Production of crystalline vinyl aromatic polymers having mainly a syndiotactic structure

The present invention relates to a process for preparing mainly syndiotactic crystalline vinyl aromatic polymers. More particularly, the present invention relates to a process for preparing crystalline styrene polymers in which the chains are at least mainly in the syndiotactic configuration. As is known, styrene polymers and particularly polystyrene is a thermoplastic material obtainable with high molecular weights. Its thermal resistance, susceptibility to be thermomolded and excellent insulating properties render it particularly suitable for the production of extruded articles, molded articles and films to be used chiefly in the fields of sound and heat insulation, of packing and as dielectric material.

Polystyrene can be produced by radical polymerization, anionic polymerizaticn or cationic polymerization.

The styrene polymerization conducted by radical mechanism, either thermally or in the presence of initiators, can be effected by the techniques of bulk polymerization, emulsion polymerization, suspension polymerization or bulk suspension polymerization. On X-ray analysis, the resulting polymer exhibits a diffraction pattern typical of an amorphous structure and on ^H-NMR analysis it exhibits a spectrum with broad bands without peaks resolution in the area of the chemical shifts, typical of the methine and methylene protons, in accordance with a random sequence of monomeric units with different steric configuation (atactic structure).

The field of use of this amorphous polymer is limited by the low glass transition temperature (Tg) which is of about 100°C. Above this temperature, in fact, the mechanical properties of the ־ 3 polymer drastically decay. Amorphous polymers are obtained also when styrene polymerization is conducted by cationic and anionic catalysis even if in some cases a prevalent syndiotactic configuration of the carbon atoms in the main chain was found.

It is also known that crystalline, mainly isotactic polystyrene can be produced by stereospecific polymerization of styrene. Styrene polymerization, in such case, is carried out in the presence of Ziegler-Natta-type catalysts based on halogenated compounds of transition metals, either as such or carried on magnesium halides, in combination with aluminium alkyls, optionally in the presence of electron donors. A detailed description of this type of polymerization is given in Italian patent No. 537,425, USA patents Nos. 3,161,624 and 2,882,263, and in British patents Nos. 826,021 and 844,944. The polystyrene prepared by means of such types of stereospecific catalysts was obtained for the first time by G. Natta et al. (J. Am. Chem. Soc. 1955, 77, 1700) and shows, in the X-ray pattern, diffraction peaks ascribr able to a mainly isotactic-type structure. On ¹H-NMR analysis, the multiplet ascribed to the methylene group is indicative of a non-equivalence of the two protons, what also is indicative of an isotactic structure (Heatley F. Bovey F.A. in Macromolecules (1968), 1,301). This crystalline mainly isotactic polymer has not found any industrial appliances as its crystallization rate is very low.

Known too (N. Ishihara et al. Macromolecules (1986), 19, 2464-2465) is a crystalline polystyrene having a low weight average molecular weight (about 82,000) and mainly a syndiotactic structure, as was attributed on the basis of the data of X ray diffraction, infrared spectroscopic analysis, ¹H-NMR and ¹^c-NMR analyses.

This polymer shows a high melting point, about 270°C, and a high crystallization rate. The thermal and structural characteristics of these mainly syndiotactic polymers permit to retain the mechanical properties unaltered even at higher temperatures than the glass transition temperature (Tg). It is well known, however, that many mechanical properties of styrene, such as tensile strength, elongation at break, tenacity and stresscracking resistance, remarkably improve as the molecular weight increases, while they drop to values, which cannot be accepted for a practical industrial utilization, when the weight average molecular weight is below 100,000 (Encyclopedia of Polymer Science and Technology, vol. 13)♦

For this reason, the commercial polymers of styrene have weight average molecular weights ranging from 200,000 to 300,000.

It has now surprisingly been found that by using part-, icular catalyst systems it is possible to obtain crystalline vinyl aromatic polymers and particularly styrene or styrene derivative polymers having mainly a syndiotactic structure and an average molecular weight even higher than 100,000.

Thus, the present invention provides crystalline vinyl aromatic polymers and particularly styrene or styrene derivative polymers having a mainly syndiotactic structure and particularly a regular structure with syndiotactic configuration at least in long portions of the chain and an average molecular weight which may be even higher than 100,000.

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The crystalline polymers with a mainly syndiotactic structure according to the present invention are endowed with improved mechanical properties in comparison with the ones of the syndiotactic polystyrene of the art, said mechanical properties being combined with a melting point of about 270°C or even higher and with a high crystallization rate.

According to the present invention, the process for preparing crystalline vinyl aromatic polymers and particularly styrene or styrene derivative polymers of mainly syndiotactic structure, consists in carrying out the polymerization of the vinyl aromatic monomer and particularly of styrene or styrene derivative either alone or in admixture with up to 30$ of another co polymerizable ethylenically unsaturated monomer, in the presence of catalytic amounts of a catalyst system comprising the product of the reaction between:

a) a transition metal compound (M) different from titanium and containing at least a bond M-0; M-C; M-N; M-P; M־S and

׳. M-halogen; and

b) an organoaluminium compound containing at least an oxygen atom bound to the aluminium atom or between two Al atoms.

