Isotactic polystyrene polymerization

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

The polymerization of styrene with less anionic butyllithium has been studied by several workers (31, 32, 33). The results of Tobolsky and Boudreau (34) showed that the butyllithium polymerization of styrene follows the electronic behavior of an anionic reaction. Electron releasing groups on the aromatic ring decreased the reactivity of the monomer. Braun and co-workers and Worsfold and Bywater (35) have studied the production of isotactic polystyrene by butyllithium catalysis. Worsfold and Bywater found that water plays an important role in the isotactic polymerization and concluded that the production of lithium hydroxide in situ is important for the isotactic steric control. Added lithium butoxide, lithium methoxide or lithium carbonate were not effective. They concluded the associated forms of butyllithium do not produce isotactic steric control but require association with lithium hydroxide. 

The alkyl derivatives of the most electropositive elements (Cs, Rb, K, Na) are highly ionic in the solid state and require only weakly solvating media (solvent plus monomer) to exist in form (III) which initiates non-stereospecific anionic polymerization. However, in hydrocarbon solvents, alkyl sodium (Alfin1) (217, 225—227), alkyl potassium (210, 217, 228) and alkyl rubidium (217) all can produce isotactic polystyrene. 

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

Isotactic Polystyrene. The familiar steam molding of pre-expanded particles has so far not been applied successfully to isotactic polystyrene. However, the polymer has been foamed, according to three disclosed methods. For example, finely divided acetone-insoluble polymer, with a melting point in excess of 200°C., is blended with a liquid selected from methylene chloride, aromatic hydrocarbons, or halogenated aromatic hydrocarbons. This blend is then heated (84). A mixture of molten polymer and methyl chloride, propane, or butane is suddenly depressurized (8). Foam may also be generated in a continuous manner directly from a butyllithium-initiated polymerization conducted in the presence of a 4/1 blend of benzene and petroleum ether (15). 

Much of the interest in polymerization initiated by lithium compounds is caused by the formation of highly specific products in nonpolar solvents. Under these conditions a highly cis-1,4-polyisoprene is formed, and methyl methacrylate is polymerized to a largely isotactic product. It is reported that isotactic polystyrene can be formed at low temperatures (2, 9), but this seems to form only in the presence of lithium hydroxide formed by catalyst destruction (28). 

Depressed rates have been observed in Ziegler-Natta systems with monomers other than 4-methylpentene-l. Bier (7) suggested that the slowly decreasing rates of propylene polymerization under polymer precipitating conditions with the catalyst system a-TiCb-Al HsbCl are caused by diffusion control. In another case Burnett and Tait (3) found depressed rates of styrene polymerization under polymer-precipitating conditions with the catalyst system a-TiCb-Al HsU. At styrene concentrations less than 3.5M in heptane (isotactic polystyrene precipitates in this region of monomer concentration) a plot of polymerization rate vs. styrene concentration falls below the extrapolated linear plot by a factor of 2. 

Styrene is a very versatile monomer. It can be polymerized by most types of polymerization mechanisms, e.g. free radical (FR), Ziegler-Natta (ZN), anionic, and cationic. Classical ZN polymerization of styrene yields isotactic polystyrene. However, if methylalumoxane (MAO) is added as a co-catalyst, syndiotactic polystyrene is formed. The resulting polymers formed using the various mechanisms of polymerization are summarized in Scheme 8.1. 

It is known from the work of Morton, on polymerization with Alfin catalysts, that the inorganic constituent of the catalysts (NaCl) plays an important part in the special effectiveness of these initiators. Hence, the question had to be examined if, likewise, the presence of inorganic substances is necessary for stereospecific polymerization with organosodium and -potassium compounds. As a result, it has been established that all the organometallic compounds derived as indicated in the above preparative methods facilitate the stereospecific polymerization of styrene in n-heptane. In addition, the nature and chain length of the residue R have no significant influence on the initiators. In fact, R can be linear or branched or aryl-aliphatic. Also, phenyl or triphenylmethylsodium yields isotactic polystyrene. 

The existence of such associated organolithium compounds has been estabhshed in various cases (19, 20, 24), In addition to isotactic polystyrene, a considerable amoimt of atactic material is always present it is formed by starting the polymerization on the nonassociated part of the organolithium compounds which probably promote a nonstereospecific anionic polymerization. The stereoregulation of the polymerization of styrene by heterogeneous alkali metal aUcyl initiators is limited by the forces on the surface of the catalyst while the dissolved organolithium initiators in their associated form cause the stereospecific polymerization. 

These equations show how a mmmm sequence in polystyrene can be converted into a mrrm sequence by a simple epimerization event. Should the configuration of the fourth carbon from the left in the last structure also be altered, a rmrm pentad would result. Thus, by a series of epimerization steps it is possible to change isotactic polystyrene gradually into a polymer that exhibits the same NMR spectra as the polystyrene that was prepared by free radical polymerization(17-19). A Monte Carlo program has... 

In 1955, for the first time, G. Natta obtained definite amounts of stereoregular polystyrene by polymerization, in the presence of titanium chloride Cl4Ti. This isotactic polystyrene has a crystallinity ratio which may reach 90 per cent. The melting temperature is 240 °C, The crystals belong to the rhombohedric system. 

