Terminal model reactivity ratios

Table 7.1 Terminal Model Reactivity Ratios for Some Common Monomer Pairs9...

Thus, the terminal model allows the copolymer composition for a given monomer feed to be predicted from just two parameters the reactivity ratios rAB and rBA- Some values of terminal model reactivity ratios for common monomer pairs are given in Table 7.1. Values for other monomers can be found in data... 

It is also possible to process copolymer composition data to obtain reactivity ratios for higher order models (e.g. penultimate model or complex participation, etc.). However, composition data have low power in model discrimination (Sections 7.3.1.2 and 7.3.1.3). There has been much published on the subject of the design of experiments for reactivity ratio determination and model discrimination.49 "8 136 137 Attention must be paid to the information that is required the optimal design for obtaining terminal model reactivity ratios may not be ideal for model discrimination.49... 

Terminal model reactivity ratios may be estimated from the initial monomer feed composition and the dyad concentrations in low conversion polymers using the following relationships (eqs. 45, 46). 

Harwood112 proposed that the solvent need not directly affect monomer reactivity, rather it may influence the way the polymer chain is solvated. Evidence for the proposal was the finding for certain copolymerizations, while the terminal model reactivity ratios appear solvent dependent, copolymers of the same overall composition had the same monomer sequence distribution. This was explained in... 

The apparent terminal model reactivity ratios are then r => aK and c =rR, K It follows that rABVBf = rABrBA - const. The bootstrap effect does not require the terminal model and other models (penultimate, complex participation) in combination with the bootstrap effect have been explored.103,1 4215 Variants on the theory have also appeared where the local monomer concentration is a function of the monomer feed composition.11[Pg.431]

We have previously reviewed ( 1, 2) the methods used to calculate structural features of copolymers and terpolymers from monomer reactivity ratios, monomer feed compositions and conversions, and have written a program for calculating structural features of copolymers from either terminal model or penultimate model reactivity ratios (3). This program has been distributed widely and is in general use. A listing of an instructive program for calculating structural features of instantaneous terpolymers from monomer feed compositions and terminal model reactivity ratios was appended to one of our earlier reviews (.1). 

Typical output from this program is shown in Figure 1. This output is for a calculation involving only terminal model reactivity ratios, which is an option of the program. 

This program functions in essentially the same manner as Program A, except it can accept as input 18 reactivity ratios, corresponding to a situation where all monomers exhibit penultimate effects. As is also the case in Program A, terminal model reactivity ratios can be used, however. The programming involves the calculation of 27 conditional probabilities. These probabilities are calculated in the same way that P(AA/AA), P(AA/BA) and P(AA/CA) are calculated in the case of Program A. 

The apparent terminal model reactivity ratios are then =t K and rT = ra follows that = = const. The bootstrap effect does... 

The kinetics of copolymerization and the microstructure of copolymers can be markedly influenced by the addition of Lewis acids. In particular, Lewis acids are effective in enhancing the tendency towards alternation in copolymerization of donor-acceptor monomer pairs and can give dramatic enhancements in the rate of copolymerization and much higher molecular weights than are observed for similar conditions without the Lewis acid. Copolymerizations where the electron deficient monomer is an acrylic monomer e.g. AN, MA, MMA) and the electron rich monomer is S or a diene have been the most widely studied." Strictly alternating copolymers of MMA and S can be prepared in the presence of, for example, dictliylaluminum scsquichloridc. In the absence of Lewis acids, there is only a small tendency for alternation in MAA-S copolymerization terminal model reactivity ratios are ca 0.51 and 0.49 - Section 7.3.1.2.3. Lewis acids used include EtAlCT, Et.AlCL ElALCL, ZnCT, TiCU, BCl- LiC104 and SnCL. 

Hence the terminal model reactivity ratios are given by... 

Values of Q = 1.00 and e = -0.80 are then defined for styrene as a reference, and other Q and e values for all other monomers are obtained by fitting the scheme to an experimental data set of terminal model reactivity ratios. 

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