Reversible deactivation molecular weight

Monomers not amenable to direct homopolymerization using a particular reagent can sometimes be copolymcrizcd. For example, NMP often fails with methacrylates (e.g. MMA, BMA), yet copolymerizalions of these monomers with S are possible even when the monomer mix is predominantly composed of the methacrylate monomer,15j This is attributed to the facility of cross propagation and the relatively low steady state concentration of propagating radicals with a terminal MMA (Section 7.4.3.1). MMA can also be copolymerized with S or acrylates at low temperature (60 C).111 Under these conditions, only deactivation of propagating radicals with a terminal MMA unit is reversible, deactivation of chains with a terminal S or acrylate unit is irreversible. Molecular weights should then be controlled by the reactivity ratios and the comonomer concentration rather than by the nitroxide/alkoxyamine concentration. 

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. 

More complex situations have been treated analytically, such as reversible deactivation of initially active catalyst either by dimerization (2C C2) or bimolecular reaction (C + C 2C) [99]. Approach to equilibrium concentration of active centres would be accompanied by a fall in rate to a steady value (assuming constant monomer concentration and a stable catalyst) and a rise in molecular weight with time either to a maximum value or to a steady rate of increase dependent on the presence or absence of transfer reactions. The effect on average molecular weight of transfer reactions in which the catalyst entities possess two active centres has been calculated [100]. Although some ionic catalysts may behave in this way there is no evidence to indicate that these mechanisms apply to any known coordination catalyst. 

Stevenson and Nettleton [71] studied the polycondensation of low-molecular-weight PET over the range 221—251°C in the presence of antimony trioxide catalyst (Table 5), essentially paralleling Challa s earlier research [68, 69, 70] on the uncatalysed reactions. The catalysed reactions were faster, but the rate coefficient for polycondensation was relatively low even though the reverse reaction had been taken into account. They considered that a vacuum-volatile constituent of the reaction mixture was responsible for deactivation of the catalyst, thus slowing the reaction. Antimony trioxide does not vigorously catalyse the polycondensation of the pure monomer, bis(2-hydroxyethyl)-terephthalate, and Stevenson and Nettleton concluded that the latter substance was responsible for the proposed deactivation of the catalyst. 

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]. 

Polymers containing pyridine can be converted into polyaldehydes, by the action of cyanogen bromide, which can then be converted back to pyridines by the amino-groups of proteins." Relatively delicate and expensive enzymes may be immobilized by condensation copolymerization of a water-soluble functionalized prepolymer, a low molecular weight a, co-diamine and the enzyme. Inclusion of substrates, cofactors, products, or reversible inhibitors during the immobilization procedure protects the enzyme active site against deactivating acylation." ... 

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