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SYNDIOTACTIC POLYSTYRENE Introduction Syndiotactic polystyrene (sPS) is a relatively new material discovery in semicrystalline polymers with a high melting point and rapid crystallization rate, which makes it possible to injection mold the material. The stereospecific polymerization was made possible by the combination of a transition metal catalyst with weakly coordinating cocatalysts, such as methylaluminoxane. The excellent balance of mechanical, electrical, solvent resistance, and dimensional stability properties combined with a relatively low price (based on styrene monomer) have made this material a competitor to existing engineering plastics. The products also have excellent heat performance and are finding application in automotive (under the hood), electrical, and electronic connector systems.

Structure, Crystallinity, and Physical Properties Syndiotactic polystyrene, as generally polymerized with monocyclopentadienyl catalysts, has a high percentage of rrrr pentad structure. The 13 C nmr chemical shift for the phenyl-1 carbon and backbone methylene carbon are approximately 145.3 and 44.9 ppm, respectively (1,2). In general these polymers are found to be greater than 99% pure in syndiotactic structure as defined by nmr. sPS can be further purified of any atactic polystyrene (aPS) developed during the polymerization procedure by extraction with solvents such as methyl ethyl ketone (MEK). The syndiotactic configuration of the backbone and the high degree of stereoregularity give rise to a semicrystalline polymer, which is able to crystallize at a high rate (3,4), and form well-structured spherulitic morphology when quiescently Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Fig. 1. Differential scanning calorimetry of homopolymer sPS. Sample was prepared by heating first to 320◦ C then quickly cooled to room temperature, and then scanned at a heating rate of 20◦ C/min.

crystallized from the melt (5–7). Peak crystallization rates of 7×10 − 2 at 180◦ C (isothermal) have been measured (3). Although sPS maintains its 100◦ C glasstransition temperature for the amorphous phase, its relatively high melting point (peak at ∼ 270◦ C, Fig. 1) (8) and high crystallization rate, along with high modulus, good electrical properties, and solvent resistance make it useful as an engineering plastic material. The equilibrium melting point has been measured between 287 and 292◦ C using Hoffman–Weeks techniques (9,10), and the heat of fusion for 100% crystalline sPS was measured as 53.2 J/g (8). Table 1 gives the general physical properties of unfilled sPS homopolymer. From the melt it is now well established that sPS crystallizes primarily in an all-trans (T4 ) form (α-form) with a planar zigzag backbone structure (11, 12) (see Fig. 2). A second (β-form) structure (T2 G2 ) is also present, but under normal crystallization conditions the level of this structure is quite low (13–15). Calculations of the ultimate elastic modulus of the α-form sPS crystal are as high as 83 GPa (16). Experimentally, using x-ray diffraction, this has been reported at 86 GPa (12.5 × 106 psi) (17). The stiffness of the crystalline sPS is due primarily to intrachain interactions involving bond bending and stretching. The applied stress Table 1. Physical Properties of Unfilled sPS Homopolymer Properties

Value ◦

Melting point, C Glass-transition temperature, ◦ C Visual appearance Density, g/cm3 Water absorption (24 h, 50% RH),% Dielectric constant at 1–1000 kHz Dissipation factor at 100 kHz Volume resistivity, ·cm

270 100 White opaque 1.05 0.01 2.6 0.0002 1.0 × 1017

Test method dsc dsc ISO 1183B ASTM D570 ASTM D150 ASTM D150 ASTM D257

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Syndiotactic Polystyrene

Fig. 2. Planar zigzag structure of sPS.

to the backbone through the all-trans structure is carried through these bending and stretching modes, with these energy terms increasing the most under the applied stress. In addition, the bulky phenyl groups prevent very close interchain packing of the molecules in the crystal. In fact, the density of the crystalline phase of sPS is very close to that of the amorphous phase at ambient temperature and pressure and is essentially the same as that of atactic amorphous polystyrene. A volume/temperature experiment for aPS and sPS is shown in Fig. 3. At 10-MPa pressure the specific volume of the two structures below T g is almost identical. Above T g the crystalline sPS domains have a lower density than the liquid phase

Fig. 3. Pressure/volume/temperature curves for sPS and aPS at 10, 100, and 200 MPa.
10 MPa aPS; + 100 MPa aPS;  200 MPa aPS; 10 MPa sPS; × 100, MPa sPS;  200 MPa sPS. To convert MPa to psi, multiply by 145.

