Styrene systems

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Billet and cowodcers [Chem. Jng. Tech., 38, 825 (1966) Jnstn. Chem. Engrs. Symp. Set No. 32, 5, 111 (1969] used the ethylbenzene/ styrene system at 100 torr and a 0.8-m column with 500-mm plate spacing. Two weir heights were used, 19 and 38 mm. Operation was at... 

In multiphase polymeric systems, the properties of the end products do not solely depend on the properties of the pure components, but other various parameters also have a great impact (Fig. 1). In order to emphasize these factors, the following systems are taken into consideration (I) elastomer toughened styrene system, (2) elastomer toughened polycarbonate blends, and (3) direct reactive blend processing. 

These observations call for comments. In the THF-styrene system the sodium salt... 

The correct explanation of the peculiar behaviour of the butadiene-styrene system was provided by O Driscoll and Kuntz 144). As stated previously, under conditions of these experiments butadiene is indeed more reactive than styrene, whether towards lithium polystyrene or polybutadiene, contrary to a naive expectation. This was verified by Ells and Morton 1451 and by Worsfold 146,147) who determined the respective cross-propagation rate constants. It is germane to stress here that the coordination of the monomers with Li4, assumed to be the cause for this gradation of reactivities, takes place in the transition state of the addition and should be distinguished from the formation of an intermediate complex. The formation of a complex ... 

The thermally-initiated styrene system is considerably simpler than most industrial applications. Though these experiments provided useful guidelines, it was difficult to develop broadly applicable design criteria without carefully evaluating a broad range of monomer, polymer and initiator systems. Hence we extended our kinetic model to some other monomer systems such as styrene and methyl methacrylate using common initiators such as benzoyl peroxide (BPO) and... 

Free radical copolymerizations of the alkyl methacrylates were carried out in toluene at 60°C with 0.1 weight percent (based on monomer) AIBN initiator, while the styrenic systems were polymerized in cyclohexane. The solvent choices were primarily based on systems which would be homogeneous but also show low chain transfer constants. Methacrylate polymerizations were carried out at 20 weight percent solids... 

Similar studies were undertaken as a comparison in the styrenic systems, although in these systems we also were able to compare the graft systems to diblock systems of similar composition. DSC also shows a depression of the high temperature Tg for all of the styrenes for the IK graft systems. The 5K systems showed a smaller depression but still a slight lowering of the Tg (Table VIII). Clearly, the t-butylstyrene system is most influenced, as anticipated. 

Subsequent mechanistic studies suggested that the abovementioned effect of ethylene on reaction efficiency is connected to a mechanistic divergence that exists for reactions of terminal styrenyl ethers versus those of disubstituted styrene systems [13b]. Whereas with monosubstituted styrenyl substrates the initial site of reaction is the terminal alkene, with disubstituted styrene systems the cyclic ji-systems react first. This mechanistic scenario suggests two critical roles for ethylene in the catalytic reactions of disubstituted styrenes ... 

Tab. 6.6. Results of allylic substitution of styrene system 163 with organocopper reagents.

F.A. Bovey. I m glad you asked that question. I should have pointed out very clearly that all of the spectra I showed for the chloroprene system vrere done at 90 MHz vMle those of the styrene system were obtained at 25 MEIz. 

In some systems it appears that the initiation step differs from the usual two-step sequence of Eqs. 3-13 and 3-14. Thus in the t-butyl hydroperoxide-styrene system only a minor part of the initiation occurs by the first-order homolysis reaction (Eq. 3-26f), which accounts for the complete decomposition of t-butyl hydroperoxide in the absence of styrene. Homolysis of the hydroperoxide occurs at a much faster rate in the presence of styrene than in its absence. The increased decomposition rate in the t-butyl hydroperoxide-styrene system occurs by a molecule-induced, homolysis reaction which is first-order in both styrene and hydroperoxide [Walling and Heaton, 1965]. The initiation reaction may be written as... 

The first system is based on a poly (norbornene)-supported Co salen and the second system is based on a poly(styrene) backbone. For the poly (norbornene) system, a homopolymers and several different copolymers were prepared (44), with varying fractions of Co salen side-chains and spacer side-chains (Figure 3, copolymer la-c and homopolymer Id). For the poly (styrene) system, both homopolymers of salen-containing monomers and copolymers with styrene (45,46) were prepared (Figure 3, copolymer 2a-c and homopolymer 2d). 

