Polystyrene polymerization, mass

In the case of propylene homopolymer, the universal calibration procedure established by Grubisic et al. [2] can be applied using anion-ically polymerized polystyrene molar mass standards with narrow MMD for TCB at 135°C, the following values of the parameters K and a in the equation ... 

The kinetic modeling of styrene controlled radical polymerization, initiated by 2,2 -asobis(isobutimitrile) and proceeding by a reversible ehain transfer meeha-nism was carried out and accompanied by addition-fragmentation in the presenee dibenzyltiitiocarbonate. An inverse problem of determination of the unknown temperature dependences of single elementary reaction rate eonstants of kinetic scheme was solved. The adequacy of the model was revealed by comparison of theoretical and experimental values of polystyrene molecular-mass properties. The influence of process controlling factors on polystyrene molecular-mass properties was studied using the model. 

The results of polystyrene molecular-mass properties calculations by the introduced mathematical model are presented in Figs. 10.3 and 10.4. Mathematical model of styrene RAFT-polymerization in the presence of trithiocarbonates, taking into account the radicals and intermediates cross termination, adequately describes the experimental data that prove the process mechanism, built in the model. The essential proof of the mechanism correctness is that in case of conceding the absence of radicals and intermediates cross termination the experimental data wouldn t substantiate theoretical calculation by the mathematical model, introduced in this assumption (Fig. 10.5). 

We now briefly consider a. flash devolatilizer (see Figure 6.3.11(b)) (Biesenberger, 1983 Meister and Platt, 1989) to remove residual monomers and solvents from a polymeric liquid. The polymeric liquid is, however, nonvolatile, whereas the other species are volatile and are to be removed from the polymer. For example, a molten viscous feed from a polystyrene reactor at around 170 °C and containing 85% polymer, the rest being the monomer styrene, ethylbenzene, cumene, styrene dimers and trimers, is pumped into a heater. The polymeric liquid and the associated species are heated up to 230 °C and introduced into the flash tank maintained under vacuum (-4-30 Torr). As the foamy polymeric mass falls to the liquid level (just as in the case of a hydrocarbon liquid in the earlier example), the monomer and the volatile solvent species escape the polymer phase into the gas phase and ate withdrawn from the tank at the top. The net dlrei tlon of both phases flow is perpendicular to the liquid surface the force i ausing volatilization is also perpendlimlar to the liquid surfai e. 

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Impact polystyrene (IPS) is one of a class of materials that contains mbber grafted with polystyrene. This composition is usually produced by polymerizing styrene (by mass or solution free-radical polymerization) in the presence of a small amount (ca 5%) of dissolved elastomer. Some of the important producers of impact-resistant polystyrenes are BASE (Polystyrol), Dow (Styron), and Monsanto (Lustrex). The 1988 U.S. production of impact polystyrene was more than 1 million t (92). 

In this stage of the investigation, poly(methyl methacrylates) (PMMAs) were selected as the polymeric probes of intermediate polarity. Polymers of medium broad molar mass distribution and of low tacticity (14) were a gift of Dr. W. Wunderlich of Rohm Co., Darmstadt, Germany. Their molar masses ranged from 1.6 X 10" to 6.13 X 10 g-mol. For some comparative tests, narrow polystyrene standards from Pressure Co. (Pittsburgh, PA) were used. 

In Ref. [107] the procedure above has been employed for the measurement of the molar mass distribution of a broad molecular weight polystyrene, obtained by radical polymerization with ethylacetate as solvent. The scaling parameters for this polystyrene in this marginal solvent have been determined to be a 2.8 x 10-4 cm2/s and b 0.52 [107]. The upper curve in Figure 17 shows the resulting molar mass distribution in comparison with the one obtained by SEC. 

In the early days of polymer science, when polystyrene became a commercial product, insolubility was sometimes observed which was not expected from the functionality of this monomer. Staudinger and Heuer [2] could show that this insolubility was due to small amounts of tetrafunctional divinylbenzene present in styrene as an impurity from its synthesis. As little as 0.02 mass % is sufficient to make polystyrene of a molecular mass of 2001000 insoluble [3]. This knowledge and the limitations of the technical processing of insoluble and non-fusible polymers as compared with linear or branched polymers explains why, over many years, research on the polymerization of crosslinking monomers alone or the copolymerization of bifunctional monomers with large fractions of crosslinking monomers was scarcely studied. 

