Copolymer comonomer

With the purpose of increasing the range of available block copolymers, comonomers other than methacrylates and acrylates can also be involved in sequential polymerization, provided that they are susceptible to anionic polymerization. Dienes, styrene derivatives, vinylpyridines , oxiranes and cyclosiloxanes are examples of such comonomers. The order of the sequential addition is, however, of critical importance for the synthesis to be successful. Indeed, the pX a of the conjugated acid of the living chain-end of the first block must be at least equal to or even larger than that of the second monomer. Translated to a nucleophilicity scale, this pK effect results in the following order of reactivity dienes styrenes > vinylpyridines > methacrylates and acrylates > oxiranes > siloxanes. 

Table I. Ratio of Quantum Efficiencies of Excimer and Normal Emission in Acenaphthylene Homopolymer and Equimolar Copolymers Comonomer IdZIM—...

Mole fraction of AOTcp in copolymer Comonomer Molecular weights ... 

Figure 2.24 Effect of copolymer comonomer sequence length on CCD according to Stockmayer s distribution (x = 1000, = 0.5, r = 1000).

There are several cases where NMR spectroscopy has been used to investigate copolymers which deviate from the terminal model for copolymerisation (see also chapter 3). For example, Hill and co-workers [23, 24] have examined sequence distributions in a number of low conversion styrene/acrylonitrile (S/A) copolymers using carbon-13 NMR spectroscopy. Previous studies on this copolymer system, based on examination of the variation of copolymer composition with monomer feed ratio, indicated significant deviation from the terminal model. In order to explain this deviation, propagation conforming to the penultimate (second-order Markov) and antepenultimate (third-order Markov) models had been proposed [25-27]. Others had invoked the complex participation model as the cause of deviation [28]. From their own copolymer/comonomer composition data. Hill et al [23] obtained best-fit reactivity ratios for the terminal, penultimate, and the complex participation models using non-linear methods. After application of the statistical F-test, they rejected the terminal model as an inadequate description of the data in comparison to the other two models. However, they were unable to discriminate between the penultimate and complex participation models. Attention was therefore turned to the sequence distribution of the polymer. 

In a number of the examples discussed in the preceding section, comonomer reactivity ratios were used to predict sequence distributions. A number of procedures exist for deriving reactivity ratios based on copolymer/comonomer composition data. Recently, a new method for determining reactivity ratios, based on in situ NMR measurements has been derived. This method is described. In addition, some of the mathematical techniques available to calculate sequence distributions using reactivity ratios are mentioned briefly, since their use can impinge on a number of the NMR studies of sequence distributions. 

Equation 42 is an empirical relation, which is based on the experimental observation that the Tg values of abinaiy copolymer (i = 2) vary monotonically with composition. Tg values are important because they determine many application properties of polymers and in relation to the apphcation temperature whether it is rubbery or glassy. In case of film formation and coatings applications of polymer dispersions, the Tg values determine at which temperature interdiflfusion of polymer molecules across particle boimdaries can start. Besides this application as building blocks for the main polymer (which is a random copolymer) comonomers are also applied to act with the aim either to contribute to colloidal properties or to allow subsequent chemical reactions. Monomers frequently employed in radical heterophase polymerizations are put together in Table 17. The monomers listed in Tables 9, 10, 16, and 17 suggest that there is an almost infinite number of combinations that can be applied in radical heterophase copolymerizations. 

Figure 7. Dynamic modulus vs. temperature of propylene oxide copolymers. Comonomer 2-4 mole %. Modulus values corrected to 80°C.

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