Styrene, radical anion/cation

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). 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

Styrene is one of the few monomers that may be polymerized by free-radical, anionic, cationic, or coordination (Ziegler-Natta) methods. This property, common to styrene and most of its derivatives, is the consequence of the availability of a benzylic position in these monomers, which is capable of stabilizing a radical, carbanionic, or carbocationic center, as well as possessing a polarizability amenable to the charge distributions required by coordination methods of polymerization. 

Polystyrene was first prepared by E. Simon in 1839 (7). At the time, he thought that he had produced an oxidation product fiom styrene which he called "styrol oxide." Since then, the polymerization of styrene has been studied extensively. Today, because of the stabilization effect of the phenyl group on styrene, styrene can be polymerized using fi ee radical, anionic, cationic, and even coordination initiatois to make polystyrene with dilferent properties. In general, a polymerization reaction consists of the following steps ... 

Styrene is unique in that it can be polymerized by free-radical, anionic, cationic or Ziegler mechanisms. However, only the first two processes are used... 

What is the composition of the first copolymer chains produced by the copol5Tnerization of equimolar quantities of styrene and methyl methacrylate in (a) free radical, (b) cationic, and (c) anionic copolymerization ... 

Many ionogenic monomers containing a polymerizable carbon double bond have been reported in the literature, and therefore a wide variety of anionic, cationic, and amphophilic polyelectrolytes may be synthesized using free radical polymerizations. Examples of anionic ionogenic monomers which have been used to synthesize anionic polyelectrolytes include acrylic acid [4-10], methac-rylic acid [6-8,11,12], sodium styrenesulfonate [7,13,14], p-styrene carboxylic... 

As described in the previous chapter, in the work on electrolytic polymerization which has appeared in the literature, the active species were formed by an electrode reaction from the compounds added to the reaction system and thus initiated polymerization. However, the possibility has been considered of direct electron transfer from the cathode to monomer or from monomer to the anode forming radical-anion or -cation, followed by initiating polymerization. Polymerization of styrene initiated by an electron has been observed when the monomer was exposed to the electric discharge of a Tesla coil (74), y-radiation (75, 16) and to cathode rays from a generator of the resonant transformer type (77). 

Styrene is an interesting monomer, as it is polymerizable in any one of the radical, anionic and cationic mechanisms, depending on the catalyst used. Its radiation-incuded polymerization, however, had long been recognized to be radical polymerization, until it was suggested that cationic polymerization was also possible in alkylhalide solutions at low temperature, as mentioned in the previous chapter (2, 3, 4). 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

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. 

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

On the basis of the profound difference in copolymer composition from a free radical or cationic type polymerization, it was stated that the sodium and potassium initiated polymerizations were carbanionic in nature. This has been one of the strongest arguments in favor of the anionic nature of the sodium and potassium polymerizations. The authors also suggested that the composition of a styrene-methyl methacrylate copolymer might be used as a criterion of the type of propagation induced by a given initiator. 

In an earlier report Mazzocchi and his coworkers reported that the photo-reaction of A) methylnaphthalimide (325) with phenyIcyclopropane involved the production of a radical cation/radical anion pair. The product from the reaction was the cyclic ether (326). - A study of the mechanism of this reaction using suitably deuteriated compounds has demonstrated that the reaction is not concerted and takes place via the biradical (327). - Other systems related to these have been studied. In the present paper the photoreactivity of the naphthalimide (328) with alkenes in methanol was examined. Thus, with 1-methylstyrene cycloaddition occurs to the naphthalene moiety to afford the adducts (329) and (330). The mechanism proposed for the addition involves an electron transfer process whereby the radical cation of the styrene is trapped by methanol as the radical (331). This adds to the radical anion (332) ultimately to afford the observed products. Several examples of the reaction were descr ibed. 