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aPS, which is a very unusual property leading to improved fabricating properties for sPS. This all stems from a relatively high average cross-sectional area 70 ˚A2 (0.7 nm2 ) for each molecule of sPS because of the bulky nature of the phenyl rings. Polyethylene cross-sectional area by comparison is only 18 ˚A2 (0.18 nm2 ) (16). This high cross-sectional area leads to considerable space between polymer chains even in the crystalline phase and therefore to lower density. Crystallized from the melt, sPS forms a crystalline morphology of wellordered spherulites nucleated homogeneously. In neat noncontaminated sPS with 20–50◦ C/min cooling rates from the melt the spherulites formed are approximately 7–10 µm in diameter. Lamella thickness is generally 8–15 nm (7). It is a typical practice to add nucleators to the material to improve its apparent overall crystallization rate (18). These nucleated materials show smaller well-impinged spherulitic structures of about 2 µm in diameter. Crystallization kinetics of the bulk unnucleated polymer has been measured by several techniques and shows a dependence on molecular weight with a broad maximum rate between 190 and 240◦ C (3,4). When measured in a dsc experiment with a cooling rate of 10◦ C/min the unnucleated sPS will show a maximum for the crystallization peak of 234– 236◦ C. Nucleated with certain organic salts [such as aluminum p-tert-butyl benzoate (pTBBA)] this maximum is shifted to 247–249◦ C (19).

Mechanical Properties Failure and deformation mechanisms have been studied and published for sPS and have been compared to aPS (20). The semicrystalline sPS material fails with a slow, controlled crack growth while aPS fails with considerable crazing and some yield. Micrographs of fracture surfaces show that the spherulites are relatively stable structures, and it is the material at the interface of spherulites that draws and fails. A mechanism of constrained crazing and void coalescence is proposed with the spherulites themselves acting as the stress concentrators for the initiation of failure. The damage zone is then highly confined to the interspherultic regions of the semicrystalline morphology. Fig. 4 shows typical stress–strain behavior of sPS. The lack of change in Poisson’s ratio for sPS is a further evidence of the lack of yielding mechanisms available in the stiff sPS material. Higher impact sPS products can be obtained using styrene–ethylene/butene–styrene block copolymers (21) or polyolefin elastomers (22) as dispersed phase toughening aids. These are added during the extrusion compounding of the formulated products. The basic mechanical properties of unfilled sPS are given in Table 2. Reinforcement materials (such as glass fibers) or filler materials are generally compounded into sPS in order to take advantage of the high melting point and high modulus of the base resin. The addition of a well-dispersed, chemically coupled glass fiber above the percolation threshold volume creates a unique material. In general, these products are 30 and 40 wt% glass-filled products. The tensile strength of glass-filled sPS is increased to 100–130 MPa with a modulus from 7500 to 11,000 MPa. Impact properties, such as Gardner impact, are shown to be 4–7 J and notched impact strengths are 70–120 J/m. Table 3 gives typical physical and mechanical properties for commercial formulations of sPS including impact modified and ignition resistant formulations.

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Fig. 4. Example of the tensile dilatometry results for injection molded sPS (349 kg/mol) at 23◦ C (20).
Stress; Poisson’s ratio. To convert MPa to psi, multiply by 145.

Table 2. Mechanical Properties of Unfilled sPS Homopolymer Properties

Value

Test method

Tensile strength at yield, MPaa Tensile modulus, MPaa Elongation at break,% Flexural strength, MPaa Flexural modulus, MPaa Izod impact (notched at 23◦ C), J/mb

41 3450 1.3 71 3950 10

ISO R527 ISO R527 ISO R527 ISO 178 ISO 178 ISO 180

a To b To

convert MPa to psi, multiply by 145. convert J/m to ft·lbf/in., divide by 53.38.

