Bulk polymerization acrylonitrile

Acrylonitrile and its comonomers can be polymerized by any of the weU-known free-radical methods. Bulk polymerization is the most fundamental of these, but its commercial use is limited by its autocatalytic nature. Aqueous dispersion polymerization is the most common commercial method, whereas solution polymerization is used ia cases where the spinning dope can be prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacryhc compositions where a high level of a water-iasoluble monomer is used or where the monomer mixture is relatively slow reacting. 

Bulk Polymerization. The bulk polymerization of acrylonitrile is complex. Even after many investigations into the kinetics of the polymerization, it is stiU not completely understood. The complexity arises because the polymer precipitates from the reaction mixture barely swollen by its monomer. The heterogeneity has led to kinetics that deviate from the normal and which can be interpreted in several ways. 

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

Similarly, Garcia-Rubio and Hamielec (17) conducted bulk polymerizations of acrylonitrile at various temperatures and initiator levels in glass ampoules. Their plots of the rate of polymerization as a function of conversion are typical of the extensive radical occlusion in this very glassy polymer. 

Poly(acrylic acid) is not soluble in its monomer and in the course of the bulk polymerization of acrylic acid the polymer separates as a fine powder. The conversion curves exhibit an initial auto-acceleration followed by a long pseudo-stationary process ( 3). This behaviour is very similar to that observed earlier in the bulk polymerization of acrylonitrile. The non-ideal kinetic relationships determined experimentally in the polymerization of these two monomers are summarized in Table I. It clearly appears that the kinetic features observed in both systems are strikingly similar. In addition, the poly(acrylic acid) formed in bulk over a fairly broad range of temperatures (20 to 76°C) exhibits a high degree of syndiotacticity and can be crystallized readily (3). 

Comparison of the kinetic features observed in the bulk polymerization of acrylic acid and acrylonitrile... 

The bulk polymerization of acrylonitrile has been studied by numerous workers (for a literature survey on the problem see ref. j O and JL1J. The kinetic features of this reaction at room temperature are summarized in Table I. It is one of the typical examples of polymerization under heterogeneous conditions in which the anomalies are generally assumed to arise as a result of non-stationary conditions caused by the "occlusion" of growing chains in the precipitated polymer (10). The presence of occluded radicals was indeed demonstrated by 5R measurements (12) and by... 

The cfata presented in Table I show that the kinetic features of the bulk polymerization of acrylonitrile are very similar to those observed with acrylic acid. It therefore seems pertinent to query whether atrix effect could not arise in the polymerization of acrylonitrile through a regular orientation of monomer molecules along the polymeric matrix involving dipole interaction of the -CsN groups (structure IV). 

The bulk polymerization of acrylonitrile in this range of temperatures exhibits kinetic features very similar to those observed with acrylic acid (cf. Table I). The very low over-all activation energies (11.3 and 12.5 Kj.mole-l) found in both systems suggest a high temperature coefficient for the termination step such as would be expected for a diffusion controlled bimolecular reaction involving two polymeric radicals. It follows that for these systems, in which radicals disappear rapidly and where the post-polymerization is strongly reduced, the concepts of nonsteady-state and of occluded polymer chains can hardly explain the observed auto-acceleration. Hence the auto-acceleration of acrylonitrile which persists above 60°C and exhibits the same "autoacceleration index" as at lower temperatures has to be accounted for by another cause. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

Bulk polymerization of acrylonitrile initiated by gamma radiation was described by Chapiro at The process is greatly complicated by other reactions. Occlusion of... 

Example Bulk polymerization of Bulk polymerization of acrylonitrile... 

Acrylonitrile and other olefinic monomers can be grafted on polycaprolactam by an indirect method in which the polyamide powder is first submitted to nitro-sation with dinitrogen trioxide and then bulk polymerized at 60° C (12). 

Because of the highly exothermic nature of acrylonitrile polymerization, bulk processes arc not normally used commercially. Howevei. a commercially feasible process lor bulk polymerization in a continuous stirred lank reactor has been developed. The heat nl reaction is controlled hy operating at relatively low conversion levels and supplementing the normal jacket cooling with reflux condensation of umcaclcd monomer... 

This review deals with current ideas on the mechanisms operative in acrylonitrile polymerization. The topic is of importance in its own right and also because the study of acrylonitrile has cast light on heterogeneous polymerizations in general. It is an active field of research and the interpretations are still controversial. We shall look first at free-radical polymerization in homogeneous solution, where the monomer behaves in a more or less classical fashion. Next we shall consider the complications that arise where the monomer is at least partially soluble in the reaction medium but where the polymer precipitates. These conditions are encountered in bulk polymerization and in most aqueous or organic diluents. Finally we shall examine the less extensive literature on anionic polymerization and show important differences between the radical and the ionic processes. 

Bensasson 30) has polymerized acrylonitrile even in the solid state at —196° and finds that it polymerizes more rapidly at —196° than in the liquid at —78°. Very recently Sobue and Tabata 121) have investigated polymerization induced in bulk by ionizing radiation over the range +15 to —196°. They suggest that both ionic and radical polymerization may occur. 

Bulk polymerization was also studied briefly (Table II), adapting a technique useful in production of impact styrene (I). Solid polybuta-diene was dissolved in styrene, mixed with acrylonitrile and/or methyl methacrylate, charged into a 12-ounce crown-cap bottle, flushed with nitrogen, and tumbled end-over-end at 42 rpm in a constant-temperature water bath for 16 hours at room temperature, 48 hours at 80 °C, and 48 hours at 90°C, then heated in an oven 24 hours at 110°C and 24 hours at 150°C. ABS polymerization produced a grainy, inhomogeneous, light-... 

Table I. Specific Surface of Particles Formed in Bulk Polymerization of Acrylonitrile at 50°C.

Figure 4. Bulk polymerization of acrylonitrile at 50°C. AIBN concentration, 2.07 X 10 sM...

Figure 5. Rate of bulk polymerization of acrylonitrile at 50°C. as a function of AIBN concentration...

Application To produce a wide range of styrene acrylonitrile (SAN) copolymer with excellent chemical resistance, heat resistance and suitable property for compounding with ABS via the continuous bulk polymerization process using Toyo Engineering Corp. (TEC)/Mitsui Chemicals Inc. technology. 

JSR Corp. Styrene acrylonitrile (SAN) Styrene and acrylonitrile Bulk polymerization with high productivity and yield for many SAN grades 5 1995... 

Suspension polymerization is frequently employed as the second stage following a preliminary bulk polymerization, such as in the manufacture of some HIPS and ABS polymers. Polybutadiene or another elastomer is dissolved in liquid styrene, and this monomer or a mixture of styrene and acrylonitrile is polymerized in a batch kettle. The syrup produeed at 30-35% conversion is too viscous for effective mixing and heat transfer. It is therefore dispersed in water, and the polymerization is finished as a suspension reaction. 

Precipitation polymerization. In precipitation polymerization, monomer is polymerized either in bulk or in solution (aqueous or organic), however, the polymer formed is insoluble in the reaction media. As such, the forming polymer precipitates and the viscosity of the medium does not change appreciably. This pol5mierization is often referred to as powder or granular polymerization because of the forms in which the polymers are produced. Solution polymerization of acrylonitrile in water, and bulk polymerization of vinyl chloride are examples of precipitation polymerization. 

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