Styrene homopolymerization

The thermal polymerization of S has a long history.310 The process was first reported in 1839, though the involvement of radicals was only proved in the 1930s. Carefully purified S undergoes spontaneous polymerization at a rate of ca 0.1% per hour at 60 C and 2% per hour at 100 °C. At 180 aC, 80% conversion of monomer to polymer occurs in approximately 40 minutes. Polymer production is accompanied by the formation of S dimers and trimers which comprise ca 2% by weight of total products. The dimer fraction consists largely of cis- and trans-1,2-diphenylcyclobutanes (90 and 91) while the stereoisomeric tetrahydronaphthalenes (92 a nd 93) are the main constituents of the trinier fraction.313 

The Mayo mechanism involves a thermal Diels-AIder reaction between two molecules of S to generate the adduct 95 which donates a hydrogen atom to another molecule of S to give the initiating radicals 96 and 97. The driving force for the molecule assisted homolysis is provided by formation of an aromatic ring. The Diels-AIder intermediate 95 has never been isolated. However, related compounds have been synthesized and shown to initiate S polymerization. 110 

The identification of both phenylethyl and l-phenyl-1,2,3,4-letrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMF provides the most compelling evidence for the Mayo mechanism. The l-phenyl-1,2,3,4-tetrahydronaphthalenyl end group is also seen amongst other 

Thermal initiation of styrene has been shown to be third order in monomer. The average rate constants for third order initiation determined by Hui and Hamielec is ki= (M s ). The rale constant for formation of the 

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Goodwin et al. (Z2.), Figures 2 and 3, styrene homopolymerization in a batch reactor at 70°C with no added surfactant. 

Badder and Brooks (2A), Figure 7, styrene homopolymerization in a CSTR at 50 °C with added surfactant. 

Copolymerization. The copolymerization of butadiene-styrene with alkyllithium initiator has drawn considerable attention in the last decade because of the inversion phenomenon (12) and commercial importance (13). It has been known that the rate of styrene homopolymerization with alkyllithium is more rapid than butadiene homopolymerization in hydrocarbon solvent. However, the story is different when a mixture of butadiene and styrene is used. The propagating polymer chains are rich in butadiene until late in reaction when styrene content suddenly increases. This phenomenon is called inversion because of the rate of butadiene polymerization is now faster than the styrene. As a result, a block copolymer is obtained in this system. However, the copolymerization characteristic is changed if a small amount of polar solvent... 

We consider styrene homopolymerization by a free-radical mechanism. Styrene, like any other vinyl monomer, is bifunctional, because the double bond opens two arms in the polymerization processs (one of the carbons is attacked by a free radical, activating the other carbon atom, which may continue to propagate the chain). 

Figure 1. Styrene homopolymerization— limiting conversions (Equations 1 and 2 with parameters Tgp = 93.5°C Tym = -88.2°C atp = 0.48 X lO C 1 am = 1.0 X 10 3oC 1). Experimental limiting conversions (bulk, suspension, and emulsion polymerization)—(O) present data (%)from Ref. 12.

Kuntz (33) reported on the copolymerization of butadiene and styrene in n-heptane at 30° using n-butyl lithium. Although styrene homopolymerized six times faster than butadiene, the copolymerization rate was initially the same as that of butadiene homopolymerization and then increased markedly. It was found that about 80% of the styrene remained when 90% of the butadiene was consumed and that the increase in rate coincided with the almost complete consumption of butadiene. With added tetrahydrofuran, the rate of polymerization was faster and about 30% styrene was found in the initial copolymer. 

In order to develop a sound optimization policy, a good understanding of styrene polymerization kinetics is necessary. In the following section the general kinetic scheme of styrene homopolymerization is introduced. 

The basic reaction scheme for free-radical bulk/solution styrene homopolymerization is described below. A complete description of copolymerization kinetics involving styrene is not given here however, the homopolymerization kinetic scheme can be easily extended to describe copolymerization using the pseudo-kinetic rate constant method [6]. Such practice has been used by many research groups [7-10] and has been used extensively for modelling of copolymerization involving styrene by Gao and Penlidis [11]. In this section, all rate constants are defined as chemically controlled, i.e. they are only a function of temperature. 

In early work, Prisyazhnyuk and Ivanchev [24] studied the fundamentals of the polymerization using bifunctional initiators. Recent studies on modelling and simulation can be found in a number of publications [25-30]. Model testing results of styrene homopolymerization using an extensive list of homo- and bifunctional initiators can be found in Dhib et al. [31]. 

