Monomer Removal

Nearly all polymerization processes and products require a post-reaction process to remove and reduce to an acceptable level residual monomer(s), solvent or diluent. End use properties can be adversely affected by high levels of residuals through toxicity, odor, or poor physical properties. In the cases of residual solvent or diluent, a separation process involving the evaporation of the volatile components (devolatilization) can be used. Devolatilization can be used for residual monomer removal, but completing the polymerization of monomer is an attractive alternative when applicable. Polymerization finishing is usually accomplished with an increase in temperature to kick-off a finishing initiator or the addition of an initiator. (D For the dispersion... 

Since the ester is relatively insensitive to water (and probably other similar impurities [la, 2b, c]) but the propagating ions are excessively so [lb, 28] we think that in the earlier work the ions formed by the slow dissociation of the styrene-solvated ester were inactivated as quickly as they were formed ions became evident as chain-carriers only when the depletion of monomer removed the stabilising coordinated monomer molecules from the ester and this then ionised very rapidly, producing the final very fast reaction-kick observed by Gandini and Plesch [1]. 

Another problem is that PS is not thermally stable. Therefore, even if special techniques for monomer removal are implemented by the manufacturer, the level of monomer in the final fabricated article is oftentimes higher than that in the raw polymer granules. 

Poly( inyl chloride), poly(acrylonitrile) and the high acrylonitrile copolymers have presented the major problems with respect to reducing the residual monomer content to extremely low levels. These are in the glas state tmder the conditions where monomer removal must be carried out. In principle, the temperature should be raised above the glass temperature to facilitate monomer removal. In practice, however, the systems are usually lattices or slurries of suspension polymer and coagulation could become a problem. In any case, both the rubbery and glassy states must be considered in any discussion of the monomer removal problem. The basic principles of the transport of gases in both situations have been presented briefly and with appropriate literature references in the introductory section of this review. 

In the final stages of the synthesis (Scheme 21), the dimerization protocol developed earlier (Scheme 5, vide supra) was followed, with the slight modification that triphenyltin hydride was used for reduction of the azide functional group. Cyclization of the a-aminoketone with PPTS afforded dimer 46 in 79 % yield, accompanied by 17 % of deazidoketone monomer. Removal of the... 

The Himont spheripol loop reactor process is initiated by injecting specially prepared supported catalyst and cocatalyst into liquid propylene circulated in a relatively simply high L/D ratio loop reactor, followed by monomer removal (Figure 4.2). The homopolymers so produced can he circulated through ethylene and ethylene/propylene gas phase reactors for insertion of copolymer fractions before final... 

When the degree of polymerization is large, the fraction of monomer removed by these other steps is negligible, so that the rate of polymerization, is given by... 

A promising technique for residual monomer removal is pervaporation, as no additional chemicals are needed for this membrane process and the energy costs are typically low. It has been shown that pervaporation can remove a considerable amount of acryhc monomer from polymethylmethacrylate (PMMA) latexes [15]. Apparently, the Hmiting factor for mass transfer does not occur in the polymer particles, mainly because of the high specific area of the polymer-water interface as compared to the membrane area. Although the high initial costs, as well as fouling of the membrane surface with the polymer particles, are potential drawbacks, pervaporation may thus be expected to provide a viable alternative. 

Although flash devolatilization has been applied for high volatility monomers removal, such as vinyl chloride from poly(vinyl chloride) [103] and butadiene from polybutadiene [104], devolatihzation of aqueous polymer dispersions is usually carried out using a stripping agent (steam and nitrogen are the most commonly used air can also be used, but explosive vapor mixtures can be produced). Devolatilization of aqueous phase dispersions is a mass-transfer process, which involves the following steps in series ... 

No.23, 1st Dec.2002, p.4245-9 KNOWLEDGE-BASED CHOICE OF THE INITIATOR TYPE FOR MONOMER REMOVAL BY POSTPOLYMERIZATION... 

The mechanisms involved in monomer removal by post polymerisation were studied. Three redox initiator systans which generate radicals with different hydrophobidties were investigated tert-butyl hydroperoxide, hydrogen peroxide and potassium persulphate. Ascorbic acid was used as a reductant in all cases. The efficiency of these initiator systems for the removal of residual monomers from commercial latexes was studied. The examples exauuued were removal of unreacted vinyl acetate from a vinyl acetate/butyl aerylate/acrylie acid latex, methyl methacrylate from a methyl aaylate, butyl aaylate/acryhc acid latex and butyl acrylate from a butyl aaylate/styrene/ acrylic acid latex. Efficieucy of monomer ranoval by post polymerisation increased with the hydrophobidty of the radical formed from the initiator system and this was independent of the water solubihty of the residual monomer. Reasons for the observations were discussed. 35 refs. 

Between the two devolatilisation steps, an injection of water (stripping) can be added to improve monomer removal. After condensation, unreacted styrene and ethyl benzene are recycled to the feed line, either directly using a recycle loop or through a storage tank. A purge of undesirable components is carried out on this stream. 

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