Popcorn polymer

Butadiene is also known to form mbbery polymers caused by polymerization initiators like free radicals or oxygen. Addition of antioxidants like TBC and the use of lower storage temperatures can substantially reduce fouling caused by these polymers. Butadiene and other olefins, such as isoprene, styrene, and chloroprene, also form so-called popcorn polymers (250). These popcorn polymers are hard, opaque, and porous. They have been reported to... 

Uninhibited chloroprene suitable for polymerisation must be stored at low temperature (<10° C) under nitrogen if quaUty is to be maintained. Otherwise, dimers or oxidation products are formed and polymerisation activity is unpredictable. Insoluble, autocatalytic "popcorn" polymer can also be formed at ambient or higher temperature without adequate inhibition. For longer term storage, inhibition is required. Phenothiasine [92-84-2] / fZ-butylcatechol [2743-78-17, picric acid [88-89-17, and the ammonium salt of /V-nitroso-/V-pheny1hydroxy1 amine [135-20-6] have been recommended. 

Chloroprene monomer will autoxidise very rapidly with air, and even at 0°C it produces an unstable peroxide (a mixed 1,2- and 1,4-addition copolymer with oxygen), which effectively will catalyse exothermic polymerisation of the monomer. The kinetics of autoxidation have been studied [1], It forms popcorn polymer at a greater rate than does butadiene [2],... 

An unusually highly crosslinked, and therefore insoluble, form of polystyrene called popcorn polymer may also contain stuck free radicals. It has the lifelike property of growing when put into fresh... 

Usually, popcorn polymers are formed in the radical polymerization of monomers containing small amounts of crosslinkers. However, the polymerization of SAN is a remarkable exception (14). 

Popcorn polymers are hard, brittle, highly crosslinked porous masses, named as such because of their physical appearance. The formation of popcorn polymers in industrial polymerization processes is highly undesirable. The formation can be suppressed by suitable crosslinking inhibitors. However, in order to avoid the formation on the walls of a reactor that are mainly in contact with the gas phase, volatile crosslinking inhibitors must be chosen. Examples for volatile crosslinking inhibitors are nitric oxide and sulfur dioxide (15,16). 

S.M. Campbell and C.-Y. Sue, Nitric oxide for vapor phase elimination of styrene and acrylonitrile popcorn polymer in bulk SAN production, US Patent 5 272 231, assigned to General Electric Company (Pittsfield, MA), December 21,1993. 

This paper reviews some of the more basic contributions over the past 20 years or so to the study of popcorn polymers. Some results from the authors9 laboratories are then presented in an attempt to characterize further the nature of these polymers and their growth mechanisms. These more recent results include those obtained by studies of the popcorn polymers using the polarizing and electron-scanning microscopes and ESR measurements. The importance of crosslinking and entanglements is discussed, and industrial applications—present and potential— are considered. 

Hr he first observations of popcorn (PC) polymer formation go back to the beginning of this century when Kondakow (J) studied the polymerization of dimethylbutadiene. The name popcorn polymers, however, is of much more recent origin. It was first used in the U.S. in 1940, where the similarity of appearance of these materials to a sponge or to cauliflower was replaced by the obviously more popular popcorn. Today, it is possible to prepare polymers of this type having no similarity at all to popcorn, but the name is retained. 

Breitenbach and Frank (5) showed that with styrene-divinylbenz-ene, no further additives (such as peroxides) are necessary for popcorn polymer formation. Breitenbach and Fally (6) found, in methyl acrylate polymerization, the possibility of crosslinking in the polymerization of a monovinyl compound. Miller and coworkers (7) developed the kinetics of the process Pravednikow and Medvedev (8) studied the chain scission, and assumed radical formation by that process as an important step. 

The occurrence of straining in a microscopic range was shown by Breitenbach, Preisinger, and Tomschik (9), and further microscopic studies of popcorn polymer formation and growth were carried out by Breitenbach and coworkers (10). 

This paper presents some results obtained with the polarizing microscope and the scanning electron microscope. The importance of cross-linking and entanglements is discussed, and results of ESR measurements on popcorn polymer systems are presented. 

At later growth stages, strains are also induced in the surrounding polymer gel. Figure 6 may show the first step in the transformation of a preformed glassy polymer into a popcorn polymer. 

The primary popcorn particles give a variety of different shapes in the final product. Electron microphotographs are given in Figures 7-9. The morphological elements of the popcorn polymer have dimensions of several /on. 

Figure 1. Formation of popcorn polymer particle styrene with 0.6 wt % p-DVB 17 hours at 60°C.

Popcorn polymer only forms in those polymerizing systems where a certain amount of crosslinking takes place. In all systems investigated, an optimum crosslinking range for popcorn polymer formation exists (11) In thermal polymerization of a styrene-p-divinylbenzene mixture... 

Figure 5. Formation of popcorn polymer particles n-butylacrylate with 3.6 mole % glycol dimethacrylate, 0.1 mole % AIBN at 20°C 3 hours after first particles formed.

Figure 7. Surface of a popcorn polymer proliferated in styrene...

Figure 6. Formation of popcorn polymer particles m-bromostyrene with 1.9 wt % p-DVB 144 hours at 70°C.

Figure 9. Surface of an acrylonitrile-styrene-glycol dimethacrylate popcorn polymer...

Figure 10. Thermal polymerization of styrene-p-divinyl benzene at 70°C optimum for popcorn polymer formation...

Under the most favorable conditions, the formation of a popcorn polymer particle is a highly improbable process for 1016 molecules of polymer produced in the system, only one popcorn polymer particle has been grown, possibly from a single macromolecular precursor. By contrast, growth under optimum conditions is a fast process and approximately obeys the relation—given by Welch et al. (4)—for butadiene popcorn polymer ... 

Popcorn polymers are also formed by some pure monovinyl compounds—for example, methyl acrylate, ethyl acrylate, and n-butyl methacrylate. In these cases, chemical crosslinking by combined polymer chain transfer and combination termination also seems to take place. 

One reaction of popcorn polymers is their very rapid, proliferous growth in appropriate monomers. The rapid growth reaction corresponds to a relatively high content of the growing material on radical chain ends. It is possible to measure the growth rate directly by ob-... 

The intensity of the ESR signal remains nearly unchanged after complete consumption of the monomer so long as the popcorn polymers are prevented from coming in contact with air. ESR signals have been observed in methyl methacrylate popcorns after 14 months at room... 

Figure 12. ESR spectra of growing popcorn polymer chains...

It is possible to obtain popcorn polymers in the form of flakes (24). For this purpose, a pulverized popcorn polymer is brought into the monomer feed in a stirred reactor. The proliferous growth of the seed material in the stirred reactor leads to a finely distributed material... 

Figure 13. ESR spectra of ortho-substituted styrene popcorn polymers...

The popcorn polymer flakes may be used as supporting materials for ion exchangers. Depending on the monomers units, the polymers have functional groups of different kinds and may be used for synthetic purposes. The materials also adsorb and absorb a variety of organic vapors and thus may be used as filter materials. 

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