Styrene monomer purification

UOP Styrene monomer Styrene monomer Purification of styrene monomer by selective hydrogenation. Less than 10 ppm of PA in SM product 3 1996... 

Styrene monomer was purified by vacuum distillation over CaHi. Inhibitor in styrene was removed using activated alumina. N-heptane was purified by distillation over sodium to remove tire trace of residual moisture. The purified slyreaie and n-heptane were stored over activated alumina under nitrogen blanket. Et[Ind]2ZrCl2 Strem Chem.), MAO (modified methylaluminoxane, type 3A, Akzo Novel) wee used without fiirtha" purification. 

S-b-MM was prepared according to the published procedures (4-6). Molecular weights in the desired range and with narrow, unimodal distibutions were obtained without resorting to extensive monomer purification (ljL) or capping of the styrene block with diphenylethylene (4,5,7-10). The S-b-MM contained about 10 mol% MM, and was conveniently characterized by 1H NMR and IR spectroscopy. 

Monomer Purification. All polymers were prepared from either column purified or distilled monomers. The acrylate and methacrylate esters, styrene, and vinyl nitrile type monomers were purified by passing them through Rohm and Haas Amberlyst exchange resins (salt forms), while the diene monomers were either distilled directly from cylinders and condensed in a dry ice trap or alternatively caustic washed to remove the inhibitor. 

Styrene-containing block copolymers are commercially very important materials. Over a billion pounds of these resins are produced annually. They have found many uses, including reinforcement of plastics and asphalt, adhesives, and compatibilizers for polymer blends, and they are directly fabricated into articles. Most styrene-containing block copolymers are manufactured using anionic polymerization chemistry. However, anionic polymerization is one of the more costly polymerization chemistries because of the stringent requirements for monomer and solvent purity. It would be preferred, from an economic cost perspective, to have the capability to utilize free radical chemistry to make block polymers because it is the lowest cost mode of polymerization. The main reasons for the low cost of FR chemistry are that minimal monomer purification is required and it can be carried out in continuous bulk polymerization processes. 

Free-radical polymerization is the preferred industrial route because (1) monomer purification is not required (158) and (2) initiator residues need not be removed firom polymer as they have minimal effect on polymer properties. The exceptions are the styrene-butadiene block copolsrmers and very low molecular weight PS. These polymers are manufactured using anionic and cationic polymerization chemistry, respectively (159). Anal5dical standards are available for PS prepared by all four mechanisms (see Initiators). 

The SPS process is divided into eight sections. They are monomer purification section, catalyst section, polymerization section, styrene removal from SPS, deactivation section, pelletizing section, blending section, and shipping section. Each section will be explained from the patent information. 

Figure 12.1 Flow diagram of the monomer purification section. V-100 styrene storage tank V-110 oxygen stripping column V-120 alumina column V-130 hydrogenation column V-140 surge vessel F-140 filter V-150 styrene feed tank.

With the improvement of refining and purification techniques, many pure olefinic monomers are available for polymerization. Under Lewis acid polymerization, such as with boron trifluoride, very light colored resins are routinely produced. These resins are based on monomers such as styrene, a-methylstryene, and vinyltoluene (mixed meta- and i ra-methylstyrene). More recently, purified i ra-methylstyrene has become commercially available and is used in resin synthesis. Low molecular weight thermoplastic resins produced from pure styrene have been available since the mid-1940s resins obtained from substituted styrenes are more recent. 

The dehydrogenation of the mixture of m- and -ethyltoluenes is similar to that of ethylbenzene, but more dilution steam is required to prevent rapid coking on the catalyst. The recovery and purification of vinyltoluene monomer is considerably more difficult than for styrene owing to the high boiling point and high rate of thermal polymerization of the former and the complexity of the reactor effluent, which contains a large number of by-products. Pressures as low as 2.7 kPa (20 mm Hg) are used to keep distillation temperatures low even in the presence of polymerization inhibitor. The finished vinyltoluene monomer typically has an assay of 99.6%. 

Styrene (Fisher), p-methylstyrene (Mobil), and t-butylstyrene (DOW) were purified by passing through a column of activated alumina and then carefully degassed to remove all traces of 0. Further purification by vacuum distillation from dibutyl magnesium resulted in anionically pure monomers. 

Materials and Purifications. 2-Methyl pentene-1 (Aldrich) was distilled under normal pressure at 62°C after refluxing for one hour 1n the presence of A1L1H.. Methyl methacrylate and para-methyl styrene (Aldrich) were distilled under reduced pressure at about 60°C the purified monomers were sealed and stored 1n refrigerator before use. The compressed gas, sulfur dioxide (Matheson), was led through a P208 tower before Introduced Into reaction system. Hydroxyethyl acrylate (Aldrich) was used for polymerization without further purification. 

In our laboratory we have observed similar behavior in the case of styrene. With styrene, a more detailed study of the effects of drying has been made, and all of the kinetic results can be readily explained by postulating the coexistence of more than one ionic process under the conditions of the most exhaustive purification and drying (21,22), reverting to a single ionic species under the conditions of moderate dryness (22). This behavior reverts further to the normal free radical process when no extraordinary means of drying the monomer are employed (4, 21). 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

Divinylbenzene, styrene, a,a -azoisobutyronitrile and methacrylic monomers di(methacryloyloxymethyl)-naphthalene (DMN), methacrylic ester of p,p -dihydroxydiphenylpropane diglycidyl ether (MEDDE), and dimethacrylglycol-ethylene (DMGE) were received from Fluka and used without further purification. Polyfvinyl alcohol), /7-dccanol, ethanol, acetone and toluene (Merck), fumed silica with specific surface area of 300 m2/g (Kalush), triethoxysilane (Kremnepolimer) and hexamethyldisilazane (Fluka) were applied. 

Recent work on the dimerisation of 1,1-diphenylethylene by aluminium chloride produced conclusive evidence that direct initiation does not lead to the total ctmsump-tion of the catalyst. This excellent piece of research diowed that about 2.5 aluminium atoms are needed to give rise to one carbenium ion. Similar indications were reported by Kennedy and Squires for the low temperature polymerisation of isobutene by aluminium chloride. They underlined the peculiar feature of limited yields obtained in flash polymerisations with small amounts of catalyst. The low conversions could be increased by further or continuous additions of the Lewis acid. Equal catalyst increments produced equal yield increments It was also shown that introductions of small amounts of moisture or hydrogen chloride in the quiescent system did not reactivate the polymerisation. This work was carried out in pentane and different purification procedures for this solvent resulted in the same proportionality between polymer yield and catalyst concentration. Experiments were also performed in which other monomers (styrene, a-methylstyrene, cyclopentadiene) were added to the quiescent isobutene mixture. The polymerisation of these olefins was initiated but limited yields were again obtained. Althou the full implications of these observations must await more precise data, we agree with the authors interpretation that allylic cations formed in the isobutene polymerisation, while incapable of activating that monomer, are initiators for the polymerisation of the more basic monomers added to the quiescent mixture. The low temperature polymerisation of isobutene by aluminium chloride was also studied... 

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