WO1997012920A1

WO1997012920A1 - Copolymer of an olefinic monomer and 1,2-polybutadiene

Info

Publication number: WO1997012920A1
Application number: PCT/NL1996/000363
Authority: WO
Inventors: Johannus Antonius Maria Van Beek; Nicolaas Hendrika Friederichs; Joseph Anna Jacob Hahnraths
Original assignee: Dsm N.V.
Priority date: 1995-09-19
Filing date: 1996-09-18
Publication date: 1997-04-10
Also published as: NL1001237C2; NO981219D0; MX9802137A; AU7146296A; NO981219L; EP0851878A1; EA199800311A1; CN1202180A; KR19990045746A; CA2232456A1

Abstract

The invention relates to a thermoplastic polyolefin which is a copolymer of at least one olefinic monomer and from 0.005 to 10 wt.% 1,2-polybutadiene referred to the copolymer and a process for producing a copolymer of at least one olefinic monomer and 1,2-polybutadiene under the influence of a cyclopentadienyl-containing transition metal complex.

Description

COPOLYMER OF AN OLEFINIC MONOMER AND 1,2-POLYBUTADIENE

The invention relates to a thermoplastic polyolefin which is a copolymer of at least one olefinic monomer and from 0.005 to 10 wt.% 1 ,2-polybutadiene. One of the characterizing parameters of polyolefins is their molecular weight dist ibution, expressed as the quotient of the weight average molecular weight, Mw, and the number average molecular weight, Mn. This parameter has an influence on product properties, for instance, tensile strength and impact resistance. An important parameter which influences the processing behaviour of polyolefins is the Melt Flow Ratio (MFR). This is usually calculated as the quotient of the melt index (MI) determined according to ASTM D-1238 using a weight of 21.6 kg and the melt index determined using a weight of 2.16 kg. The MFR of common polyolefins is known to increase with wider molecular weight distributions or, in other words, with increasing values of Mw/Mn. As a consequence, in some applications a compromise needs to be sought when a particular MFR is desired for the processing properties and a particular Mw/Mn ratio for the desired product properties.

From O-A-93/08221 it is known to produce thermoplastic polyolefins with the aid of constrained- geometry catalysts. The disclosed process allows the MFR to be varied while the width of the molecular weight distribution remains almost constant. However, the polyolefins disclosed in the application mentioned all appear to possess a molecular weight distribution which is very near to 2. It is not taught how polyolefins with other molecular weight distributions should be produced. However, to enable optimum material selection for more applications there is a need for thermoplastic polyolefins with other combinations of MFR and molecular weight distribution than those according to the prior art. The object of the invention is to provide such polyolefins.

This object of the invention is achieved with a copolymer of at least one olefinic monomer and from 0.005 to 10 wt.% 1,2-polybutadiene. The 1,2-polybutadiene will hereinafter also be referred to as (co)monomer. The amount of 1,2-polybutadiene is referred to the total of the copolymer.

From DE-A-2.123.911 it is known to copolymerize polybutadiene in the production of sulphur-crosslinkable ethylene-propylene-diene rubbers. In that application no mention is made of a possible influence of the 1,2- polybutadiene on the aforementioned important properties of polyolefins and, moreover, no distinction is made between the types of polybutadiene; as 1,4- or 1,2- polybutadiene.

DE-A-2.917.403 discloses copolymers of propylene and polybutadiene containing from 10 to 20 % 1,2- (vinyl)unsaturations and having a molecular weight of from 10^s to 10⁶ g/mol. This application does not mention a difference between the types of polybutadiene, too. The described copolymers behave as a thermoplastic with elastomeric properties.

The polyolefin of the invention, in contrast, has essentially thermoplastic and no distinct elastomeric properties. The polyolefin of the invention contains at least one olefinic monomer. As olefinic monomer ethylene, optionally in combination with one or more of a C₄-C₂₀ c.- olefin is used. The olefinic monomer can contain 0-50 wt.% of a C₄-C₂₀ α-olefin with respect to the total amount of olefinic monomer present in the polyolefin of the invent ion .

