KR19990045746A - Copolymer of olefin monomer and 1,2-polybutadiene - Google Patents

Abstract

The present invention relates to a process for the preparation of 1,2-polybutadiene, which comprises reacting said at least one olefin monomer with 1,2-polybutadiene under the influence of a thermoplastic polyolefin and a cyclopentadienyl-containing transition metal complex which are copolymers of at least one olefin monomer and 0.005 to 10% ≪ / RTI >

Description

Copolymer of olefin monomer and 1,2-polybutadiene

The present invention relates to a thermoplastic polyolefin which is a copolymer of at least one olefin monomer and 0.005 to 10 wt% 1,2-polybutadiene.

One of the characteristic parameters of the polyolefin is a molecular weight distribution represented by a ratio of a weight average molecular weight (Mw) and a number average molecular weight (Mn). These variables affect product properties such as, for example, tensile strength and impact resistance. An important parameter influencing the processing behavior of polyolefins is the melt flow rate (MFR). This is normally calculated as the ratio of the melt index measured using the melt index (MI) measured according to ASTM D-1238 and the weight of 2.16 kg using a weight of 21.6 kg. It is known that the MFR of polyolefins is increased by increasing the molecular weight distribution, that is, the Mw / Mn value. As a result, it is necessary to compromise when the specific MFR in use is favorable to the processing characteristics and when the specific Mw / Mn ratio is favorable to the product properties.

WO-A-93/08221 discloses the production of thermoplastic polyolefins with constrained-geometry catalysts. While the method described above allows for a variety of MFRs, the width of the molecular weight distribution is nearly constant. However, the polyolefins described above all have a molecular weight distribution close to two. A method of producing a polyolefin having a different molecular weight distribution is not obtained.

However, there is a need for thermoplastic polyolefins having different molecular weight distributions and MFR combinations than those in the prior art, in order to be able to select the materials that are more suitable for use.

It is an object of the present invention to provide the polyolefin.

This object of the present invention is to obtain copolymers of at least one olefin monomer with from 0.005 to 10 wt% 1,2-polybutadiene. Also, 1,2-polybutadiene will be referred to as (co) monomer. The amount of 1,2-polybutadiene is referred to as the total amount of copolymer.

DE-A-2.123.911 discloses copolymerization of polybutadiene during the production of sulfur-crosslinkable ethylene-propylene-diene rubbers. In this document, the influence of 1,2-polybutadiene on the above-mentioned important characteristics of the polyolefin is not mentioned, and the distinction between the types of polybutadiene such as 1,4-polybutadiene or 1,2-polybutadiene I do not.

DE-A-2.917.403 describes copolymers of polybutadiene and propylene containing 10 to 20% 1,2- (vinyl) unsaturation and having a molecular weight of 10 ⁵ to 10 ⁶ g / mol. The literature also does not mention the differences between the forms of polybutadiene. The copolymer acts as a thermoplastic with elastomeric properties.

In contrast, the polyolefins of the present invention are inherently thermoplastic and do not have distinct elastomeric properties. The polyolefin of the present invention contains at least one olefin monomer. As olefin monomer ethylene it is used in combination with one or more C ₄ -C ₂₀ alpha-olefins. The olefinic monomers may contain 0-50 wt% of C ₄ -C ₂₀ alpha-olefins relative to the total amount of olefinic monomers present in the polyolefin of the present invention.

The? -Olefins combined with ethylene are preferably C ₄ -C ₂₀ ? -Olefins. Examples of the? -Olefins are butene, hexene and octene. Copolymers of ethylene with at least one alpha -olefin in which the diene is copolymerized as the third monomer are known to exhibit elastomeric properties. Since it has thermoplastic properties, the polyolefin of the present invention does not contain diene-derived units having different physical mass from those generated from 1,2-polybutadiene. The total polymer of the other diene-derived units contained is preferably at most 1 wt%, but most preferably, they are completely absent.

As described above, the polyolefin of the present invention does not contain an olefin monomer and 1,2-polybutadiene and a monomer having a different mass.

The molecular weights Mw and Mn of the polyolefin are measured by size-exclusion chromatography combined with a polyethylene calibration sample and a viscosity detector used as reference.

For purposes of the present invention, 1,2-polybutadiene is a polymer having the number of [-CH ₂ -CH-CH = CH ₂ -] units [-CH ₂ -CH = CH-CH ₂ -] units and no more unsaturation Is understood to be a polymer of butadiene that is larger than the number of other butadiene-derived units such as units.