Solubility and Solvent Resistance At room temperature there are no known solvents for sPS. Under ambient conditions of temperature and pressure the amorphous regions can be swollen with good solvents for aPS such as toluene, benzene, tetrahydrofuran, or dichloroethane. Chlorinated aromatics (such as trichlorobenzene, TCB) will dissolve sPS at temperatures above the glass-transition temperature (>100◦ C). Typically a 5–10 wt% solution can be prepared by heating the polymer with TCB at 160◦ C, with agitation. This solvent is generally used for solution viscosity or gel permeation chromotography. The excellent solvent resistance of the base sPS material is useful in many applications for the formulated product. Table 4 shows the basic solvent resistance data for commercial products (23).

Table 3. Physical and Mechanical Properties of Commercial Grade Formulated sPS Productsa

Properties

423

Tensile strength, MPab Tensile modulus, MPab Elongation, % Flexural strength, MPab Flexural modulus, MPab Notched izod impact, J/mc (23◦ C) Notched izod impact, J/mc (−18◦ C) Gardner impact (23◦ C), J Gardner impact (−29◦ C), J Vicat softening point, ◦ C Deflection temperature @ 1.82 MPa, ◦ C Deflection, ◦ C temperature @ 0.45 MPab Coefficient of linear thermal expansion, 10 − 6 cm/cm·◦ C) Dielectric constant Dielectric strength, kV/mm Dissipation factor Specific gravity Water absorption, % (24 h, 50% RH) Mold shrinkage, cm/cm a Ref.

23. convert MPa to psi, multiply by 145. c To convert J/m to ft·lbf/in., divide by 53,38 b To

ASTM method

30% glass

40% glass

30% glass, impact modified

30% glass, ignition resistant

40% glass, ignition resistant, impact modified

D638 D638 D638 D790 D790 D256 D256 D3029 D3029 D1525 D648 D648 D696

121 10,000 1.5 167 10,000 96 96 4.2 5.2 263 249 263 38.7

132 11,170 1.5 185 10,480 112 112 4.9 5.2 263 249 263 31.5

105 7,580 3.4 163 7,930 117 96 4.0 7.3 263 232 263 42.3

97 9,650 1.8 137.9 10,340 70 70

122 10,340 1.8 188 12,760 91 91

263 227 263 30.5

263 240 263 25.2

D150 D149 D150 D792 D570 D955

3.1 23.6 0.0002 1.25

3.1 23.6 0.0002 1.32 0.01 0.0015–0.0025

3.1 23.6 0.0002 1.21 0.01 0.003–0.004

3.1 23.6 0.001 1.41 0.01 0.025–0.035

3.1 23.6 0.001 1.47 0.01 0.001–0.002

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Table 4. General Solvent Resistance Properties for sPS Formulationsa Solvent Acids Bases Glycols (antifreeze) Automotive oils Chlorinated hydrocarbons Aromatic hydrocarbons Aliphatic hydrocarbons Gasoline Alcohols Salt solutions Water and hot water a Ref.

General performance Excellent Excellent Excellent Good Poor Fair Good Fair Excellent Excellent Excellent

23.

Polymerization The development of coordinative polymerization of olefins during the second half of the twentieth century has been one of the most important achievements in polymer chemistry. Extensive studies have been carried out on the heterogeneous polymerizations of ethylene and propylene (24–26). Controlling the stereochemistry of polypropylene is of critical importance for the control of polymer properties. The introduction of methylaluminoxane (MAO) was a significant development in homogeneous coordinative polymerization of olefins (27,28). By utilizing MAO as a cocatalyst, metallocene catalysts exhibited outstanding polymerization activities. Furthermore, the structure of the metallocene complex could be modified to yield changes in polymer stereoregularity, molecular weight, and molecular weight distribution (29–31). Metallocenes, together with the MAO cocatalyst, have allowed the synthesis of highly stereoregular polypropylene (32,33), polyethylene with improved rheological properties (34), and ethylene copolymers with high comonomer incorporation (35). Indeed, Ishihara and co-workers succeeded in the first preparation of sPS through activation of a transition metal complex with MAO (36,37). Typically, Group IV metallocene complexes have been used as catalysts for the polymerization of sPS. Of these, the monocyclopentadienyl-type complexes of titanium have been found to give the highest polymerization activity based on transition metal (38,39). Subsequent to the development of MAO as the sPS cocatalyst, it has been found that highly electrophilic activators, such as the tetrakis(pentafluorophenyl) borate type, can be used as cocatalysts for the production of sPS (40,41). The syndiotacticity of sPS results from the homogeneous coordinative polymerization process (1,42). Styrene monomer complexes at a vacant coordination site on the transition metal, typically titanium, and inserts into a titanium carbon or hydride bond (Fig. 5). In the case of the growing polymer chain, the insertion occurs via cis-addition with secondary insertion so that the titanium is attached to the carbon bearing the phenyl substituent (43). Chain transfer occurs typically via β-hydride elimination, forming a titanium hydride, or via reaction with an