Preferably the α-olefin combined with ethylene is a C₄-C₁₀ α-olefin. Examples of such α-olefins are butene, hexene and octene. It is known that in particular copolymers of ethylene with one or more α-olefins in which dienes are copolymerized as third monomer exhibit elastomeric properties. Since it possesses thermoplastic properties, the polyolefin of the invention does not contain any substantial amounts of diene-derived units other than those originating from the 1,2-polybutadiene. It is preferred for at most 1 wt.% referred to the total polymer of these other diene-derived units to be present but most preferably they are completely absent. The polyolefin of the invention does not contain substantial amounts of monomers other than the olefinic monomer and the 1,2-polybutadiene as described before.

The molecular weights, Mw and Mn of the polyolefin are determined by means of Size-Exclusion Chromatography in combination with a viscosity detector, with the polyethylene calibration samples being used as a reference.

For the purposes of the invention, 1,2- polybutadiene is understood to be a polymer of butadiene in which the number of [-CH-.-CH-] units is

I HC=CH₂ greater than the number of other butadiene-derived units such as [-CH₂-CH=CH-CH₂-] units and units in which there are no longer any unsaturations.

The length of the incorporated polybutadiene chains, expressed as the number of polymerized butadiene units, should be at least 4. Preferably, this length is at least 10, more preferably at least 25. The number of 1,2-vinyl unsaturations per chain is at least 3, preferably at least 6 and more preferably at least 13. If the requirements relating to the chain length and the number of vinyl unsaturations remain satisfied, it is within the scope of the invention also permissible for the 1,2-polybutadiene to be partially saturated.

Saturation may be effected by, for instance, hydrogenation or copolymerization of butadiene with α-olefins. The chain length of the polybutadiene preferably is not more than 5000. Greater chain lengths result in deterioration of the normal properties of the polyolefin and the polyolefin begins to exhibit strongly inhomogeneous behaviour.

It has been found that the MFR is relatively strongly dependent on the amount of incorporated 1,2- polybutadiene in that the MFR increases with increasing amounts of polybutadiene, whereas the Mw/Mn ratio appears to be relatively less sensitive. However, the Mw/Mn ratio does tend to increase with increasing 1,2-polybutadiene content, especially with copolymers incorporating different α-olefinic monomeric units, so that this content can be used to control this ratio as well.

In general, the Mw/Mn ratio of the polyolefin of the invention is higher than that of a (co)polymer produced under otherwise equal conditions but not incorporating any 1,2-polybutadiene. It is preferred for this increase to amount to not more than a factor of 3.5. Any higher increases involve the risk of gel formation in the copolymer. On being processed, especially on being processed into films, such a copolymer yields products whose appearance is less attractive. In view of the requirement of a limited increase in the Mw/Mn ratio, the amount of 1,2-polybutadiene is preferably at most 10 wt.% if the chain length of the 1,2-polybutadiene is at least 4, more preferably this amount is at most 5 wt.% when the chain length is 10 or more, most preferably this amount is at most 3 wt.% when the chain length is 25 or more. The effect of the 1 , 2-polybutadiene is very readily appreciable even at an amount of 1,2-polybutadiene of at most 5 wt.% at a chain length of 10 or more. Polyolefins containing a larger amount of 1,2-polybutadiene than 10% may exhibit increased susceptibility to oxidation.

Furthermore, it has been found that the presence of partially saturated 1 ,2-polybutadiene brings about an increase in the Mw/Mn ratio in that the Mw/Mn ratio increases as the number of double bonds in the polybutadiene decreases. In addition, in this case as well the MFR increases with increasing amounts of partially saturated 1, 2-polybutadiene.

The polyolefins of the invention possess good processability and melt strength and are suited to application in a wider range of products, both thin-walled objects, for instance films, and thick-walled objects.

