Process for producing crystalline vinyl aromatic polymers having a predominantly syndiotactic structure.

0001The present invention relates to a process for preparing pre­dominantly 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.

0002As is known, styrene polymers and particularly polystyrene are thermoplastic materials which may show high molecular weights. Its thermal resistance, thermomoldability and excel­lent insulating properties render polystyrene particularly suitable for the production of extruded articles, molded ar­ticles and films to be used mainly in the field of sound and heat insulation, of packing and as dielectric material.

0003Polystyrene can be produced by radical polymerization, anionic polymerization or cationic polymerization.

0004The styrene polymerization conducted by the radical mechanism, either thermally or in the presence of initiators, can be ef­fected by bulk polymerization, emulsion polymerization, sus­pension polymerization or bulk suspension polymerization. On X-ray analysis, the resulting polymer exhibits a diffraction pattern typical for an amorphous structure, and on ¹H-NMR ana­lysis it exhibits a spectrum with broad bands without peak resolution in the area of the chemical shifts typical for the methine and methylene protons. This is in accordance with a random sequence of monomeric units with different steric con­figuration (atactic structure).

0005The use of this amorphous polymer is limited by its low glass transition temperature (Tg) of about 100°C. Above this tempe­rature there is a drastic deterioration of the mechanical pro­perties of the polymer.

0006Amorphous polymers are also obtained 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.

0007It 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 sup­ported 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 IT-PS- 537 425, US-A-3 161 624 and 2 882 263 and in GB-A-­826 021 and 844 944. 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 which may be assigned to a structure which is mainly isotactic. On ¹H-NMR analysis, the multiplet assigned to the methylene group is indicative of a non-equivalence of the two protons, what also indicates an isotactic structure (Heatley, F. and Bovey, F.A., Macromolecules (1968), 1, 301). This crystalline, mainly iso­tactic polymer has not found any industrial use as its cry­stallization rate is very low.

0008Also known (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 concluded on the basis of the data of X-ray diffraction, infrared spectroscopic analysis, ¹H-NMR and ¹³C-NMR analyses.

0009This polymer shows a high melting point, about 270°C, and a high crystallization rate. The thermal and structural charac­teristics of this mainly syndiotactic polymer allow retention of the mechanical properties even at higher temperatures than the glass transition temperature (Tg). It is well known, how­ever, that many mechanical properties of polystyrene, such as tensile strength, elongation at break, tenacity and stress-­cracking resistance, improve remarkably as the molecular weight increases, while when the weight average molecular weight is below 100 000 the values of said properties are un­acceptable for a practical industrial utilization. (Encyclo­pedia of Polymer Science and Technology, vol. 13).

0010For this reason, the commercially available polystyrenes have weight average molecular weights ranging from 200 000 to 300 000.

0011It has now surprisingly been found that by using particular catalyst systems it is possible to obtain crystalline vinyl aromatic polymers and particularly styrene or styrene deriva­tive polymers having a mainly syndiotactic structure and an average molecular weight even higher than 100 000.

0012Thus, the present invention provides crystalline vinyl aroma­tic polymers, and particularly styrene or styrene derivative polymers, having a predominantly 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.

0013The crystalline polymers with a mainly syndiotactic structure according to the present invention are endowed with improved mechanical properties as compared to 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.

0014According 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 % (particu­larly up to 15 %) of another copolymerizable ethylenically unsaturated monomer, in the presence of catalytic amounts of a catalyst system comprising the product of the reaction bet­ween: <ul id="ul0001" list-style="none"><li>a) a compound of a transition metal (M) different from tita­nium and containing at least one M-O; M-C; M-N; M-P; M-S or M-halogen bond; and</li><li>b) an organoaluminium compound containing at least one oxygen atom bound to the aluminium atom or located between two Al atoms.</li></ul>

0015The transition metal compounds are utilizable as such or sup­ported on organic or inorganic carriers, such as for example SiO₂ or Al₂O₃ or Mg halides (e.g. MgCl₂ and MgBr₂). Also the organoaluminium compound can be used as such or supported on the same carriers utilized for the transition metal compounds.

0016The catalyst components carried on Mg halides are well known in literature and are described, for example, in US-A-4 298 718 or in GB-A-1 292 853 and 1 305 610 in the name of the app­licant hereof, the disclosures of which are an integral part of the present description.

0017The molar ratio of Al to M is not critical and is in the range from 1 to 1000, preferably from 10 to 200. The concentration of the aluminium compound preferably ranges from 10⁻⁴ to 1, particularly from 10⁻³ to 10⁻¹, mole/liter, based on the reac­tion mixture.