The hindrance of desorption does not affect the mobility of radical-anions on the metal surface. Hence, their dimerization with formation of still adsorbed dimeric dianions is very likely, and these may grow and form living oligomers. Degree of polymerization of the attached oligomers depends on their lifetime on the surface, and the lifetime is shortened by a cationsolvating solvent that facilitates removal of the cation from the metal lattice and therefore the desorption. This is demonstrated by Overberger (13), who studied the co-polymerization of styrene and methyl methacrylate initiated by a fine suspension of particles of metallic lithium. 

Can any kind of initiator produce the cyclopolymer from St-C3 St Since the monomer is a kind of styrene derivative, so it could be polymerized by a variety of initiators cationic, radical, anionic, and coordination catalysts, and the question could be answered by the results of the polymerization. 

Donor/Acceptor Systems Upon excitation, a monomer donor (e.g., styrene) undergoes an electron transfer with a monomer acceptor (e.g., maleic anhydride). Then, the donor radical cation and the acceptor radical anion can recombine to form a biradical a recent review was provided in Ref. [193]. 

Removal of metal cation from lattices of alkaline-earth is very difficult. Consequently, initiation is extremely slow and the concentration of radical-anions released into the solvent can be very low. Under such conditions radical growth might compete with their dimerization. The reported formation of random co-polymers of styrene and methyl-methacrylate when the mixture of these monomers reacts with dispersions of alkaline-earth metals might be evidence for such a polymerization95. Confirmation of these observations is desirable. 

Some alkenes undergo polymerization by more than one mechanism. For example, styrene can undergo polymerization by radical, cationic, and anionic mechanisms because the phenyl group can stabilize benzylic radicals, benzylic cations, and benzylic anions. The particular mechanism followed for the polymerization of styrene depends on the nature of the initiator chosen to start the reaction. 

TSE s have been used to prepare new families of engineering materials of high performance, polymers and their blends. Polycondensation, free radical, anionic and cationic polymerizations were conducted to obtain, e.g., PA, PEST, POM, styrenic or acrylic resins. When the reaction is conducted in a low molecular weight liquid (e.g., solution or emulsion polymerization), usually devolatilization and compounding are carried out in a cascade second extruder. Functionalization and chemical modifications have been performed in TSE on virtually all polymers. 

Styrene is one of the oldest and most studied monomers. It spontaneously generates free radials upon heating above 100 °C and polymerizes yielding amorphous polystyrene (PS). Styrene can also be polymerized by other mechanisms (anionic, cationic, or Zeigler-Natta) with the aid of chemical initiators. Commercially, over twenty billion pounds of PS are produced annually worldwide. All of this polystyrene is produced via free radical (FR) chemistry, and mostly via continuous solution polymerization processes. The commercial preference for the continuous solution process is due mainly to economic factors. Non-solution polymerization processes (suspension and emulsion) have lower reactor efficiency (product/reactor volume) due to reactor volume occupied by the water which adds to the manufacturing cost. 

The photoreactions (X >435 nm in dichloromethane) of tetranitromethane (261) with styrenes (260) to yield nitro-trinitromethyl adducts (269), diastereo-meric oxazolidines (270), nitro ketones (271) and nitronic esters (272) may be rationalised in terms of the interlinked pathways in Scheme 3. SET yields nitrogen dioxide (262), the styrene radical cation (263) and the trinitromethanide ion (264). Addition of nitrogen dioxide (262) to the styrene (260) initiates a radical chain process. Benzylic radical (266) is oxidised by tetranitromethane (261) to the benzylic cation (265). Nitro-trinitromethyl adducts (269) result from coupling of the cation (265) with the anion (264). A competing pathway involves reaction of radical cation (263) with anion (264). C clisation of the resulting radical (268) yields the aminoxyl (267), from which loss of nitrogen dioxide yields the nitronic ester (272), whereas coupling of (267) with the radical (266) forms oxazolidine diastereomers (270). The nitroketones (271) may arise, at least in part, from secondary photolysis of the oxazolidines (270). ... 

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