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Fig. 5. Generalized schematic of sPS polymerization mechanism. Ligands on titanium omitted for clarity. A − represents counterion, such as MAO or borate type.

aluminum alkyl species, forming a titanium alkyl (44,45). In either case, reinsertion by styrene monomer generates a new growing polymer chain at the titanium site. The characteristic backbone defects in sPS were analyzed by 13 C nmr spectroscopy in order to establish the mechanism of syndiospecificity (46). The defect structures were found to be consistent with a chain-end control mechanism in which the last monomer added to the growing polymer chain directs the insertion of the next monomer added to the chain.

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Transition Metal Complexes. The transition metal is the site of the homogeneous coordinative polymerization of sPS and can consequently influence the polymerization activity, polymer molecular weight, and syndiospecificity, as well as other parameters. Various transition metals have been tested and reported for the polymerization of styrene monomer to sPS (1,39). Syndiotactic stereospecificity is observed for TiCl4 , TiBr4 , Ti(OCH3 )4 , Ti(OC2 H5 )4 , [C5 H5 ]TiCl3 , [C5 H5 ]2TiCl2 , [(CH3 )5 C5 ]TiCl3 , [(CH3 )5 C5 ]2 TiCl2 , [C5 H5 ]ZrCl3 , and Ti(acac)2 Cl2 (acac = acetylacetonate). Polymerization with the following catalysts leads to atactic materials: ZrCl4 , [C5 H5 ]2 ZrCl2 , [C5 H5 ]2 HfCl2 , Nb(OC2 H5 )5 , Ta(OC2 H5 )5 , Cr(acac)3 , Co(acac)3 , and Ni(acac)2 . Titanium complexes are more active than other transition metals for syndiotactic polymerization of styrene. The catalytic activity of titanium complexes varies significantly with the ligands bonded to the titanium. Among those complexes active for sPS polymerization, monocyclopentadienyl-type complexes of titanium yield the highest polymerization activity based on titanium. For substituted cyclopentadienyl complexes, the polymerization activity decreases in the following order (1,47): [(C2 H5 )(CH3 )4 C5 ]Ti(OCH3 )3 – 100 > [(CH3 )5 C5 ]Ti(OCH3 )3 – 95 > [(CH3 )4 HC5 ] Ti(OCH3 )3 – 64 > [((CH3 )3 Si)2 H3 C5 ]Ti(OCH3 )3 – 12 > [C5 H5 ]Ti(OCH3 )3 – 5. Electron-releasing substituents on the cyclopentadienyl ligand generally yield complexes with higher polymerization activities, suggesting stabilization of the active site by the electron-donating ligands. However, pendant donor ligands, such as aminoalkyl (48), methoxyethyl (49), or even phenethyl (50), on the cyclopentadienyl ring result in lower polymerization activities. It is possible that these pendant donors hinder coordination or insertion of the styrene molecule. Furthermore, coordination of the pendant donor to MAO may occur, causing destabilization of the active site. Biscyclopentadienyl complexes and bridged metallocene complexes of titanium show lower polymerization activities than the monocyclopentadienyl complexes (39). The catalytic activity and syndiospecificity for bridged metallocenes does increase by decreasing the bite-angle (51). Although ansamonocyclopentadienyl-amido titanium complexes are essentially inactive for homopolymerization of styrene, efficient sPS formation with nonbridged amido cylcopentadienyl complexes of titanium has been reported (52). Other cyclopentadienyl-type ligands on titanium have been investigated for their effect on polymerization of styrene monomer to sPS. Indenyltitanium trichloride has been reported to be a significantly more active catalyst than cyclopentadienyltitanium trichloride (53). This has spawned additional work on substituted indenyl complexes (54,55), benzindenyl complexes (56), and cyclopentaphenanthrene titanium derivatives (57). Several non-cyclopentadienyl complexes of titanium have been investigated for the polymerization of sPS. These have included benzamidinate ligands (58), bridged bisphenolato ligands (59), and pyrazoylborate ligands (60). While these complexes do yield sPS, the activities are lower than those of the monocyclopentadienyl-type complexes. For the monocyclopentadienyl complexes of titanium, [(CH3 )5 C5 ]Ti(X)3 , the polymerization activities decrease in the following order for the ancillary ligand X: OCH3 – 100 > OC6 H5 – 98 > Cl – 45 > O-t-C4 H9 – 11 > O-i-C3 HF6 – 4 (1). The bulky tert-butylalkoxide and the electron-withdrawing alkoxide cause a decrease in conversion, which may be due to a decrease in the number of active sites for