0018Typical examples of transition metal compounds corresponding to the definition of component a) are: bis-indenyl zirconium dichloride; zirconium tetrabenzyl; ZrCl(O-i-C₃H₇)₃; vanadyl bis-acetylacetonate; vanadium triace­tylacetonate; dibenzyl zirconium dichloride,ZrCl(OC₄H₉)₃ and Ni(1,5-cyclooctadiene)₂.

0019Examples of organoaluminium compounds corresponding to the definition of component b) are: <chemistry id="chem0001" num="0001"><img file="EP0272584A2_D0001.tif" /></chemistry>     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 containing 1 to 20, preferably 1 to 8 carbon atoms, an aryl radical containing 6 to 20, pre­ferably 6 to 12 carbon atoms, an arylalkyl radical containing 7 to 20, preferably 7 to 12 carbon atoms or a cycloalkyl ra­dical containing 3 to 20, preferably 5 to 8 carbon atoms; a O-Rʹ radical, where Rʹ may be an alkyl radical having 1 to 8, preferably 1 to 4, carbon atoms or an aryl radical having 6 to 20, preferably 6 to 12, carbon atoms or halogen (prefer­ably F, Cl or Br) provided that not all radicals R are halo­gen or O-Rʹ.

0020R can be the same or different in the various positions of the above cited components (A) and (B).

0021Representative examples are linear methylaluminoxane with n = 20; cyclic methylaluminoxane with n = 20.

0022These aluminium compounds can be used either alone or blended with aluminium trialkyls AlRʺ₃ or aluminium alkyl halides AlRʺ₂X and AlRʺX₂, wherein the alkyl groups Rʺ contain prefer­ably 1 to 8 carbon atoms and may be the same or different and X may be the same or different and represent F, Cl, Br and J, particularly Cl and Br.

0023The 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 is preferably in the range of from -80°C to +200°C, most preferably from -20°C to +100°C.

0024The molecular weight of the polymer can be adjusted by using the techniques which are usually adopted in the Ziegler-Natta catalysis, for example by addition of hydrogen.

0025The modalities concerning the addition of the various catalyst system components, of the monomer and of the solvent, if any, are not critical.

0026The term "vinylaromatic polymers", whenever used in the pre­sent description and in the claims, comprises styrene polymers, styrene derivative polymers and the related copolymers contai­ning up to 30 % by weight of another copolymerizable, ethyle­nically unsaturated monomer, suitably of general formula:     CH₂ = CH - R₃ wherein R₃ is hydrogen or an aliphatic group containing 1 to 6, preferably 1 to 4, carbon atoms; an aryl group containing 6 to 20, preferably 6 to 12, carbon atoms; an aryl group sub­stituted with alkyl or halogen radicals in the nucleus and containing altogether 6 to 30, preferably 6 to 18 carbon atoms, or a cycloaliphatic group, optionally alkyl- or halogen-sub­stituted, containing altogether 3 to 20, preferably 5 to 12, carbon atoms.

0027The styrene derivatives include alkylstyrene, halogenated styrene, vinyl-naphthalene and vinyltetrahydro-naphthalene containing from 9 to 12 carbon atoms in the molecule. Repre­sentative examples of styrene derivatives are alpha- or beta­vinyl naphthalene, 1,2,3,4-tetrahydro-6-vinyl naphthalene, styrene ring substituted with one or more (up to 5) alkyl groups containing from 1 to 4 carbon atoms and/or with one or more (up to 5) 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.

0028Typical examples of ring substituted styrene are: methylstyrene,ethylstyrene, butylstyrene, p-tert-butylstyrene, and dimethylstyrene, chlorostyrene, bromostyrene and fluoro­styrene, chloromethylstyrene, alkoxystyrene, preferably C_1-4 alkoxystyrene, such as methoxystyrene, carboxymethylstyrene (e.g. acetoxymethylstyrene), alkyletherstyrene, alkylsilyl­styrene, vinylbenzenesulfonic acid esters, and vinylbenzyl­dialkoxy phosphide.

