Copolymerization azeotropic composition

Styrene Copolymers. Acrylonitrile, butadiene, a-methylstyrene, acryUc acid, and maleic anhydride have been copolymerized with styrene to yield commercially significant copolymers. Acrylonitrile copolymer with styrene (SAN), the largest-volume styrenic copolymer, is used in appHcations requiring increased strength and chemical resistance over PS. Most of these polymers have been prepared at the cross-over or azeotropic composition, which is ca 24 wt % acrylonitrile (see Acrylonithile polya rs Copolyp rs). 

Cases have been reported where the application of the penultimate model provides a significantly better fit to experimental composition or monomer sequence distribution data. In these copolymerizations raab "bab and/or C BA rBBA- These include many copolymerizations of AN, 4 26 B,"7 MAH28" 5 and VC.30 In these cases, there is no doubt that the penultimate model (or some scheme other than the terminal model) is required. These systems arc said to show an explicit penultimate effect. In binary copolynierizations where the explicit penultimate model applies there may be between zero and three azeotropic compositions depending on the values of the reactivity ratios.31... 

Azeotropic compositions are rare for terpolymerization and Ham 14 has shown that it follows from the simplified eqs. 38-40 that ternary azeotropes should not exist. Nonetheless, a few systems for which a ternary azeotrope exists have now been described (this is perhaps a proof of the limitations of the simplified equations) and equations for predicting whether an azeotropic composition will exist for copolymerizations of three or more monomers have been formulated.20113 This work also shows that a ternary azeotrope can, in principle, exist even in circumstances where there is no azeotropic composition for any of the three possible binary copolymerizations of tire monomers involved. 

Using copolymerization theory and well known phase equilibrium laws a mathematical model is reported for predicting conversions in an emulsion polymerization reactor. The model is demonstrated to accurately predict conversions from the head space vapor compositions during copolymerization reactions for two commercial products. However, it appears that for products with compositions lower than the azeotropic compositions the model becomes semi-empirical. 

Copolymerizations were performed at 70 C using an ampoule technique similar to that used for MMA. Monomers were purified by distillation. Most of the runs had an initial weight fraction styrene of 0.767 and 1.45 mole % AIBN initiator. Also utilized is one run using 0.235 wt. fraction styrene (0.350 mole % AIBN) and one at 0.557 (1.45 mole % AIBN). Gruber and Knell (10) used both the former compositions. The latter one is the calculated azeotropic composition using their values of the reactivity ratios. 

Under homophase synthesis in real systems the azeotrop (a) exists only provided n < 1 and r2 < 1. In this case, however, it is a repeller, unlike in the case of interphase copolymerization where the azeotrop (b) is an attractor. This means that at the final stage of homophase copolymerization homopolymer molecules are primarily formed in all real systems whereas under the interphase synthesis the majority of copolymer chains formed at p —> 1 have the azeotropic composition x. ... 

Corresponding data for the alternating radical copolymerization of styrene (Mi)-diethyl fumarate (M2)(n = 0.30 and r2 = 0.07) are shown in Figs. 6-6 and 6-7. This system undergoes azeotropic copolymerization at 57 mol% styrene. Feed compositions near the azeotrope yield narrow distributions of copolymer composition except at high conversion where there is a drift to pure styrene or pure fumarate depending on whether the initial feed contains more or less than 57 mol% styrene. The distribution of copolymer compositions becomes progressively wider as the initial feed composition differs more from the azeotropic composition. 

In such cases the polymerization can be taken to relatively high conversion without change in composition of the copolymer formed (see Example 3-37). In the copolymerization diagram the azeotrope corresponds to the intersection point of the copolymerization curve with the diagonal. For example, from Fig. 3.4 it may be seen that in the radical copolymerization of styrene and methyl methacrylate the azeotropic composition corresponds to 53 mol% of styrene. 