Fig. 55. Gel-permeation chromatogram(GPC) of a microgel sample of Mw = 10X106 g/mol obtained in the anionic polymerization of EDMA in toluene. Microgel concentration = 1 g/L solvent = butyl acetate elution temperature = 70 °C is the weight-average molar mass of linear polystyrene used for comparison. [Reproduced from Ref. 256 with permission, Huthig Wepf Publ., Zug, Switzerland].

Union Carbide (34) and in particular Dow adopted the continuous mass polymerization process. Credit goes to Dow (35) for improving the old BASF process in such a way that good quality impact-resistant polystyrenes became accessible. The result was that impact-resistant polystyrene outstripped unmodified crystal polystyrene. Today, some 60% of polystyrene is of the impact-resistant type. The technical improvement involved numerous details it was necessary to learn how to handle highly viscous polymer melts, how to construct reactors for optimum removal of the reaction heat, how to remove residual monomer and solvents, and how to convey and meter melts and mix them with auxiliaries (antioxidants, antistatics, mold-release agents and colorants). All this was necessary to obtain not only an efficiently operating process but also uniform quality products differentiated to meet the requirements of various fields of application. In the meantime this process has attained technical maturity over the years it has been modified a number of times (Shell in 1966 (36), BASF in 1968 (37), Granada Plastics in 1970 (38) and Monsanto in 1975 (39)) but the basic concept has been retained. 

The second large-scale process was the batch mass suspension process. Monsanto did the pioneer work on this (41). In this process, prepolymerization is carried out in bulk and main polymerization in suspension the latter is taken to conversions of over 99%. In contrast to the continuous mass process, peroxide starters are used in order to achieve a high conversion at tolerable reaction times. Figure 3 shows a basic flow diagram of such a plant. A detailed discussion of advantages and disadvantages of the two processes can be found in R. Bishop s monograph published in 1971 (42), and it is continued in a paper by Simon and Chappelear in 1979 (43). It was a decisive factor for the economic success of impact polystyrene that these processes had been completely developed and mastered in theory and practice. 

One of the earliest published studies on extraction in twin-screw extruders was conducted by Todd (1974). In this work devolatilization was conducted under vacuum using two different polymeric systems, polystyrene in one and polyethylene in the other. In the case of polystyrene, styrene was not used as the volatUe component so as to avoid problems associated with further polymerization or depolymerization instead, use was made of mixtures of thiophene and toluene or ethylbenzene. Todd found good agreement between the measured exit concentrations of the volatile component and the predicted values using Pe = 40 in the solution to Eq. (38) (see Fig. 15). The value of 5 in Eq. (39) was not reported and it is not known whether a value was chosen to provide a fit with the data or whether it was known a priori. In any event, what is clear is that the exit concentration varies with IVwhich suggests that mass transfer is occur-... 

As explained in Sections 16.3.4, 6.4.1, and 16.4.2, SEC is a nonabsolute method, which needs calibration. The most popular calibration materials are narrow molar mass distribution polystyrenes (PS). Their molar mass averages are determined by the classical absolute methods—or by SEC applying either the absolute detection or the previously calibrated equipment. The latter approach may bring about the transfer and even the augmentation of errors. Therefore, it is recommended to apply exclusively the certified well-characterized materials for calibrations. These are often called PS calibration standards and are readily available from numerous companies in the molar mass range from about 600 to over 30,000,000g moL. Their prices are reasonable and on average (much) lower than the cost of other narrow MMD polymers. Other available homopolymer calibration materials include various poly(acrylate)s and poly(methacrylate)s. They are, similar to PS, synthesized by anionic polymerization. Some calibration materials are prepared by the methods of preparative fractionation, for example, poly(isobutylene)s and poly(vinylchloride)s. 

R. B. Bishop, Practical Polymerization for Polystyrene, CB1. Boston. Mass. (1971). 

Polyfmethyl methacrylate), initiated and polymerized at 250 by t-butylmagnesium bromide in toluene-THF solution (—). Mole fraction of monomer, X.VJM = 0.1 OM. XTHF is indicated in each case. A mixture of standard polystyrene samples of indicated molar mass (------). All traces are aligned so that the elution volumes correspond. 

Table 1. Distribution coefficient, elution volume, and the ratio of phase volumes rv = vp s/vp g calculated as a function of the mass of three samples of polystyrene with various degree of polymerization Pi = 1,300 P2 = 4,000 P3 = 80,000. Total interstitial volume ca. 250 ml Temperature 25, JC. vp s = 20.0 ml, m3 = 124.5 mg...

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

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