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polymerization. Increased activity for the trifluoride monometallocene titanium complexes, X = F, relative to the analogous trichlorides has been reported (61). Similar results have been reported for indenyl and substituted indenyl complexes of titanium (62). Several investigations of supported catalysts for the polymerization of sPS have been reported (63–66). Generally, two approaches have been evaluated for preparing a supported catalyst system for sPS. Either a homogeneous catalyst, such as (C5 H5 )TiCl3 , is attached to a solid support and activated by MAO or the MAO cocatalyst is attached to a support followed by reaction with the metallocene. Under certain conditions, high activity and syndiotacticity have been reported for these supported catalyst systems (67). Cocatalysts. The MAO cocatalyst, basically a reaction product of (TMA) and water, was critical in the discovery of sPS (68). Prior to the introduction of MAO, the homogeneous metallocene, (C5 H5 )2 TiCl2 , activated with an aluminum alkyl species yielded only aPS with low activity (38). While the exact characterization of MAO has not been achieved analytically, it has generally been accepted to have an oligomeric structure with the molecular formula (CH3 AlO)n (69,70). Currently, MAO is believed to act as a cocatalyst by alkylation of the titanium catalyst and abstraction of a ligand from the catalyst. The active species formed by this process is a titanium(III) cationic species in combination with an MAO counterion (39). Evidence for a titanium(III) cationic active species in styrene polymerization with MAO activation has been obtained by esr spectroscopy (71). In sPS polymerizations, the polymer yield can be affected by the properties of the MAO cocatalyst, such as its molecular weight and residual TMA content. The catalytic activity has been reported to increase with increasing MAO molecular weight, reaching high activity with a molecular weight in the range of approximately 500 (72). Because of the nature of the preparation of MAO, it usually contains residual TMA, which can vary with the synthesis. The residual TMA generally decreases the activity of the catalyst system (1); however, the effect is dependent upon the catalyst. In the case of (C5 H5 )TiCl3 and Ti(OC2 H5 )4 as catalysts, the polymerization activity is reduced by TMA in molar ratios of TMA/MAO from about 0.5 to 2 (73). The activity of the catalyst system can also depend significantly upon molar ratio of MAO/Ti (Fig. 6). The results, however, are also dependent upon the catalyst used in the polymerizations. The polymer yield, with [(CH3 )5 C5 ]Ti(OR)3 as the catalyst, increased with increasing MAO/Ti molar ratio, reaching a maximum at a ratio of approximately 300–500 to 1 (47). In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74–76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76).

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Fig. 6. Percent conversion to sPS as a function of MAO-to-titanium mole ratio for [(CH3 )5 C5 ]Ti(OCH3 )3 (47).

