Patterns of reactivity scheme

The patterns of reactivity scheme is a more advanced treatment of copolymerization behavior. It follows the general form of the Q-e scheme but does not assume that the same intrinsic reactivity or polarity factors apply both to a monomer and its corresponding radical [Bamford and Jenkins, 1965 Jenkins, 1999, 2000 Jenkins and Jenkins, 1999]. The monomer reactivity ratio for monomer 1 is expressed in terms of four parameters 

The reactivity ris is the monomer reactivity ratio for copolymerization of monomer 1 with styrene. Since styrene has very little polar character, ris measures the intrinsic reactivity of Mi- radical. The polarity of Mi- radical % is obtained from 

The monomer reactivity ratio for monomer 2 follows from Eq. 6-60a as 

The values of v and u for a monomer are obtained from monomer reactivity ratios for copolymerization of that monomer with a series of reference monomers. A plot of the data according to Eq. 6-60a as [log r 2 — log ns] versus 71] yields a straight line whose slope is u2 and intercept on the y-axis is —v2. 

Appropriate manipulation of Eqs. 6-60 and 6-62 gives alternate expressions for the two monomer reactivity ratios 

Bamford, Jenkins and coworkers concluded that many of the limitations of the Q-e scheme stemmed from its empirical nature and proposed a new scheme containing a radical reactivity term, based on experimentally measured values of the rate constant for abstraction of benzylic hydrogen from toluene (A 3. i), a polar tenn (the Hammett o value) and two constants a and p which are specific for a given monomer or substrate (eq. 57)  

In the revised Patterns scheme reactivity ratios involving S are used as reference reactions. Reactivity ratios are then given by eqs. 60 and 61  

The Pallerns scheme has been tested for its capacity to predict C NMR 

The Patterns scheme has been tested for its capacity to predict nC NMR chemical shifts of the CH7- carbon of monomers (CH =CXY)159 and in evaluating the reactivities of small radicals towards monomers.160 

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Many researchers have applied similar approaches to develop or apply linear free energy relationships, when the substituent is directly attached to the double bond, with some success. Two of the more notable examples can be found in the Patterns of Reactivity Scheme (Section 7.3.4) and the works of Giese and coworkers.16 19... 

The basic Hammett scheme often does not offer a perfect correlation and a number of variants on this scheme have been proposed to better explain reactivities in radical reactions.-0 However, none of these has achieved widespread acceptance. It should also be noted that linear free energy relationships are the basis of the Q-e and Patterns of Reactivity schemes for understanding reactivities of propagating species in chain transfer and copolymerization. 

Chain transfer is kinetically equivalent to copolymerization. The Q-e and Patterns of Reactivity schemes used to predict reactivity ratios in copolymerization (Section 7.3.4) can also be used to predict reactivities (chain transfer constants) in chain transfer and the same limitations apply. Tabulations of the appropriate parameters can be found in the Polymer Handbook 3 ... 

The most popular methods are the Q-e (Section 7.3.4.1) and Patterns of Reactivity schemes (Section 7.3.4.2). Both methods may also be used to predict transfer constants (Section 6.2.1). For furtherdiscussionontheapplicationofthe.se and other methods to predict rate constants in radical reactions, see Section 2.3.7. 

Patterns. of Reactivity scheme 365 6 Q-c scheme 363-5 radical-radical term inaiirm... 

PAM sec polyacrylamide PAM see polyacrylonitrile palladium complexes, as catalysts for ATRP 492 Patterns of Reactivity scheme 11,26, 31 for prediction of reactivity ratios 365-6 lor prediction of iransfer constants 287 PB see polybutadieue PE see polyethylene... 

Table 6-8 shows values of the various parameters needed to calculate monomer reactivity ratios from Eqs. 6-60 and 6-62 [Jenkins and Jenkins, 1999]. The monomers in Table 6-8 are lined up in order of their u values. The Patterns of Reactivity scheme, like the Q e. scheme, is an empirical scheme. Monomer reactivity ratios calculated by the patterns of reactivity scheme are generally closer to experimental values than those calculated by the Q e scheme, which supports the rationale of assigning different polarity values to a monomer and the radical derived from the monomer. 

The patterns of reactivity scheme has also been extended to chain transfer constants. 

Calculate the monomer reactivity ratios for chloroprene-2-vinylpyridine using the data from Table 6-8 for the patterns of reactivity scheme. 

Note 2 The patterns of reactivity scheme is known also as Jenkins scheme. 

The relation between monomer reactivity ratios and the Alfrey-Price Q-e parameters is explained in the introduction to the tables of monomer reactivity ratios and Q-e values, compiled by Robert Z. Greenley and published in the present volume (1,2). Although very widely used, the Q-e scheme is well known to have serious limitations (3), which have prompted several attempts to improve upon it. Qne such endeavour was the Patterns of Reactivity scheme, first described as long ago as 1959 (4-7), when the Q-e scheme was only about ten years old despite the indisputably more satisfaetory basis of this procedure, it did not achieve popularity but recent revisions have greatly improved both its accessibility and its accuracy (3,8,9). 

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