The reaction diffusion regime was further clarified by Russell et al. [42] According to their model, the actual residual termination rate constant lie between two limiting values, a minimum, corresponding to a rigid chain, sue as polystyrene, and a maximum, corresponding to a flexible chain. It has beer found that the expression of the reaction diffusion controlled kt from Stickler e> al. [41] is the same as the minimum value proposed by Russell et al [42]. Both approaches share some common characteristics. Reaction diffusion control plays an important role in styrene homopolymerization since it is the main method of termination in later stages of the polymerization. 

Table 5.1 Optimization policies in styrene homopolymerization. Reprinted from J. Gao and A. Penlidis, J. Macromolecular Sci., Reviews in Macromol. Chem. and Phys., C36(2), 199(1996), by courtesy of Marcel Dekker, Inc.

Other complexes that have been used in ethylene-styrene co-polymerizations are 42, which is inactive in styrene homopolymerization but has been claimed to produce ethylene/styrene co-polymers with styrene content in the range 35-87 mol%.6 2 Living ethylene/styrene co-polymerization can be achieved using the MAO-activated complex 43. Although a relatively low amount of styrene was incorporated (about 10 mol%), NMR analysis indicated that trace amounts of pseudo-random tail-to-tail S-S or S-E-S sequences were observed.6 Marks and co-workers showed that bimetallic catalysts based on 44 can effectively yield ethylene/styrene co-polymers and, in contrast to the monometallic CGCs, styrene incorporation can be higher than 50%.664... 

The ylide nickel-catalyzed oligo-/polymerization of ethylene in the presence of styrene or substituted styrenes with unpolar or polar substituents results predominantly in styrene-terminated ohgo-/polyethylenes. Usually, no styrene homopolymerization takes place. The ethylene pressure has to be adjusted relative to the styrene concentration to reduce the competing formation of simple a-olefms. High styrene concentration and low ethylene pressure favor homologous series of the composition aryl-C ,H2, . While the aryl group is frequently located at one... 

I, vinylruthenocene, 66, vinylosmocene, and the T)5-(vinylcyclopentadienyl)metal carbonyl monomers in radical-initiated polymerizations summarized in Scheme 1.1 no longer exists for anionically initiated addition polymerizations. Styrene is readily initiated by such anionic species as BuLi and Na1 Naphth. Living anionic styrene homopolymerizations and block copolymerizations have been extensively commercialized for many years (e.g., Kraton thermoplastic elastomers). However, the exceptionally electron-rich vinyl metal-containing monomers 1, 8-18, 24-30, and 66 were never successfully initiated by anionic systems in our laboratory despite many attempts. In these systems, the a-carbocations are very stable, but the a-carbanions are quite unstable. Thus, the addition of an anion to tbe vinyl function of these monomers is unfavorable. 

It has been established [79, 80] that the initial stage of copolymerization in hydrocarbon media is determined only by the influence of diene. Styrene virtually does not take part in the reaction. As a result, the chain propagation rate is close to that in the polymerization of diene alone. Only after almost all the diene has been consumed, does the insertion of styrene into the growing chain begin. In this case the polymerization rate is higher than in styrene homopolymerization (presumably, because of the higher concentration of the monomer form of active centers). Finally, instead of the expected random copolymer, a block copolymer is formed in this system. Its formation mechanism has not yet been completely explained in the literature. 

Harkins conclusions relate to the polymerization of hydrocarbon monomers which are miscible with their polymers and which have very low s(4ubilities in water, but which can be solubilized in larger quantities in the interior of the micelles of ionic surfactants. Most of the wartime work relates to the etmilsion copolymerization of butadiene and styrene, monomers which fulfil these critma. Model experiments concentrate on styrene homopolymerization because it can be handled more conveniently and is less toxic than some other common monomers. However, conclusions based on experiments with styrene may need modification before they can be extended to more polar monomers (e.g. methyl methacrylate, vinyl acetate) which have significantly higher solubilities in water or which have only limited miscibility with their polymer (e.g. acrylonitrile, vinyl chloride) or which produce polymras with a significant degree of crystallinity (e.g. vinylidene chloride, tetrafluoroethylene). 

Poly(styrene-c i-t-butyl acrylate). One of the major issues with TEMPO mediated "living free radical polymerizations is the very different reactivities of st5n ene and acrylates. It has been observed that TEMPO mediated styrene homopolymerization achieve high conversion, with low polydispersity and excellent molecular weight control. In contrast acrylate homopolymerizations exhibit considerably lower conversion with much broader polydispersities. Figure 2. However, it has been shown that "living" free radical polymerization permits the synthesis of well defined... 

Styrene homopolymerization at the same temperature. From this, it is plausible to postulate that there are two parallel and competing mechanisms for radical generation in the copolymerization case i) the one operating in styrene homopolymerization, ii) an additional mechanism due to the combined presence of styrene and maleic anhydride. 

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