The invention also relates to a process for the manufacture of a thermoplastic polyolefin by contacting ethylene and optionally one or more C₄-C₂₀ α-olefins with a cyclopentadienyl-containing transition metal complex as catalyst under conditions whereby the monomers polymerize in the presence of the catalyst.

It is generally known to produce polyolefins with the aid of catalysts containing non-cyclopentadiene- derived ligands, such as Phillips and Ziegler catalysts. The molecular weight distribution, Mw/Mn, and the Melt Flow Ratio of polyolefins so produced appear to be interdependent. Given a certain value of the Mw/Mn ratio, the MFR is virtually fixed so that when a polymer with a particular desired Mw/Mn ratio is to be produced, there is no more scope for selecting a particular MFR.

From O-A-93/08221 it is known to produce polymers of ethylene and copolymers thereof with α-olefins under the influence of a cyclopentadienyl-containing transition metal compound as catalyst. The process disclosed in that application yields polyolefins with an Mw/Mn ratio of 1.86-2.32 and with values of 5.6-16 for a quantity equivalent to the MFR. It is not taught how polyolefins having other molecular weight distributions can be produced.

Thus, there is a need for a process for producing thermoplastic polyolefins having other combinations of Mw/Mn ratio and MFR than the known polyolefins. This need is met by the invention in that polymerization takes place in the presence of 1,2- polybutadiene.

It has been found that the MFR and the Mw/Mn ratio can be controlled by the amount and type of 1,2- polybutadiene present in the polymerization medium. The manner in which this amount influences the properties mentioned is indicated in the discussion of the polyolefins in the foregoing.

The amount of 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, this is at the expense of the thermoplastic properties of the polyolefin obtained. It is surprising that a clearly measurable effect on the polymer structure and end-product properties obtained is observed even when very small amounts of 1,2-polybutadiene are added as comonomer and/or termonomer. The permissible amounts depend on the chain length of the 1,2- polybutadiene as indicated in the above description of the polyolefins of the invention.

The amount of 1,2-polybutadiene that must be present in the reaction mixture in order for polyolefin having the desired amount of copolymerized 1,2- polybutadiene to be obtained depends on, inter alia, the activity of the catalyst used and can easily be determined experimentally. The effect of for instance the 1,2- polybutadiene content on for instance the MFR will be apparent from the foregoing and from the examples. Those skilled in the art can determine this relationship from a few experiments under the reaction conditions chosen and subsequently determine the right amount and/or the right molecular weight for a copolymer having the desired properties.

The 1,2-polybutadiene appears to be incorporated in a highly efficient manner. This is surprising because the vinyl-unsaturated side groups may be regarded as γ- substituted olefins, which have an alkyl group substituent on the 3-site. Such substituents are known to strongly suppress the reactivity of an olefin in catalytic polyolefin processes due to what is known as steric hindering around the olefinic bond. Such a γ-substituted olefin is therefore not an obvious choice as comonomer for effecting a high degree of incorporation of the 1,2- polybutadiene with a high degree of conversion in a catalyzed polyolefin process.

Furthermore, incorporation of only a minor amount of 1,2- polybutadiene appears to have a significant effect on the MFR. Minor here means: in an amount at which the detectable number of unsaturations in the polyolefin of the invention is in the same range as the number of unsaturations of an otherwise similar thermoplastic polyolefin in which no 1,2-polybutadiene is copolymerized.

A further advantage of the process of the invention is the following. In WO-A-93/08221 the variation in the MFR is effected by suitably choosing the amount of catalyst. This entails that the reaction conditions must be adapted also. The process of the invention, in contrast, can be practised with a constant amount of catalyst and otherwise equal conditions because the amount of 1,2-polybutadiene in the reaction mixture is in principle the determining parameter.

For the definitions of the copolymers to which the process of the invention relates and for the requirements for the 1,2-polybutadiene to be copolymerized, see the foregoing.