At 20 °C, for y-ray induced copolymerizations, r, 0.04 for monomer compositions containing 8-39% CO 7). At 120-130 °C, for (C2HsO)2 initiated copolymerizations, tj si 0.15 9). As Eq. (6) indicates, there exists one monomer ratio for which the copolymer composition equals the monomer composition, namely if + [C]/[E]) = 1. Using the above values of r, this azeotropic composition corresponds to 48.5 mol % CO for the y-ray induced copolymerizations at 20 °C (Fig. 1) 7), and si 46 mol % CO for the free radical initiated copolymerizations 9). The value of rj is dependent on the reaction temperature. For example, for the y-ray induced copolymerizations, the value of r2 increases from 0.04 at 20 °C to 0.31 at 157 °C 7). As expected, the value of rt at 135 °C was close to that observed for the free-radical initiated polymerization at that temperature. These results indicate that the copolymerization should be carried out at low temperatures in order to get copolymers with high CO contents. The azeotropic composition is also altered by pressure. For example, for (C2HsO)2 initiated copolymerizations the %CO in the azeotropic composition drops from 46% to 36% when the total gas pressure is lowered from 100 to 13.6 MPa (from 1000 to 136 atm) 9). 

Note that the expressions (4.15) can be obtained even without the formulae (4.11), if we employ an algorithm similar to the one used for the derivation of formulae (4.11). Actually, the proper diverging from the point i directed tree corresponds to each of the items in the expression for Af. A weighting factor o ay corresponds to each of the arcs in there digraphs that leaves point i and enters point j. The sum of all so weighted trees directed from a root of type i directly gives an expression similar to expressions (4.15) for the value of Af, which is equal (when all coj are positive) within the accuracy of the normalizing factor A to the component xf = Xf = Af/A of azeotropic composition under the copolymerization of an arbitrary number (m) of monomers. 

Hence, within the framework of the traditional kinetic model (2.8) there is a mathematically rigorous solution of the problem of the calculations of the azeotropic composition x under the copolymerization of any number of monomer types knowing their reactivity ratios, i.e. the elements of matrix ay. However, since the values of au can be estimated from the experiment with certain errors Say, the calculated location of azeotrope x is also determined with an accuracy, the degree of which is characterized by vector 8x with components 8xj (k = 1,2,..., m) and modulus 8X ... 

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

Note that f in the above equation is physically meaningful (0 < /i < 1) only if ri and r2 are both either greater or less than unity. (If Ti = T2 = 1, all values of f are azeotropic compositions.) Since the case of ri > 1, T2 > 1 is unlmown in free-radical systems, the necessary conditions for azeotropy in such copolymerizations is that ri < 1, T2 < 1 (see Fig. 7.2). 

The existence of an azeotropic composition has some practical significance. By conducting a polymerization with the monomer feed ratio equal to the azeotropic composition, a high conversion batch copolymer can be prepared that has no compositional heterogeneity caused by drift in copolymer composition with conversion. Thus, the complex incremental addition protocols that arc otherwise required to achieve this end, are unnecessary. Composition equations and conditions for azeotropic compositions in ternary and quaternary copolymerizations have also been defined. " ... 

Some copolymerization systems are not strictly alternating, but still they show a tendency toward alternation. This occurs when both and r2 < 1. The alternating trend increases as the reactivity ratios approach zero. An interesting feature of these systems is that they present the so-called azeotropic composition, at which Fj = /j. At this composition, the copolymer formed has the same composition as the monomers in the feed and, therefore, systems copolymerizing at this condition do not show compositional drift. It can be shown that a necessary condition that the reactivity ratios have to satisfy in order for a copolymerization system to show an azeotropic point is that either both and r2 < 1 or both and T2 > 1. 

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... 

Einally, as per Equation 18.27b, if a copolymer has a very narrow composition distribution, will be very close to (M) using a single solvent the same applies to mixed solvents if we follow the procedure mentioned earlier. This can be the case, for instance, of a radical copolymerization in a true azeotrope composition or of a block copolymer synthesized by a controlled anionic or living radical polymerization. 

Azeotropic copolymerization occurs when ta < 1 with ra < 1 (the situation defined by ta > 1 with ra > 1 is not observed in practice). The Fa vs. /a curves are characterized by their intersection of the Fa = /a line at one point which corresponds to the azeotropic composition (/A)azco. Substituting Fa = /a = (/A)azeo uito Equation (1.33) leads to... 

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