The polymerization activity of tris(pentafluorophenyl)boron has been reported to be higher than that of dimethylanilinium tetrakis(pentafluorophenyl) borate with [(CH3 )5 C5 ]Ti(CH3 )3 as the catalyst (77). It was proposed that the dimethylaniline coordinates to the active site and decreases the polymerization activity. The use of dimethylanilinium tetrakis(pentafluorophenyl)borate with triisobutylaluminum (TIBA) and [(CH3 )5 C5 ]Ti(CH3 )3 as the catalyst has been reported to yield high activity for sPS polymerization (74). Additives. Certain additives may be included with the catalyst system of the transition metal catalyst and MAO or borate cocatalyst in order to improve the polymerization activity or perhaps to adjust a polymer property into a useful range. Activators that have been reported for sPS polymerization include aluminum alkyls, hydrogen, and organometallic compounds of tin and zinc. Aluminum alkyls that have been investigated in catalyst systems for sPS polymerization include TMA, triethylaluminum (TEA), and TIBA (1,73,78). In conjunction with [(CH3 )5 C5 ]TiCl3 and MAO, the polymerization activity increases with the addition of aluminum alkyl in the following order: TMA < TEA < none < TIBA (Table 5). Aluminum alkyls can act as both reducing agents and alkylating agents for the titanium catalyst. In this case, the polymerization activity is decreased by the strong reducing agents, TMA and TEA. Hydrogen is a well-known chain transfer agent in classical Ziegler–Natta heterogeneous polymerization and also causes a decrease in molecular weight when used in sPS polymerization (79). Additionally, as shown in Fig. 7, hydrogen significantly improves catalyst efficacy in sPS polymerization (80,81). The activation by hydrogen in sPS polymerization may be due to reactivation of dormant polymerization sites.

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Table 5. Relative Polymerization Activities of Aluminum Alkyls with the Catalyst System [(CH3 )5 C5 ]TiCl3 /MAOa Aluminum alkyl None TMA TEA TIBA a Ref.

Relative activity

Mw

100 13 23 560

750,000 64,000 84,000 580,000

1.

Fig. 7. Effect of hydrogen pressure on conversion for [(CH3 )5 C5 ]Ti(OCH3 )3 /MAO catalyst system (80). ♦ no H2 ;
13.8 kPa H2 ; ◦ 41.4 kPa H2 ; × 55.2 kPa H2 . To convert kPa to mm Hg, multiply by 7.5.

Two other activators have been investigated for sPS catalyst systems. Tetraphenyltin, when added to the titanium and MAO catalyst system, has been reported to significantly improve the yield of sPS and cause an increase in the sPS molecular weight (82). It was proposed that mixed aluminum–tin sites may form in the MAO structure, which could affect the active site formation. The influence of diphenylzinc on the metallocene MAO catalyst system for sPS polymerization has been reported for titanium and zirconium (83). Molecular Weight Control. Chain transfer under the usual sPS polymerization conditions occurs primarily via β-hydride elimination or chain transfer to aluminum alkyl (Fig. 5). For the (C5 H5 )Ti(OC4 H9 )3 /MAO catalyst system at low monomer concentration, it has been reported that the rate of β-hydride elimination is 76 times faster than chain transfer to MAO (84). Additional evidence for β-hydride elimination as the main mechanism of chain transfer has been reported from analysis of the tert-butylcyclopentadienyltitanium trichloride catalyst