The polymerization is effected by contacting ethylene and optionally one or more C₄-C₂₀ α-olefins with a cyclopentadienyl-containing transition metal complex as catalyst. This metal complex contains a transition metal preferably from group 3 or 4 of the Periodic System of Elements in the new IUPAC version as shown in the cover of Handbook of Chemistry and Physics, 70th Edition CRC Press, 1989-1990. If the valency of the metal is 3⁺, the complex can be represented as R¹MX¹X² or R^MX¹. If the valency of the metal is 4*^* the complex can be represented as R¹R²MX¹X² or R¹MX¹X²X³.

In the formulae given, R¹ is a substituted or unsubstituted cyclopentadienyl ligand, for instance indenyl, fluorenyl, methyl-cyclopentadienyl, pentamethylcyclopentadienyl or a heteroatom-containing derivative of the cyclopentadienyl ligand. In the group last mentioned, the heteroatom may be an element from group 15 or 16 of the Periodic System of Elements, for instance N, P, As, 0 or S. The heteroatom may form part of the cyclopentadienyl ring or may be located outside thereof. R² may be a Cp derivative as defined for R¹ but may also be a substituent containing an element from group 15 or 16 of the Periodic System, for instance N, P, As, 0 or S, which element is linked to the metal via a covalent or coordinate bond. R¹ and R² may be linked to one another by an -Si(R)₂ group, where R represents an aliphatic or aromatic group, by an aliphatic or aromatic group or by a group which contains an element from group 15 or 16 of the Periodic System. If R² is neutral and is not a cyclopentadiene- derived compound and, also, is linked to R¹ in one of the aforementioned manners, this combination of R¹ and R² is considered a heteroatom-containing derivative of the cyclopentadienyl ligand as meant in the description of R¹.

X¹, X² and X³ may or may not be the same and are chosen from the group of:

- halogens

- aliphatic or aromatic substituents - substituents which contain an element from group 15 or 16 of the Periodic System such as OR³, NR³ and the like, where R³ may be an aliphatic or aromatic substituent which may optionally contain silicon.

As catalyst a cyclopentadienyl-containing transition metal complex with the formula R⁴MX¹X² is preferably used, where M is a transition metal from Group 4 of the Periodic System, not having the highest valency, preferably Ti³⁺, where X¹ and X² have the same meaning as in the foregoing and where R⁴ is equal to

[Cp'-Y-Z(R⁵)_n]^'

where Cp'is a cyclopentadienyl derivative substituted with aliphatic or aromatic groups or with groups containing a heteroatom, Y is an aliphatic or aromatic group or a group containing silicon or a heteroatom, Z is an element from group 15 or 16 of the Periodic System, preferably N or P, R⁵ is an aliphatic, aromatic or silicon-containing group and n is equal to the valency of Z minus 1. As a rule, the metal complex is employed as catalyst in conjunction with an activator. As activator use may be made of substances known to be suitable for the purpose, for instance methylaluminoxane (MAO) or possibly (per)fluorinated boron compounds, for instance tris- penta luorophenylborane and tetrakispentafluoro- phenylborate compounds. In addition, organometal compounds with a metal from group 1, 2, 12 or 13, preferably aluminium-alkyl compounds or magnesium-alkyl compounds, may be applied in the catalyst system, for instance trimethylaluminium, triethylaluminium, triisobutylaluminium, trioctylaluminium, diethylaluminiumethoxide, dieth Imagnesium, dibutylmagnesium, ethyl-butylmagnesium and butyl- octylmagnesium. The activity of the catalyst system can be increased further by adding these main-group-metal/alkyl compounds.

The olefinic monomers are contacted with the catalyst under conditions at which the monomers polymerize in the presence of the catalyst. The polymerization of olefins with the aid of metal catalysts, for instance classical Ziegler-Natta or Phillips catalysts and the conditions to be chosen form a technique known per se which may also be employed for the polymerization of olefins with the aid of catalysts as prescribed for the process of the invention.