The crystalline content of a polymer has a profotmd effect on its properties, and it is important to know how the rate of crystallization will vary with the temperature, especially drrring the processing and manrrfacturing of polymeric articles. The chemical structure of the polymer is also an important featirre in the crystallization for example, polyethylene crystallizes readily and carmot be quenched rapidly enough to give a largely amorphous sample, whereas this is readily accomplished for isotactic polystyrene. However, this aspect will be discussed more fully later. 

Commercial polystyrene is manufactured only by free-radical polymerization. Isotactic polystyrene formed with Ziegler-Natta catalysts was introduced conunercially in the sixties, but failed to gain acceptance. 

Anionic polymerization of styrene with amyl sodium yields an isotactic polymer. Polymerizations catalyzed by triphenylmethylpotassium also yield stereospecific polystyrene. The same is true of oiganolithium compounds. ... 

There is a small interest in forming isotactic polystyrenes with vary narrow molecular weight distributions, because of some very limited practical applications, and from purely academic interests. Several preparations of virtually monodisperse polystyrenes of MJMn = 1.06 by anionic polymerizations were developed. The materials are available commercially [181-186], small quantities for use as standards for GPC. 

When small amounts of water were deliberately added to butylhthiiun in hydrocarbon solution, it was possible to prepare polystjrrene with as much as 85% polymer that was insoluble in refluxing methyl ethyl ketone and identified as isotactic polystyrene by x-ray crystallography (164). Isotactic polystyrene (10-22% crystalline) can be prepared when lithium f-butoxide is added to w-C4H9Li initiator and the polymerization in hexane (styrene/hexane = 1) is effected at —30°C (165). This polymerization becomes heterogeneous and is quite slow (after 2-5 days, 50% monomer conversion 20-30% conversion to isotactic polymer). 

In comparison with the extensive and fruitful investigations in the area of homogeneous syndiose-lective styrene polymerization, research in the field of isoselective polymerization is still trudging. Isotactic polystyrene, known for almost a half century, is best produced by heterogeneous Ziegler-Natta catalysis. Recently, the homogeneous isoselective polymerization of styrene by soluble metallocenes and nonmetallocene transition metal complexes has been gradually developed. 

The Ni(acac)2/MAO/Et3N system produces partially isotactic polystyrene with moderate molecular weight (Mw = 9,000-25,000 Mw/A/n = 1.6-4.2 75-85% m diads). Ni(a-naph)2 (a-naph = a-nitroacetophenonate. Figure 14.21, 101) and Ni(hfacac)2 (hfacac = hexafluoroacetylacetonate. Figure 14.21, 102) in combination with MAO and PCys as an ancillary ligand (PCys ligates in situ Cy = cyclohexyl) can also catalyze the polymerization of styrene to produce partially isotactic... 

Some catalyst systems based on rare earth metal complexes, such as Nd(P507)3/H2O/TIBA (P507 = RP(0R)(0)0-, R = CH3(CH2)3CH(Et)CH2-), are also active for styrene polymerization and give mixtures of atactic and isotactic polystyrenes (Tn, = 220 C). ... 

Styrene can be polymerized to stereoregular structures by coordination catalysts. Highly isotactic polystyrene is prepared using Ziegler-Natta-type catalysts obtained from the reaction between TiC and AlEts [104,105] and of a TiCl3/Al(i-Bu)3 mixture [106] in a temperature range of 0°C to 10 °C. The Al/Ti ratio has to be 3 1 for the formation of isotactic polystyrene [107,108]. A detailed description of preparations for isotactic polystyrene is given in Ref. [109]. 

Armistead JP, Hoffman JD (2002) Direct evidence of regimes I, II, and III in linear polyethylene fractions as revealed by spherulite growth rates. Macromolecules 35(10) 3895-3913 Armstrong SR, Offord GT, Paul DR, Freeman BD, Hiltner A, Baer E (2014) Co-extruded polymeric films for gas separation membranes. J Appl Polym Sci 131(2) 39765 Azzurri F, Alfonso GC (2008) Insights into formation and relaxation of shear-induced nucleation precursors in isotactic polystyrene. Macromolecules 41(4) 1377-1383 Baekeland LH (1909) Method of making insoluble products of phenol and formaldehyde. US Patent 942,699... 

Polystyrene produced by free radical polymerization techniques is atactic and therefore non-crystalline. However, isotactic polystyrene has been prepared by the use of Ziegler-Natta catalysts and n-butyllithium. Isotactic polystyrene has a high crystalline melting point of 230°C, which makes it a difficult material to process also it is less transparent and more brittle than the atactic polymer. For these reasons isotactic polystyrene has not achieved commercial importance. 

The main structural results on these copolymers were obtained for products of low styrene content 41,154) polymerized with Ti-based systems. In all cases the total polymer products can be separated into two parts atactic polystyrene (soluble in ketones) and the true copolymers. These products do not contain isotactic polystyrene 41), which verifies the suggestion that this atactic polystyrene is formed on cationic active sites rather than cm the usual Ziegler-type centers. Copolymers of low styrene content have the styrene units isolated independently from the rit2 value. This was proved by IR and NMR spectra studies. Styrene units absorb at frequencies characteristic for isolated groups at 550-560cm" and at 1075cm" when the styrene content is 5.7-19.1% 41,154), and give rise to a resonance at t 298 in the NMR spectrum (154). 

Is it possible to make isotactic polystyrene by radical polymerization ... 

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