The length of the inserted polybutadiene chain expressed by the number of polymerized butadiene units should be at least 4. [ The length is preferably at least 10, more preferably at least 25.

The number of 1,2-vinyl unsaturation per chain is at least 3, preferably at least 6, more preferably at least 13. [ If the need for number of vinyl unsaturation and chain length is met, then 1,2-polybutadiene is partially saturated within the scope of the present invention. Saturation can be effected, for example, by hydrogenation or copolymerization of butadiene with alpha-olefins. The chain length of the polybutadiene is preferably 5,000 or less. The longer the chain length, the lower the normal properties of the polyolefin, and the polyolefin begins to exhibit strongly non-uniform behavior.

It has been found that in terms of increasing the amount of polybutadiene and simultaneously increasing the MFR, the MFR is relatively strongly dependent on the amount of 1,2-polybutadiene incorporated, while the Mw / Mn ratio appears to be relatively less sensitive. However, the Mw / Mn ratio increases the 1,2-polybutadiene content and tends to increase, especially with copolymers incorporating other? -Olefin monomer units, so that the content can be used to adjust the ratio.

Generally, the Mw / Mn ratio of the polyolefin of the present invention is higher than the Mw / Mn ratio of the (co) polymer produced under the same conditions as those of the other without incorporating 1,2-polybutadiene. It is preferable to increase the amount to 3.5 elements or less. There is a risk of gel formation in the copolymer. When processed, especially when processed into a film, the copolymer gives a product with poor appearance. In view of the requirement to increase the Mw / Mn ratio in a limited manner, if the chain length of 1,2-polybutadiene is at least 4, the amount of 1,2-polybutadiene is preferably at most 10 wt%, and when the chain length is greater than 10, More preferably at most 5 wt%, and most preferably at most 3 wt% when the chain length is 25 or more. The effect of 1,2-polybutadiene can be evaluated very easily with an amount of 1,2-polybutadiene of at most 5 wt% at chain lengths of 10 or more. Polyolefins containing greater than 10% of 1,2-polybutadiene exhibit increased sensitivity to oxidation.

In addition, the presence of partially saturated 1,2-polybutadiene increases the Mw / Mn ratio in that the Mw / Mn ratio increases as the number of double bonds in the polybutadiene increases. In addition, the MFR in this case increases by increasing the amount of partially saturated 1,2-polybutadiene.

The polyolefin of the present invention has good processability and melt strength and is suitable for use in a wide range of two products, a thin walled material such as a film and a thick walled material.

The present invention also relates to a process for preparing a thermoplastic polyolefin by contacting a cyclopentadienyl-containing transition metal complex as a catalyst with ethylene and optionally one or more C ₄ -C ₂₀ alpha-olefins as catalysts under the condition of polymerizing the monomers in the presence of a catalyst .

It is generally known to prepare polyolefins with catalysts containing non-cyclopentadiene-derived ligands such as Phillips and a Ziegler catalyst. The molecular weight distribution, Mw / Mn and melt flow rate of the resulting polyolefin appear interdependently. Given a specific value of the Mw / Mn ratio, the MFR is actually fixed such that there is no range for selecting a specific MFR when a polymer having a particularly desirable Mw / Mn ratio is produced.

WO-A-93/08221 discloses the preparation of polymers of ethylene and copolymers thereof with? -Olefins under the influence of cyclopentadienyl-containing transition metal compounds as catalysts. The process described in this document yields polyolefins having a value of 5.6-16 and an Mw / Mn ratio of 1.86-2.32, equivalent to MFR. A method by which a polyolefin having a different molecular weight distribution can be produced is not known.

There is therefore a need for a process for preparing thermoplastic polyolefins having a combination of known polyolefins and other Mw / Mn ratios and MFR.

This need is met by the present invention in which polymerization takes place in the presence of 1,2-polybutadiene.

It has been found that the MFR and Mw / Mn ratio can be controlled by the amount and type of 1,2-polybutadiene present in the polymerization medium. The way in which the amount affects the mentioned properties is shown in the description for the polyolefin.

