [14, 15]. The synthesis and process development of ATRP, as well as some new hybrid materials made of amphiphilic polymers, have been reported in the literature (Figure 2.3) [16, 17]. 

Reversible addition-fragmentation chain transfer polymerization is a reversible deactivation radical polymerization and it represents one of the most versatile methods for providing living characteristics to radical polymerization and polymers of predictable chain length and narrow molecular weight distribution. 

Figure 2.5 shows plots of Mw/Mn vs. conversion for polymerization systems possessing a Poisson molecular weight distribution (eqn (2.30)) and three distributions predicted by eqn (2.31) (P = kd/kp[l]f) with kpjk = 5, 2, and 0.5 L moU For the system with slow exchange kpjk = 5 L moU ), the final distribution at p=1.0 is 1.05, significantly broader than Poisson. For A p/ (j = 0.5LmoU the distribution of the reversible-deactivation polymerization is very nearly Poisson at high conversions but deviates from Poisson at low conversions. For p/ d<0-5L mol , eqn (2.31) predicts a distribution that is more narrow than Poisson, especially at low conversions. 

Deactivation of catalysts, particularly by coke deposition (the main means of reversible FCC catalyst deactivation) has been the subject ofintensive study over the past 50 years (2-4). Initially, the loss of activity was correlated with the time on stream, but it is now generally accepted that a more appropriate approach to understanding the effect of deactivation by coke is to relate deactivation to the deposited coke concentration (5). Furthermore, few studies on the effect of catalyst formulation on both the product distribution and coke formation have appeared in the open literature. 

Living radical polymerization (LRP) has attracted growing attention as a powerful synthetic tool for well-defined polymers 1,2). The basic concept of LRP is the reversible activation of the dormant species Polymer-X to the propagating radical Polymer (Scheme la) 1-3). A number of activation-deactivation cycles are requisite for good control of chain length distribution. 

---