Copolymer composition azeotropic

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). 

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 eopolymerizations have also been defined.211,21... 

Equation (13.42) gives the condition that a copolymer azeotrope exists. The azeotropic composition is... 

The Asahi process for wet-spinning the copolymer involves water dilution at below 0°C of a solution of the copolymer in aqueous 68 wt% nitric acid (the azeotropic composition). The potential for slow self heating and decomposition have been investigated experimentally with variations in several parameters in a Sikarex safety calorimeter. At 20% polymer content, the slow self heating starts even at ambient temperature, and later involves evolution of 30 mol of uncondensable gas per kg of copolymer with comcommittant boiling of the nitric acid. A 2 step mechanism has been proposed. 

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). 

For the symmetrical copolymers the formula x4 = A, /A of the azeotropic composition has a surprisingly simple form since in this case A- = cOj/Bj = G)ia.iJa.Vv where the values of tOj = D4/D were determined above. Their expressions in a particular case of terpolymerization obtained by means of relations (4.16) and (4.17) lead to the well-known formulae [119, 166], where the azeotropic composition is expressed explicitly through the reactivity ratios ry. Note, that the terms partial and limited azeotropes were introduced initially for the processes of the production of symmetrical copolymers [126, 127],... 

The calculations of the statistical characteristics of such polymers within the framework of the kinetic models different from the terminal one do not present any difficulties at all. So in the case of the penultimate model, Harwood [193-194] worked out a special computer program for calculating the dependencies of the sequences probabilities on conversion. Within the framework of this model, Eq. (5.2) can be integrated in terms of the elementary functions as it was done earlier [177] in order to calculate copolymer composition distribution in the case of the simplified (r 2 = Fj) penultimate model. In the framework of the latter the possibility of the existence of systems with two azeotropes was proved for the first time and the regions of the reactivity ratios of such systems [6] were determined. In a general version of the penultimate model (2.3-24) the azeotropic compositions x = 1/(1 + 0 ) are determined [6] by the positive roots 0 =0 of the following... 

Figure 3.6 Relationship between feed composition and copolymer composition of syrene-acrylonitrile. Point A indicates the azeotropic composition. Reproduced with permission from Encyclopedia of Polymer Science and Engineering, Mark (Ed.), John Wiley Sons, NY. Copyright John Wiley Sons...

Figure 7-2 includes some representative copolymer-feed composition curves for r, varying with rj constant at 0.8. Azeotropic feed compositions containing appreciable quantities of both polymers can be achieved only when the reactivity ratio values are close to each other. 

Ek]uation (7-23) calculates the feed composition that yields an invariant copolymer composition as the conversion proceeds in a batch polymerization. Note that comonomer ratios that are near but not equal to the estimated azeotropic value may produce copolymers whose compositions are constant for all practical purposes. The permissible range of feed compositions for which this approximate azeotropy occurs is evidently greater the closer the two reactivity ratios are to each other. 

The copolymer equations, Eqs. (7.11) and (7.18), give the instantaneous copolymer composition, i.e. the composition of the copolymer formed from a given feed composition at very low degrees of conversion (approximately < 5%) such that the composition of the monomer feed may be considered to be essentially unchanged from its initial value. For all copolymerizations except when the feed composition is an azeotropic mixture or where rj = T2 — the comonomer feed and copolymer product compositions are different. The comonomer feed changes in composition as one of the monomers preferentially enters the copolymer. Thus there is a drift in the comonomer composition, and consequently a drift in the copolymer composition, as the degree of conversion increases. It is important to be able to calculate the course of such changes. 

Note that /i in the above equation is physically meaningful (0 < /i < 1) only if both ri and rz are either greater or smaller than unity. (If ri = rz = 1, all values of/i are azeotropic compositions.) Since the case of ri > 1, rz > 1 is uncommon in free-radical systems, the necessary conditions for azeotropy in such copoly-meiizations is that ri < 1, rz < 1 (see Fig. 7.2). Equation (7.19) then predicts the feed composition that would yield an invariant copolymer composition as the conversion proceeds in a batch reactor. 

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