The amount of the copolymerized 1,2-polybutadiene is at most 10 wt%, preferably at most 5 wt%, more preferably at most 3 wt%. If too much 1,2-polybutadiene is copolymerized, it consumes the thermoplastic properties of the resulting polyolefin. The measurable effect on the polymer structure and end product properties obtained is observed when very small amounts of 1,2-polybutadiene are added as comonomers and / or tri-comonomers. The tolerance depends on the chain length of the 1,2-polybutadiene as shown in the above description of the polyolefin of the present invention.

1,2-polybutadiene, which should be present in the reaction mixture in the order of the desired amount of the copolymerized 1,2-polybutadiene-containing polyolefin, depends on the activity of the catalyst used and can be easily measured empirically. For example, the effect of the 1,2-polybutadiene content on the MFR will be clear from the above and the examples. Those skilled in the art will be able to experiment under selected reaction conditions to determine the relationship and to substantially measure the desired amount and / or the desired molecular weight for the copolymer with the desired properties.

1,2-polybutadiene appears to be incorporated in a highly effective method. This can be regarded as a y-substituted olefin in which the vinyl-unsaturated side group has an alkyl group substituent on the 3-position. Such substituents are known to strongly inhibit the reactivity of olefins in the catalytic polyolefin process due to what is known as steric hindering around the olefinic bonds. The y-substituted olefins are not selected as comonomers which influence the highly entrained 1,2-polybutadiene highly converted in the catalyzed polyolefin process.

In addition, incorporation of only a minor amount of 1,2-polybutadiene appears to have only a minor effect on the MFR. Minor means here: the detectable unsaturated number in the polyolefin of the present invention is in the same range as the number of unsaturations of other similar thermoplastic polyolefins in which the 1,2-polybutadiene is not copolymerized.

Other advantages of the method of the present invention are as follows. The change in MFR in WO-A-93/08221 is influenced by an appropriate choice of catalyst amount. This means that the reaction conditions must also be modified. Conversely, the process of the present invention can be carried out under the same conditions as a certain amount of catalyst, since the amount of 1,2-polybutadiene in the reaction mixture is on the principle of measurement variables.

The requirement of 1,2-polybutadiene, in which the process of the present invention is copolymerized with the definition of the copolymer to which it relates, is shown above.

The polymerization is effected by contacting the cyclopentadienyl-containing transition metal complex as a catalyst with ethylene and optionally with at least one C ₄ -C ₂₀ alpha-olefin.

The metal complex contains a transition metal of Group 3 or Group 4 of the Periodic Table of the Elements in the new IUPAC version as shown in the cover of Chemistry and Physics, 70th Edition, CRS Publication 1989-1990. If the valence of the metal is 3 ⁺ , the complex may be represented by R ¹ MX ¹ X ² or R ¹ R ² MX ¹ . If the valence of the metal is 4 ⁺ , the complex may be represented by R ¹ R ² MX ¹ X ² or R ¹ MX ¹ X ² X ³ .

In the above formula, R ¹ is a substituted or unsubstituted cyclopentadienyl such as indenyl, fluorenyl, methyl-cyclopentadienyl, pentamethylcyclopentadienyl, or heteroatom-containing derivatives of cyclopentadienyl ligands. Lt; / RTI > ligand. In the last mentioned group, a heteroatom may be an element of Group 15 or Group 16 of the Periodic Table of the Elements such as N, P, As, O or S. The heteroatom may form part of the cyclopentadienyl ring or be located external to it. R ² may be a Cp derivative as defined for R ¹ but may also be a substituent containing an element of Group 15 or Group 16 of the Periodic Table of the Elements such as N, P, As, O or S, Covalent bond or coordination bond. R ¹ and R ² are bonded to one another by an aliphatic or aromatic group or by a group containing an element of Group 15 or Group 16 of the Periodic Table of the Elements by a -Si (R) ₂ group (wherein R represents an aliphatic or aromatic group) .

And R ² is a neutral, cyclopentadiene - if bonded to is not a derived compound, R ¹ in one of the above methods, a combination of R ¹ and R ² are cyclopentadienyl heteroatom dienyl ligand as described for R ¹ - Containing derivatives.

X ¹ , X ² and X ³ are the same or different and are selected from the following groups:

-halogen

- aliphatic or aromatic substituents

A substituent containing an element of Group 15 or Group 16 of the Periodic Table of the Elements, such as -OR ³ , NR ^3, etc., wherein R ³ is an aliphatic or aromatic substituent optionally containing silicon.