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for sPS polymerization (45). Increasing the polymerization temperature significantly decreases the polymer molecular weight, presumably by increasing the rate of β-hydride elimination (1). However, sPS molecular weight may also be decreased by the addition of external additives, such as aluminum alkyls or hydrogen. The effect on the molecular weight depends upon the titanium catalyst but aluminum alkyls (TMA, TEA, or TIBA) are all effective at decreasing the sPS molecular weight (Table 5) (1,73). Hydrogen, in addition to increasing the catalyst efficiency, is effective as a chain transfer agent in reducing the sPS molecular weight (79–81). Copolymerization. Polymerization of various ring-substituted styrene monomers using the characteristic sPS catalyst system (C5 H5 )TiCl3 /MAO yielded the corresponding syndiotactic substituted polystyrenes (38,85). Electron releasing substituents, tertiary-butyl or methyl, gave higher conversions than styrene monomer, which in turn gave higher conversion than electron withdrawing substituents, such as fluorine or chlorine. Therefore, syndiospecific copolymers of styrene with alkyl-substituted styrenes have been conveniently prepared and their physical properties studied (86–88). The copolymers exhibit a random incorporation of the comonomers in the polymer backbone. Indeed, for syndiotactic poly(styrene-co-p-methylstyrene), as the content of p-methylstyrene (pMS) is increased, the melting point decreases regularly from the value for sPS homopolymer. Furthermore, the lower crystallization temperatures and the breadth of the crystallization peaks by dsc suggest a slower crystallization rate for the copolymers relative to sPS. Several reports have been published on the copolymerization of styrene and olefins, ie ethylene or butadiene, utilizing the typical sPS catalyst system. The results have depended on the specific catalyst and polymerization conditions. It has been reported that the copolymerization of styrene and ethylene with (C5 H5 )TiCl3 and MAO yielded a mixture of polyethylene and sPS (89,90). An alternating copolymerization using the borane based catalyst system [(CH3 )5 C5 ] Ti(CH2 C6 H5 )3 /B(C6 F5 )3 has also been reported (91). No evidence of stereoregularity was found in the copolymer. The products of copolymerization of styrene and ethylene using the catalyst system (C5 H5 )Ti(OC6 H5 )3 /MAO have been analyzed (92), leading to the conclusion that the polymer was a random copolymer. However, the polymerization results were influenced by the nature of the MAO, the TMA content, and the polymerization conditions. Copolymerization of styrene and butadiene has been reported with the [C5 H5 ]TiCl3 /MAO catalyst system (93). Under certain polymerization conditions, block copolymers of sPS–polybutadiene are obtained without appreciable contamination by the homopolymers. dsc and 13 C nmr spectroscopy were used to confirm the block structure. Laboratory Polymerization Techniques. Syndiotactic polymerization of styrene monomer must be carried out under inert atmosphere taking utmost care to exclude oxygen, water, and other polar impurities. The monomer and any solvent must be free of oxygen, water, and polar impurities. Styrene is typically distilled from calcium hydride and stored under inert atmosphere. Alternatively, styrene is conveniently purified prior to polymerization by sparging with nitrogen or argon and passing through activated alumina (94). Furthermore, it has been reported that removal of phenylacetylene from styrene monomer via hydrogenation is beneficial for higher catalyst activity (95).

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The sPS polymerization has been reported using several different laboratory scale polymerization techniques. Slurry polymerization, using a diluent such as iso-octane, is particularly convenient since the polymer precipitates from the nonsolvent as a finely divided solid (47,96). Bulk polymerization has often been reported in small laboratory reactors (97). However, in this case, a solid mass forms at low conversions as the polymer crystallizes from the polymerizing mixture. Diffusion, heat transfer, and reactor fouling may be concerns with bulk polymerization of sPS. Suspension polymerization of sPS has also been reported utilizing a perfluorinated hydrocarbon in which both the monomer and polymer are insoluble (98). Beads of sPS are obtained in good yield with kinetics similar to bulk polymerization. Commercial Processes. Several processes have been reported in the patent literature for the production of sPS. A continuous polymerization process was disclosed using a self-cleaning type reactor comprising kneading elements arranged on rotating shafts (99,100). sPS could be produced efficiently without adhesion of the polymer to the reactor or the agitator. By distributing the catalyst addition along the axis of the reactor, improved temperature control and productivity was claimed (101). Additionally, a second stirred vessel could be added as a secondary reactor to prolong the polymerization times. Another continuous process has been disclosed for the production of sPS in which monomer and catalyst are introduced into a reactor containing previously added fluidized sPS particles (102–104). Temperature control is achieved by partial evaporation of the monomer. A similar process was disclosed in which a nonsolvent was evaporated in order to control the polymerization temperature (105). A different type of process has been disclosed in which sPS is produced by dispersion polymerization (106–108). A special block copolymer based on polybutadiene is used as the dispersant. The advantages of this process include being able to produce sPS in conventional stirred reactors at low viscosity with improved productivity and without reactor fouling.

