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Reactivity Ratios Change with Conversion

It has already been pointed out in the previous section that below compbte conversion crosslink density is not related in a unique way to the extent of reaction. [Pg.9]

The true course of the homopolymerization reaction of a multifunctional monomer is determined by the ratio of reactivities of free and pendent reactive groups. Copolymerizations are even more complicated and will not be discussed here. [Pg.9]

In Fig. 2, we have depicted the molality of crosslinks as a function of double bond conversion for a tetiafunctional monomer, 1,6-hexanediol diacrylate (HD DA) for a few special cases. The molality was chosen in order to get a plot which is independent of density changes during polymerizaticm. In the hypothetical case of [Pg.9]

This will be further discussed in Sect. 7.3 and 8 of this chapter. [Pg.10]

In the final stage of the reaction a reversal of relative reactivities seems to occur. Here, overall vitrification probably sets in, affecting the mobility of pendent double bonds more strongly than those in free monomer. [Pg.10]


The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. [Pg.430]

Reactivity ratios for the copolymerization of AN and DM WS in DMSO were found to be rj =0,53 and r2=0,036, and in water r1=0,56 and r2=0,25. The higher reactivity of DM VPS in the copolymerization with AN in aqueous medium, as compared with its reactivity in DMSO, can be explained by a higher degree of dissociation of DMVPS in aqueous medium. This fact also produces a considerable effect on the character of the distribution of monomeric units within the copolymers, which manifests itself in the change of their solubility in water. Copolymers containing 30% of monomeric units AN obtained from a 90 10 mixture of AN and DMVPS in DMSO, irrespective of the level of conversion, are completely soluble in water, whereas copolymers of the same composition, but obtained in aqueous medium with a yield 40%, are insoluble in water. [Pg.115]

Numerical approaches for estimating reactivity ratios by solution of the integrated rate equation have been described.124 126 Potential difficulties associated with the application of these methods based on the integrated form of the Mayo-kewis equation have been discussed.124 127 One is that the expressions become undefined under certain conditions, for example, when rAo or rQA is close to unity or when the composition is close to the azeotropic composition. A further complication is that reactivity ratios may vary with conversion due to changes in the reaction medium. [Pg.361]

In most cases one of the two monomers is preferentially incorporated into the copolymer, so that the composition of the monomer mixture changes with increasing conversion, as also does the composition of the polymer chains. Therefore it is important, when determining reactivity ratios, to work at low conversions, so that at the end of the experiment the ratio of monomer concentrations is essentially the same as at the beginning. However, if high conversions are needed for preparative purposes, constant composition can be achieved by adding the more reactive monomer in a programed manner (see Example 3-39). [Pg.236]

More often than not, reactivity differs from monomer to monomer. This is evident when the reactivity ratios differ from a value of one. Thus, if one is operating at concentrations other than the azeotropic composition, batch copolymerization will result in a changing copolymer composition throughout the reaction. For example, a copolymerization with rj > 1 and r2 < 1 would result in the instantaneous copolymer composition decreasing in monomer 1 as monomer conversion increases. The degree of compositional drift that leads to a heterogeneous copolymer composition depends on the ratio of reactivity ratios where heterogeneity increases with the... [Pg.120]

The importance of m,p-cresol novolak resins to photoresist formulation makes the supply of a range of copolymer compositions and molecular weights useful. In novolak resin synthesis, the growing polymer chains compete with cresol monomer for formaldehyde. As conversion increases, the polymer competes better than the monomer, so that the supply of phenolic monomer is never exhausted, and the amount of unreacted monomer changes with extent of reaction. Since different monomers react at different rates, this ensures not only that copolymer composition will not be the same as the charge ratio of the monomers, but also that it will change over the course of the reaction. The model we describe uses relative monomer reactivities to predict copolymer composition. [Pg.311]

In calculating the copolymerization reactivity ratios, experiments are very rarely carried out at any given yield, since the integrated equation (22-15) can only be solved by computer. More generally, the experiment is based on different mixing proportions [Mi]o/[M2]o of the monomers and polymerization is limited to low total conversions. Under these conditions, the composition of the monomer mixture is essentially constant. The change in the monomer ratio d[Mi]/ /[M2] is then equal to the molar ratio ([mi]o/[m2])o of the monomeric units in the copolymer, and equation (22-13) can be used. For exact determinations, particularly with very different copolymerization reactivity ratios, the ratios should be determined over a range of conversions and extrapolated to zero conversion. [Pg.767]

Methods for simulation of sequence distribution. It often proves beneficial to be able to predict sequence distribution from reactivity ratio data. For example, it may be necessary to compare the sequence distribution determined by NMR spectroscopy with one determined from reactivity ratio data obtained by some other technique. Relationships which enable monomer addition probabilities to be calculated from reactivity ratios have already been given (equations (2.20), (2.21)). The monomer addition probabilities can in turn be used to calculate sequence distribution as discussed earlier. This approach is only valid, however, for low conversion polymers. As stated before, as conversion increases, the ratio of the two unreacted monomers drifts because one monomer is normally more reactive than the other. Thus, the monomer addition probabilities are continuously changing as the polymerisation proceeds. [Pg.74]


See other pages where Reactivity Ratios Change with Conversion is mentioned: [Pg.9]    [Pg.9]    [Pg.63]    [Pg.185]    [Pg.471]    [Pg.4]    [Pg.100]    [Pg.148]    [Pg.471]    [Pg.601]    [Pg.579]    [Pg.52]    [Pg.498]    [Pg.869]    [Pg.186]    [Pg.70]    [Pg.262]    [Pg.357]    [Pg.96]    [Pg.166]    [Pg.350]    [Pg.247]    [Pg.203]    [Pg.469]    [Pg.498]    [Pg.54]    [Pg.98]    [Pg.120]    [Pg.271]    [Pg.125]    [Pg.125]    [Pg.498]    [Pg.361]    [Pg.111]    [Pg.297]    [Pg.442]    [Pg.63]    [Pg.1907]    [Pg.50]    [Pg.110]    [Pg.261]   


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