As the catalyst, a cyclopentadienyl-containing transition metal complex having a structure of R ⁴ MX ¹ X ² is preferably used;

( ^Wherein M is a transition metal of Group 4 of the Periodic Table of the Elements having no high valence such as Ti ³⁺ , X ¹ and X ² are as defined above, and R ⁴ is represented by the following Chemical Formula 1:

^{[Cp'-YZ (R 5)} n] -

(Wherein Cp 'is a cyclopentadienyl derivative substituted with a group containing an aliphatic or aromatic group or a heteroatom, Y is an aliphatic or aromatic group or a group containing silicon or a hetero atom, Z is N Or P, and R ⁵ is an aliphatic, aromatic or silicon-containing group, and n is equal to the valence of Z of -1), or an element of Group 15 or Group 16 of the Periodic Table of Elements,

Essentially, the metal complex is used as a catalyst in combination with an activator. Materials known to be suitable for purposes such as methylaluminoxane (MAO) or (per) boron fluoride compounds such as trispentafluorophenylborane and tetrakispentafluorophenylborate compounds can be used as activators. Furthermore, 1,2, 12 or 13 such as trimethylaluminum, triethylaluminum, triisobutylaluminum, trioctylaluminum, diethylaluminum ethoxide, diethylmagnesium, dibutylmagnesium, ethyl-butylmagnesium and butyl- Group metal, preferably an aluminum-alkyl compound or an organometallic compound having a magnesium-alkyl compound, can be used in the catalyst system. The activity of the catalyst system can be increased by adding the metal / alkyl compound of the main group.

The olefin monomer is contacted with the catalyst under conditions in which the monomer polymerizes in the presence of the catalyst. The conditions selected to polymerize the olefin with a metal catalyst such as a Ziegler-Natta or Phillips catalyst and the conditions selected form a technique known to be usable for polymerizing olefins with catalysts such as those described in the process of the present invention.

The polymerization reaction can be effected, for example, in a gas phase, in a suspension or solution (semi-continuously) or batchwise. In addition, a plurality of active agents arranged in parallel, in series or in combinations thereof, can be used. The solution process is preferably used because it is very suitable for the production of very low crystalline polymers which are soluble in the hydrocarbons, at least to some extent.

As the dispersant or solvent for the polymerization reaction, a liquid which does not have an effect opposite to the activity of the catalyst system can be used. Saturated, branched or branched aliphatic hydrocarbons such as butane, pentane, heptane, pentamethylheptane or petroleum fractions or gasoline or refined gasoline, naphtha, kerosene or gas oil, mixtures of such materials . Aromatic hydrocarbons such as benzene and toluene are suitable, but are undesirable in cost and safety drawings. For plant-scale polymerization, aliphatic hydrocarbons or mixtures thereof used in the petrochemical industry are preferably used as dispersants and solvents after drying and refining.

The polymer obtained by the method of the present invention can be worked in a known manner. In general, the catalyst is deactivated in a manner known per se to the point of operation of the polymer.

Polymerization is affected at atmospheric pressure as well as at increased pressure. If polymerization is effected at increased pressure, the polymer yield per unit time can still be increased. The polymerization is preferably carried out at pressures between 0.1 and 60 MPa, especially between 1 and 30 MPa. When polymerization takes place in so-called autoclaves, higher pressures of more than 100 MPa are used.

The molecular weight of the polymer can be adjusted by adding hydrogen or other chain stopper or adjusting the polymerization conditions.

The 1,2-polybutadiene is preferably added as a solution in a suitable dispersant which has no adverse effect on the polymerization reaction. In a continuous process, 1,2-polybutadiene is preferably added to the polymerization activator on a continuous basis. In systems using multiple active agents, 1,2-polybutadiene may be added to only a few active agents used. In batch polymerization, the 1,2-polybutadiene may be added before or during the polymerization. The amounts and molecular weights of 1,2-polybutadiene are described above.

The present invention will be described without being limited to the embodiments.

Density (D ₂₃ ) was measured according to ASTM Standard D 792-66. The melt index (MI) was measured according to ASTM standard D1238 using a weight of 2.16 kg. The melt flow rate (MFR) was measured by an index of the melt index measured by ASTM D1238 using the weights of 21.6 kg and 2.16 kg, respectively. The Mw / Mn ratio was determined by combining the Viscotek type 502 viscometer as a viscometer with water M150C gel permeation chromatography with a DRI-detector as size exclusion z chromatography using a polyethylene calibration sample.