Processing Syndiotactic polystyrene formulated products are generally injection molded into useful parts for automotive, electrical, electronic, medical, and other market applications (109,110). The melt viscosity of sPS at normal processing temperatures is very low, especially in comparison to other engineering thermoplastic polymers. Syndiotactic polystyrene rheology allows it to flow in to very thin molds at relatively low pressure, and crystallize quickly with low warpage providing for fast cycle times. Table 6 shows the mold shrinkage data typical for formulated commercial products. Since sPS has very low moisture absorbance, there are no additional changes in part dimension with humidity. Syndiotactic polystyrene will strain-induce crystallize, as well as quiescently crystallize as discussed previously. Furthermore, under certain conditions, it may also be quenched to the amorphous state. In injection molded parts, these processes may lead to skin/core differences in morphology that can be observed (111,112). With higher temperature molds (>150◦ C) the parts are generally fully

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Table 6. Typical Mold Shrinkage for Formulated sPS Commercial Productsa Formulation Type 30% glass filled (GF) 40% glass filled (GF) 30% GF, impact modified (IM) 30% GF, ignition resistant (IR) 30% GF, IR, IM 40% GF, IR, IM a Ref.

Flow shrinkage,%

Cross-flow shrinkage,%

0.2–0.4 0.1–0.3 0.1–0.3 0.27–0.37 0.15–0.3 0.1–0.2

0.5–0.9 0.4–0.9 0.2–1.0 Not available 0.7–0.9 0.6–0.8

23.

crystallized to the first 10 µm of the surface. Fig. 8 shows a plot of percent crystallinity as a function of distance from the surface of the mold for sPS with a mold temperature of 105◦ C and increasing level of nucleator (pTBBA). The low crystallinity at the surface leads to higher absorbance of good solvents such as benzene. Fig. 9 illustrates the effect of mold temperature and nucleator level on the ability of molded parts to imbibe solvent (benzene). It is clear that low crystallinity at the surface correlates to higher levels of benzene up-take and is consistent with the fact that a skin/core structure is present. Syndiotactic polystyrene is generally nucleated in commercial formulations and molded at higher mold temperatures in applications where solvent resistance is critical. Syndiotactic polystyrene can also be extruded into sheet, quenched quickly, and then tentered to form biaxially oriented film. It can also be thermoformed

Fig. 8. Plot of percent crystallinity versus depth for tensile bars injection molded with a mold temperature of 105◦ C.  0.1 % Al p-TBBA;  0.3% Al p-TBBA; • 1.0% Al p-TBBA; 1.3% Al p-TBBA.

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Fig. 9. Benzene absorption (wt. gain after 24-h immersion). Injection molded tensile bars at various mold temperatures and nucleator (Al p-TBBA) levels. Control; 0.1; 0.3%; 0.6%; 1.0%; 1.3%.

from the quenched sheet to form cups, bowls, and other parts. In this case unfilled (neat) or in some cases a low level copolymer (such as pMS) can be used to decrease the quiescent crystallization rate allowing for thicker sheets and films to be made. The copolymer made with pMS disrupts the crystallization of homopolymer sPS such that the melting point is decreased. Fig. 10 shows the peak melting point as

Fig. 10. Plot of melting point (as determined by dsc) versus the level of pMS in a syndiotactic copolymer with styrene.

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a function of pMS content in the polymer. The decrease in melting point is a result of smaller crystallites and thinner lamellae. At pMS contents greater than 20% it is very difficult to crystallize the copolymer even with long annealing times.

Economic Aspects Syndiotactic polystyrene is manufactured by Dow Chemical Company in Germany at announced capacity of 36,400 t/year. Idemitsu Petrochemical Company also manufactures sPS in Japan at approximately 5000 t/year. Compounding to the formulated products is done around the world at many locations. Pricing varies by formulation and product but is generally 3.30–5.50/kg at this time.

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MICHAEL MALANGA THOMAS H. NEWMAN Dow Chemical Company

TETRAFLUOROETHYLENE POLYMERS.

See PERFLUORINATED

POLYMERS.

TRANSITIONS AND RELAXATIONS.

See GLASS TRANSITION;

VISCOELASTICITY

ULTRAHIGH MW PE.

See ETHYLENE POLYMERS, HDPE.

URETHANE POLYMERS.

See POLYURETHANES.