Transition metal compounds are utilizable as such or carried on organic or inorganic compounds, such as for example Si0₂, Α1₂0β or Mg halides. Also the organoaluminium compound can be used as such or carried on the same carriers as are utilized for the transition metal compounds.

The catalyst components carried on Mg halides are well known in literature and are described, for example, in USA patent No. 4,298,718 or in British patents Nos. 1,292,853 and 1,305,610 in the name of the Applicant hereof, the content of which is an integral part of the present description.

The molar ratios of Al to M are not critical and are in the range from 1 to 1,000, preferably from 10 to 200. The concentration of the aluminium compound ranges from 10 to 1 mole/liter with respect to the reaction mixture.

Typical examples of transition metal compounds corresponding to the definition of component a) are:

bis-indenyl zirconium dichloride; zirconium tetrabenzyl;

Examples of organoaluminium compounds the definition of component b) are:

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ZrCl(O.i-C₃Hy)₃: vanadyl bis-acetylacetonate; vanadium triacetylacetonate; zirconium-dibenzyl dichloride; ZrCKOCiiHg)^ and Ni(15_׳~ cyclooctadiene)2« corresponding to for a straight aluminoxane ־ for a cyclic aluminoxane wherein:

n is 0 or an integer from 1 to 40, preferably from 10 to 30;

R can be: an alkyl radical, an aryl radical, an arylalkyl radical or a cycloalkyl radical, each of them containing 1 to 20 carbon atoms; an 0-R' radical, where R' may be an alkyl radical having 1 to 8 carbon atoms or an aryl radical having 6 to 20 carbon atoms

־ or a halogen provided that not all radicals R are a halogen or 0R’.

R can be of the same or different nature in the various positions of the above-cited components 1-2.

More representative examples are straight methyl aluminoxane with n = 20; cyclic methylaluminoxane with n = 20.

These aluminium compounds can be used either alone or blended with aluminium trialkyls or aluminium alkyl halides, wherein the alkyl groups contain 1 to 8 carbon atoms.

Polymerization of vinylaromatic monomers, according to both the continuous and the discontinuous process, can be carried out in bulk, in suspension, in solution or in bulk-suspension. The polymerization temperature can be selected in the range of from -80°C to +200°C, preferably from -20°C to 100°C.

The polymer molecular weight can be adjusted by using the techniques which are usually adopted in theZiegler-Natta catalysis, for example by addition of hydrogen.

The modalities concerning the addition of the various catalyst system components, of the monomer and of the solvent, if any, are not critical.

The term vinylaromatic polymers, whenever used in the present description and in the appended claims, comprises styrene polymer, styrene derivative polymer and the relevant copolymers containing up to 30? by weight of another copolymerizable, ethylenically unsaturated monomer of general formula:

CH₂ = CH - r₃wherein R^ is hydrogen or an aliphatic group containing 1 to 6 carbon atoms; an aryl group containing 6 to 20 carbon atoms; an aryl group alkyl- or halogen-substituted in the nucleus containing 6 to 30 carbon atoms, or a cycloaliphatic group, optionally alkylor halogen-substituted, containing 3 to 20 carbon atoms.

These styrene derivatives include alkylstyrene, halogenated styrene, vinyl-naphthalene and vinyltetrahydronaphthalene containing from 9 to 12 carbon atoms in the molecule. Representative examples of styrene derivatives are a or β-vinyl naphthalene, 1,2,3,4-tetrahydro-6-vinyl naphthalene, ring substituted styrene with one or more alkyl groups containing from 1 to 4 carbon atoms or with one or more halogen atoms, such as chlorine. The term ring substituted styrene includes first of all and preferably p.methyl-styrene even if other alkyl derivatives may be used with the same results.

Typical Examples of ring substituted styrene are: methylstyrene, ethylstyrene, butylstyrene, p-tert-butylstyrene and dimethylstyrene, chlorostyrene, bromostyrene and fluorostyrene, chloromethylstyrene, alkoxystyrene such as methoxy^ styrene, carboxymethylstyrene, alkyletherstyrene, alkylsilylstyrene, vinyl benzenesulfonic acid esters, and vinyl benzyl dialkoxy phosphide.