Examples I-IV

A number of polyolefins of the present invention were prepared as follows.

A 2 liter autoclave was charged with a specific boiling point spirit (boiling range 65-70 占 폚) and maintained at a temperature of 160 占 폚. A mixture of specific boiling point spirits (5.5 kg / h) and ethylene (1.2 kg / h) was continuously added into the autoclave. In some cases, hydrogen was added to obtain the desired molar volume. The feeding of the autoclave to the autoclave was adjusted so that the autoclave was completely filled with the reaction medium. The catalyst solution, the active agent suspension and the triethyl aluminum solution were also continuously added into the autoclave. The conversion of ethylene was controlled by the amount of catalyst and the amount of activator, which was about 95% in each experiment.

As a catalyst for copolymers made of ethylene and 1,2-polybutadiene ^{_{ethylene-dimethylamino-tetramethyl-cyclopentadienyl-titanium-dimethyl, Cp * (CH 2 CH 2}} ) N (CH 3) 2 Ti (CH ₃ ) ₂ was added as a catalyst. A catalyst concentration of about 20 μmol per liter was required to obtain 95% ethylene conversion.

As the activator, dimethylanilinium tetrakispentafluorophenylborate was used. An activator concentration of about 40 μmol per liter was required to obtain 95% conversion. The concentration of triethylaluminum used in the autoclave was determined to be about 40 μmol per liter. In the separation vessel, a mixture of 1,2-polybutadiene and a specific boiling point spirit having a Mn of about 3000 g / mol containing about 50 vinyl groups per molecular chain (grade B-3000 of Messrs NISSOH IWAI) was prepared. A specific amount of 1,2-polybutadiene solution was continuously pumped from the vessel into the autoclave. The preferred ratio of 1,2-polybutadiene to be added is adjusted by appropriately selecting the concentration of 1,2-polybutadiene in the solution. 1 g / h of 1,2-polybutadiene corresponded to 0.088 wt% with respect to the converted ethylene. The properties of the copolymers obtained by adding 1,2-polybutadiene in various proportions are shown in Table 1.

Example Addition of 1,2-polybutadiene (g / h) MI MFR Mw / Mn D23 Ⅰ 0 6.1 25.8 2.5 958.9 Ⅱ 0.25 4.7 27.8 2.5 959.6 Ⅲ 2.5 4.7 30.2 2.6 959.1 IV 5 4.1 33.0 2.8 959.1

The results show that the MFR can be controlled at a rate at which 1,2-polybutadiene is added.

Example V-IX

Ternary copolymerization of ethylene, 1-octene and 1,2-polybutadiene:

Similar to Example I-IV, octene-1 0.2 kg / hr was added as an extra monomer in the autoclave and polymerization was carried out.

The properties of the copolymers obtained by adding 1,2-polybutadiene in various proportions are shown in Table 2. The polymers contained 15 wt% octene.

Example Addition of 1,2-polybutadiene (g / h) MI MFR Mw / Mn D23 V 0 4.2 29.1 2.5 914.9 VI 1.0 2.8 33.4 2.6 915.4 Ⅶ 5 2.4 39.8 3.2 915.6 VIII 15 0.68 56.7 5.0 916.2 IX 25 0.16 79.4 8.2 917.7

The process of the present invention also proves to be suitable for producing terpolymers. The MFR corresponds to 0.088 wt% with respect to the converted ethylene and increases markedly when 1 g / h of 1,2-polybutadiene is added.

Examples X-XIII

Similar to the above examples, in order to investigate the effect of the molar ratio of 1,2-polybutadiene on the MFR in the ternary copolymerization of ethylene, 1-octene and 1,2-polybutadiene, two different 1,2-polybutadiene Grade was used to produce a terpolymer of ethylene, 1-octene and 1,2-polybutadiene. The grade B-3000 and grade B-2000 already mentioned in Example V-IX were used from the same feedstock having a Mn of 2000 g / mol containing about 33 vinyl unsaturations per molecular chain. The results are shown in Table 3.