In the poly(methyl methaerylate)-styrene system, less than 7% of the original polymer remained as homopolymer at total conversion (77). Over 85% of the product was non-branched, single-segment block copolymer. The difference for these two systems is in part due to the higher molecular weight of the initial poiy(methyl methycrylate) (2900000 versus 495000) and in part to the preferential scission of the poly(methyl methacrylate) chain. This point was confirmed by running tests on a mixture of the two homopolymers in the presence of a radical acceptor to prevent macroradical recombination, and on the isolated block copolymers. 

The very informative semilogarithmic plot of over-all reaction rate vs. time was first published by Bartholome et al. (4, 6). Looking at the reaction rate-time curves given below one can see that even for the styrene system, which was shown to follow Smith-Ewart kinetics the best... 

Vinylidene chloride and chloroprene (Figures 7 and 8) under the given conditions produce curves which more or less resemble the styrene curve. Vinylidene chloride especially shows a long period of a rather constant reaction rate. By the theory of Harkins and Smith-Ewart this would be interpreted as a period of constant particle number and of constant monomer concentration at the reaction site—i.e., the monomer-polymer particles. The first assumption seems justified (15). The second assumption of constant monomer concentration at the reaction site can be true only in a modified sense because poly (vinylidene chloride) is insoluble in its monomer, and the monomer-polymer particles in this system therefore have a completely different structure as compared with the monomer-polymer particles in the styrene system. 

Since Schultz (7) found that ultimate conversion depended considerably on temperature (in a highly polymerized methyl methacrylate system), similar effects would be expected in the postirradiation-heating of the PVC-styrene system. Such effects have indeed been found. Figure 4 shows the effect of heating temperature on the conversion level at two different radiation doses. No increased conversion is found for a temperature higher than 75 °C. This seems to indicate that a more or less definite melting point of the partially polymerized mixture exists. When this temperature is reached during the postirradiation treatment, the reaction runs to a point of termination and is unaffected by further temperature increases. 

Melt Viscosity of Uncured Mixtures. One of the attractive features of the PVC—styrene system is the fact that the styrene acts as a temporary... 

Fig. 8.12. Dimensionless shear rate /30 locating the onset of shear dependence in the viscosity for narrow distribution poly(a-methyl styrene) systems. Symbols are (198, 199) O M = 3.3 x 106, 6 M=1.82x 106,O-M= 1.19 x 106, and 9 M = 0.444 x 106. Values for intrinsic viscosity (cM=0) are similar to those for polystyrene (see caption of Fig. 8.11)...

Funt and Gray recently reinvestigated the MMA-styrene system in detail (37). An analysis of the data at various initial feed ratios with tetrahydrofuran revealed that the copolymer composition was found to vary as the square of the feed ratios, according to the relationships proposed by O Driscoll,... 

The butadiene-styrene system alone has received the detailed study required to give a clearer picture of the mechanism. The results should, however, be similar for the other systems. The two homopolymerization rates are easily measured. The exchange rate between two active centres can be measured by forming a solution of polybutadienyllithium or of polystyryllithium and allowing it to react with the other monomer. It is convenient to measure the rate spectroscopically from the rate of... 

Although bubble sizes for large particles with the a-methyl styrene system have not been measured directly and any prediction must be regarded as highly speculative, Fig. 4.21 shows an estimate of the variation of mean bubble diameter with size of catalyst particles06 . This estimate is based on measurements with glass beads in water, with subsequent adjustments to allow for the different densities of the particles and differences in the viscosity and surface tension of the liquids. 

The initiation events involving dienes and styrene in hydrocarbon solvents have been thoroughly and accurately studied by the application of UV and visible spectroscopy. The archetype of such studies is the now classic 1960 study of Worsfold and Bywater 94) on the n-butyllithium-styrene system in benzene. The reaction was found to follow the relationship ... 

These results indicated that the conventional spectroscopic method of determining the completion of the initiation reaction is fraught with potential uncertainties, insofar as the /-butyllithium-styrene system is concerned. Hsieh s method 106) of analyzing for residual initiator (via GC measurements for isobutane), is unencumbered with such uncertainties. 

It has been found (Table 24) that in hydrocarbon solvents, the diene polymerizes preferentially in diene-styrene systems, initially to the near exclusion of the styrene even though the latter monomer exhibits the faster rate of homo-polymerization. When the diene supply nears depletion, styrene begins to become incorporated into... 

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