Impurities in styrene monomer

All the non-polymerisable volatile impurities which occur in styrene monomer used in polystyrene manufacture will also be present in the finished polymer. Analysis of the monomer is therefore important. Gas chromatographic methods, for determining up to 40 impurities in styrene monomer are discussed in methods 28 to 30. 

In Method 28 the sample was first analysed on the squalene column in the presence of an added internal standard (eg. n-butyl benzene) to determine down to 5 ppm n-propyl benzene, butyl benzenes, diethyl benzenes, ethyl toluenes, methyl styrenes, ediyl styrenes and divinyl benzenes. The sample was then re-analysed on the Carbowax column to determine benzene, toluene, xylenes, ethyl benzene, cumene, n-propyl benzene and m-/p-ethyl toluenes. By temperature programming it is possible to reduce the analysis time to about one hour using a single gas chromatographic column (Carbowax 15-20 M-Celite) and to separate up to 40 hydrocarbons up to die divinyl benzenes and benzaldehyde. Experimental conditions are given in Method 29 (hydrocarbons) and Method 30 (benzaldehyde). 

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Typical applications at Polysar included the quantification of residual solvents and monomers in finished rubber products (e.g. styrene in SBR), quality control of feedstocks such as benzene or ethyl benzene as impurities in styrene monomer, and the analysis of samples collected from environmental monitoring programs. 

METHOD 28 - DETERMINATION OF HYDROCARBON IMPURITIES IN STYRENE MONOMER. GAS CHROMATOGRAPHY. 

This method is capable of determining a wide range of alkyl benzene impurities in styrene monomer in amounts down to 5 ppm. 

In Table 7.15 are tabulated relative corrected retention distance data obtained using isothermal and temperature programmed Carbowax columns fot a range of impurities in styrene monomer. It is seen that different separations are obtained by the two methods of analysis. The selection of method is dependent on the separations it is required to achieve. 

Table 7.15 - Impurities in Styrene Monomer, Relative Corrected Retention Distances... 

It is seen in Table 7.15 that in the temperature programmed gas chromatographic method benzaldehyde with a retention distance relative to methyl styrene of 1.60 is well resolved from all other impurities in styrene monomer. 

Determination of down to 5 ppm hydrocarbon impurities in styrene monomers. Gas chromatography 28.1 Summary... 

The effect of using a borate compound together with a small amount of TIBA as cocatalyst for the polymerization of styrene to SPS was examined by Campbell [1], Tomotsu [13], and Kucht et al. [14]. TIBA was found to be not only a good scavenger of impurities in styrene monomer, but also a component to increase the number of the active sites as well as of the syndiotacticity of the resulting styrene polymers. 

Many types of commercial styrene polymerization processes are applied. However, the process for the production of syndiotactic polystyrene (SPS) is completely different from those for atactic polystyrene polymerizations. Catalysts are sensitive to the impurities in styrene monomer and SPS is insoluble in aromatic solvents. 

It is a matter of general observation that with styrene, the polymerization-rate curves will exhibit three distinct phases, the nature of which can be determined by the polymerization conditions and the purity of the monomer (1) an initial slow period at the begin ning of the reaction, known as the induction period, which appears to be associated with the presence of an inhibitor or other impurity in the monomer (2) a period of relatively rapid polymerization, which persists almost to the end of the reaction, and for which the rate is exponentially dependent upon temperature and (3) a final slowing down in rate as the reaction approaches completion and the... 

Retention data for some of the impurities found in styrene monomer are given in Table 7.12. 

According to the experiments shown (l/ r )o=2.5 XIO at 100°C, which is somewhat larger than the value of Cm for pure styrene indicated by other experiments at this temperature. The presence in the monomer of impurities which cause chain transfer would account for this discrepancy. 

It follows that each molecule of HD formed corresponds to a molecule of metal hydride. Measurements of HD showed that one percent of metal hydride was present as an impurity in the Zr (benzyl)4 solution in toluene catalyst. On adding styrene monomer the hydride did not disappear from the reaction mixture, but progressively increased as the polymerization proceeded. It was estimated that if the hydride had the empirical formula (CeH6CH2) 3ZrH] , the amount formed corresponded to one molecule per chain. The persistence of this hydride in solution probably results from dimerization giving species of the type (XXVIII). 

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. 

Berlin [69] also confirmed the importance of the presence of OH radicals in his investigation of the polymerisation of polystyrene in the presence of styrene monomer when he found the addition of water to the reaction solvent (benzene) greatly enhanced the yield of polymer. However, latterly it has been argued for these systems that the appearance ofwater decomposition products (e. g. H2O2) led to oxidation of the various impurities, which previously, may have acted as inhibitors in the polymerisation process. 

I Materials, Double distilled water and absolute ethanol were used in all polymerisations Styrene monomer was washed with 10% w/w aqueous sodium hydroxide solution and then distilled under a nitrogen atmosphere with reduced pressure to remove inhibitor and impurities. 4.4f-Azobis (4-cyanovaleric acid) (ADIB), recrystallised from absolute ethanol to remove any peroxide impurities, and benzoyl peroxide (BzP) of reagent grade were utilised. 

Other technical barriers were the need to control the exotherm of polymerization and to produce colour-free polystyrene. While the manufacture of styrene seems simple and straightforward, in the early days at Dow there were three major impurities in the styrene monomer apart from residual ethylbenzene. These were phenylacetylene (which acted as an inhibitor for styrene polymerization), divinylbenzene (which caused plugging and fouling of the distillation column for separating styrene from its precursor, ethyl benzene) and sulphur (which caused discoloration of the polystyrene). 

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