0029The crystalline styrene or styrene derivative polymers having a mainly syndiotactic structure, obtained by the process of the present invention, were characterized by the following properties and by using the methods indicated hereinbelow: <ul id="ul0002" list-style="none"><li>a) <u style="single">Residue of the extraction</u> with methylethyl ketone (MEK), carried out in a Kumagawa extractor for 24 hours.</li><li>b) <u style="single">Weight average molecular weight</u>, (<img file="EP0272584A2_D0002.tif" />_w), determined by gel permeation chromatography (G.P.C.) carried out with a Wa­ters apparatus 150 ALC-GPC in ortho-dichlorobenzene (ODCB) at 135°C. Instrument calibration was obtained by using standard samples of atactic polystyrene of known molecular weight (Waters Associates Inc.).</li><li>c) <u style="single">Melting point</u> (M.P., determined by means of a differential calorimeter, model Perkin-Elmer DSC 7, as the temperature corresponding to the maximum of the endothermal peak, at a scanning rate of 40°C/min.</li><li>d) <u style="single">Intrinsic Viscosity</u>, determined in tetrahydronaphthalene at 135°C.</li><li>e) <u style="single">Crystallinity of the polymer residue of MEK</u>, 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 crystalline substances. The values of the interplanar distances corresponding to said reflexes were determined with an error of ± 0.1 Å.</li><li>f) <u style="single">Syndiotacticity of the polymers</u>, determined by ¹H-NMR or ¹³C-NMR analysis, carried out on a BRUKER AM-300 instru­ment, in orthodichlorobenzene at 125°C, using hexamethyl­disiloxane as standard.</li></ul>

0030The following examples are given to illustrate the present invention, without being, however, a limitation thereof.

Example 1

0031Into a glass reactor having an internal volume of 100 ml and being equipped with a stirrer there were placed, in an inert atmosphere and at 20°C: 150 mg of methylaluminoxane (A) 10 ml of toluene 5 mg of Zr(CH₂C₆H₅)₄. After 5 minutes, 30 ml of styrene, passed through an alumina column and distilled over LiAlH₄, were added. Within 30 minutes the temperature was brought to 50°C and polymerization was conducted for 4 hours. After this period 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 showing a residue to MEK, a weight average molecular weight and a melting point as listed in the following Table.

0032On X-ray analysis, the residue of the extraction with MEK ex­hibited, after melting and solidification, reflexes of rela­tively high intensity corresponding to interplanar distances of 13.1 Å, 7.6 Å, 6.46 Å, 4.37 Å and 2.58 Å.

0033The ¹H-NMR pattern of the polymer showed 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:

0034Into a three-neck flask having an internal volume of 500 ml, equipped with a dropping funnel, a stop cock and a magnetic stirrer, there were placed 37.1 g of Al₂(SO₄)₃.18H₂O and 250 ml of toluene in a nitrogen atmosphere.

003550 ml of Al(CH₃)₃ were placed into the dropping funnel and were added under stirring within 30 minutes. The reaction was carried out 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

0036Example 1 was repeated, using: 140 mg of methylaluminoxane, 10 ml of toluene, 11 mg of ClZr(OC₃H₇)₃.

0037The amount of polymer obtained, the residue of MEK extraction, the weight average molecular weight and the melting point are given in the Table. The X-ray diffraction pattern and the ¹H-­NMR pattern were similar to those of Example 1.

Example 3

0038Example 1 was repeated, using: 230 mg of methylaluminoxane, 10 ml of toluene, 9 mg of VOAc₂ (*)). The amount of polymer obtained, the residue of 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 pattern were similar to those of Example 1. (*) Ac = Acetylacetonate.

Example 4

0039Example 1 was repeated, using: 200 mg of methylaluminoxane 10 ml of toluene 10 mg of ZrCl(OC₄H₉)₃. Amount of obtained polymer, residue of 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 of Example 1.

Example 5

0040Example 1 was repeated, using: 150 mg of methylaluminoxane, 10 ml of toluene, 8 mg of Zr(CH₂C₆H₅)₂Cl₂. Amount of polymer obtained, residue of MEK extraction, weight average molecular weight and melting point are reported in the Table. The X-ray diffraction pattern and ¹H-NMR pattern were similar to those of Example 1. <tables id="tabl0001" num="0001"><img file="EP0272584A2_D0003.tif" /></tables>

0041Specific examples of the various generic groups mentioned in the preceding description are: Alkyl and aliphatic radicals: methyl, ethyl, propyl, i-propyl, n-butyl, sec.-butyl, iso-butyl, tert.-butyl, pentyl, hexyl, heptyl, 2-ethylhexyl, n-octyl, nonyl, decyl, undecyl and do­decyl.

0042Cycloaliphatic and cycloalkyl radicals: cyclopropyl, cyclo­butyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl.

0043Arylalkyl: benzyl, (1- and 2-) phenylethyl, phenylpropyl, phe­nylbutyl.

0044Halogen: F, Cl, Br and J.

0045Carboxy: acetoxy, propoxy, butoxy.

0046Alkoxy: methoxy, ethoxy, propoxy, butoxy.

0047Specific examples of transition metals (M) are: Zr, V, Hf, Ni, Nb, V, Co.