The crystalline styrene or styrene derivative polymers having mainly a syndiotactic structure obtained by the process of the present invention were characterized by the following properties and by using the methods indicated hereinbelow:

a) Residue to the extraction with methylethyl ketone (MEK), carried out in a Kumagawa extractor for 24 hours.

b) Weight average molecular weight, (M_w), determined by gel permeation chromatography (G.P.C.) carried out with a Waters ־ 9 apparatus 150 ALC-GPC in ortho-dichiorobenzene (ODCB) at 135°C. Instrument calibration is obtained by using standard samples of atactic polystyrene of known molecular weight (Waters Associates Inc.).

c) Melting point (M.P.), determined by means of a differential calorimeter, model Perkins-Elmer DCS 7, as the temperature corresponding to the maximum of the endothermal peak, at a scanning rate of 40°C/min.

d) Intrinsic viscosity, determined in tetrahydronaphthalin at 135 °C.

e) Crystallinity of the polymer residue to MEK, determined after melting and solidification by means of X-ray diffraction, with the powder method, by verifying in the pattern the presence of sharp reflexes typical of the crystralline substances.

The values of the inter planar distances corresponding to said reflexes are determined with a spread of ± 0.1 Angstrom.

f) Syndiotacticity of the polymers, determined by ¹H-NMR or ¹^CNMR analysis, carried out by means of a BRUKER AM-300 instrument, in orthodi chlorobenzene at 125°C, and check standard of hexamethyl-disiloxane.

The following examples are given to illustrate the object of the present invention, without being however a limitation thereof.

Example 1

Into a glass reactor having an Internal volume of 100 ml and equipped with a stirrer there were placed, in an inert atmosphere and at 20°C:

250 mg of methylaluminoxane (1) ml of toluene mg of Zr(CH2.C5H5)4

After 5 minutes, 30 ml of styrene poured on an alumina column and distilled on L1A1H4 were added. In 30 minutes the temperature was brought to 50°C and polymerization was conducted for 4 hours. After this stretch of time, the polymer was coagulated with methanol acidified with hydrochloric acid and repeatedly washed with methanol. After drying there was obtained a polymer with a conversion and having the characteristics of residue to MEK, weight average molecular weight and melting point, which are shown in the Table.

On X-ray analysis, the residue to the extraction with MEK exhibited, after melting and solidification, reflexes of higher intensity corresponding to interplanar distances of 13.1 A, 7.6 .A, 6.46. A, 4.37 A, and 2.58 A.

The ¹H-NMR pattern of the polymer shows chemical shifts' of the methine and methylene protons respectively centered at 1.9 and 1.4 ppm, as illustrated in Fig. 1.

(1) Methylaluminoxane was prepared as follows:

Into a three-neck flask having an internal volume of 500 ml, equipped with a dropping funnel, a cock and a magnetic stirrer, there were placed 37.1 g of Al 2(804)3.18H20 and 250 ml of toluene in a nitrogen atmosphere.

ml of AHCH^jj were placed into the dropping funnel and were added under stirring in 30 minutes. It was reacted at 60°C for 3 hours. The aluminium sulphate was filtered off from the suspension and the solvent was removed. 15.5 g of a solid white product were obtained.

Example 2

Example 1 was repeated, using:

140 mg of methylaluminoxane, ml of toluene, mg of ClZr(OC₃H₇)₃.

The polymer amount obtained, the residue to MEK extraction, the weight average molecular weight and the melting point are reported in the Table. The X-ray diffraction pattern and the ¹H־NMR pat־ tern were.similar to those in Example 1.

Example 3

Example 1 was repeated, using:

230 mg of methylaluminoxane, ml di toluene, mg of V0AC₂.*

Amount of polymer obtained, residue to MEK extraction, weight average molecular weight and melting point are reported in the table. The X-ray diffraction pattern and the ¹H-NMR pattern were similar to those in Example 1.

* AC = acetylacftonate.

Example 4

Example 1 was repeated, using:

200 mg of methylaluminoxane, ml of toluene,

0 mg of ZrCl (OC1|Hg) ₃.

Amount of obtained polymer, residue to MEK extraction, weight i average molecular weight and melting point are reported in the Table. The X-ray diffraction pattern and the ¹H־NMR pattern were

־ 12 similar to those in Example 1.

Example 5

Example 1 was repeated, using:

150 mg of methylaluminoxane, ml of toluene, mg of Zr(CH₂-C6H₅)₂Cl₂.

Amount of polymer obtained, residue to MEK extraction, weight average molecular weight and melting point are reported in the table. The X-ray diffraction pattern and the ¹H-NMR pattern were similar to those in Example 1.

TABLE

<td> Example No.</td><td> Conversion 1</td><td> Residue to MEK ן</td><td> ^w (u.m.a.)</td><td> M.P. °C</td>

<td> 1</td><td> 20</td><td> 23</td><td> 23,000</td><td> 270</td>

<td> 2</td><td> 25</td><td> 41</td><td> 21 ,000</td><td> 269</td>

<td> 3</td><td> 11</td><td> 85</td><td> 48,000</td><td> 268</td>

<td> 4</td><td> 30</td><td> 49</td><td> 30,000</td><td> 270</td>

<td> 5</td><td> 25 '</td><td> 31</td><td> 31,000</td><td> 269</td>

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