Example 1,2-polybutadiene (g / h) MI MFR Mw / Mn D23 1,2-polybutadiene grade X 0 4.2 29.1 2.4 914.9 - XI 2.5 4.1 31.1 2.7 917.0 B-2000 XII 5.0 2.2 38.6 3.0 915.6 B-2000 XIII 2.5 2.3 36.5 2.8 917.0 B-3000 Ⅶ 5.0 2.4 39.8 3.2 915.6 B-3000

Example XIV-XVIII

Polymerization was carried out in the same manner as in Example I-IV except that diphenylmethylene-fluorenyl-cyclopentadienyl-hafnium-dimethyl {[(C ₆ H ₅ ) ₂ C] FluCpZrMe ₂ } was used as a catalyst.

The 1,2-polybutadiene includes a grade (grade A) supplied by Messrs Aldrich with a Mn of about 1300 g / mol, a degree of unsaturation of about 99% and a vinyl: trans unsaturation of 40:30 and a Mn of about 1800 g / mol, (Grade B) supplied by the company with the unsaturation distribution in the various possibilities provided by the% unsaturation and 45: 10: 5 vinyl: trans: cis. The results are shown in Table 4.

Example 1,2-polybutadiene (% m / m) MI MFR Mw / Mn D23 Rating XIV 0 12 n.d. 3.5 956.7 XV 0.30 2.5 52 5.6 951.5 B XVI 0.60 0.5 98 4.6 949.7 B XVIII 0.09 42 n.d. 3.8 955.8 A XVIII 0.19 9.6 38 3.8 956.7 A

n.d .: Not measured

Claims (12)

A thermoplastic polyolefin characterized in that it is a copolymer of at least one olefin monomer and 0.005 to 10 wt% 1,2-polybutadiene. The method according to claim 1, Wherein the olefin monomer is ethylene. The method according to claim 1, Wherein said olefinic monomer contains ethylene and 0-50 wt% of a C ₄ -C ₂₀ alpha-olefin with respect to the total amount of olefin monomer present. The method of claim 3, Wherein said alpha -olefin is a C ₄ -C ₂₀ alpha-olefin. 5. The method according to any one of claims 1 to 4, Wherein the chain length of the 1,2-polybutadiene expressed by the number of polymerized butadiene units is 5,000 or less. 6. The method according to any one of claims 1 to 5, Wherein the polyolefin contains 0.01 to 5 wt% polybutadiene. 7. The method according to any one of claims 1 to 6, Wherein the chain length of the 1,2-polybutadiene is at least 10. 8. The method according to any one of claims 1 to 7, Wherein the 1,2-vinyl unsaturated number per polybutadiene chain is at least 3. < RTI ID = 0.0 > 11. < / RTI > A process for preparing a thermoplastic polyolefin of at least one olefin monomer constituted by contacting a cyclopentadienyl-containing transition metal complex as a catalyst with ethylene and optionally one or more C ₄ -C ₂₀ alpha-olefins under conditions in which the monomers polymerize in the presence of a catalyst In the method, Wherein said polymerization takes place in the presence of 1,2-polybutadiene. 10. The method of claim 9, Characterized in that a cyclopentadienyl-containing transition metal complex having a structure of R ⁴ MX ¹ X ² is preferably used as a catalyst. (In the above structural formula, M is a transition metal of Group 4 of the Periodic Table of Elements having no high valence such as Ti ³⁺ , X ¹ and X ² are the same or different and selected from the following groups; -halogen - aliphatic or aromatic substituents A substituent containing an element of Group 15 or Group 16 of the Periodic Table of the Elements, such as -OR ³ , NR ^3, etc., wherein R ³ is an aliphatic or aromatic substituent optionally containing silicon, and R ⁴ is as follows: (Formula 1) ^{[Cp'-YZ (R 5)} n] - (Wherein Cp 'is a cyclopentadienyl derivative substituted with a group containing an aliphatic or aromatic group or a hetero atom, Y is an aliphatic group or an aromatic group, or a silicon-containing group or a heteroatom-containing group, Z is an element of group 15 or group 16 element of the periodic table such as N or P, R ⁵ is an aliphatic, aromatic or silicon-containing group is, n is equal to the valence of Z -1)) 11. The method according to claim 9 or 10, Characterized in that said polymerization takes place in the presence of 0.01-10 wt% 1,2-polybutadiene. 12. The method according to any one of claims 9 to 11, Characterized in that the polymerization takes place in the presence of 0.01 to 5 wt% 1,2-polybutadiene.