The polymerization reaction may for instance be effected in the gas phase, in suspension or in solution, either (semi)continuously or batchwise. Furthermore, it is also possible to apply a plurality of reactors arranged in parallel, in series or in a combination thereof.

Preferably, a solution process is used, since this process is eminently suitable for the production of very low- crystalline polymers which at least to some extent are soluble in hydrocarbons. As dispersant or solvent for the polymerization reaction any liquid that does not have an adverse effect on the activity of the catalyst system may be used. Saturated, linear or branched aliphatic hydrocarbons, for instance butanes, pentanes, heptanes, pentamethylheptane or petroleum fractions, for instance light or regular gasoline, naphtha, kerosene or gas oil or mixtures of the aforementioned substances may be used therefor. Aromatic hydrocarbons, for instance benzene and toluene, are suitable but are not preferred for reasons of cost and safety. For plant-scale polymerization, the aliphatic hydrocarbons or mixtures thereof supplied by the petrochemical industry are preferably used as dispersant or solvent after drying and purification.

The polymer obtained by the process of the invention can be worked up by methods known per se. In general, the catalyst is deactivated in a manner known per se at some point in this working-up phase of the polymer.

The polymerization may be effected at atmospheric pressure but also at increased pressure. If the polymerization is effected at increased pressure, the polymer yield per unit time can be increased still further. It is preferred for the polymerization to take place at pressures between 0.1 and 60 MPa, particularly between 1 and 30 MPa. Higher pressures of 100 MPa and above may be used when the polymerization takes place in so-called autoclaves.

The molecular weight of the polymer may be controlled in the usual manner, for instance by adding hydrogen or other chain terminating agents or by adjusting the polymerization conditions.

The 1,2-polybutadiene is preferably added as a solution in a suitable dispersant that has no adverse effect on the polymerization process. In a continuous process, the 1,2-polybutadiene is preferably added to the polymerization reactor on a continuous basis. In a system employing a plurality of reactors it also possible to add the 1,2-polybutadiene to only some of the reactors employed. In batch polymerization, the 1,2-polybutadiene may be added prior to or during polymerization. The amounts to be used and the molecular weights of the 1,2- polybutadiene are described in the foregoing.

The invention will be illustrated by the following examples without being limited thereto.

The density D₂₃ was determined in accordance with ASTM Standard D 792-66. The melt index MI was determined in accordance with ASTM Standard D1238 using a weight of 2.16 kg. The Melt Flow Ratio, MFR, was determined as the quotient of the melt indices determined to ASTM D1238 using weights of 21.6 and 2.16 kg, respectively. The Mw/Mn ratio was determined with the aid of a Waters M150C Gel Permeation Chromatograph with DRI- detector as Size Exclusion Chromatograph in combination with a Viscotek type 502 viscometer as viscosity detector and using polyethylene calibration samples as reference.

Examples I-IV

A number of polyolefins of the invention were produced as follows.

An autoclave with a capacity of 2 litres was filled with special boiling point spirit (boiling range from 65 to 70°C) and kept at a temperature of 160°C. A mixture of special boiling point spirit (5.5 kg/h) and ethylene (1.2 kg/h) was continuously added to the autoclave. Also, in some cases, hydrogen was added in order to obtain the desired molar mass. The supply to the autoclave was adjusted so that the autoclave remained completely filled with the reaction medium. A solution of a catalyst, a suspension of an activator and a solution of triethylaluminium was also continuously added to the autoclave. The ethylene conversion was controlled by means of the amounts of catalyst and activator and amounted to approximately 95% in each experiment.

Ethylene-dimethylamino-tetramethyl- cyclopentadienyl-titanium-dimethyl , Cp^*(CH₂CH₂)N(CH₃)₂Ti(CH₃)₂, was added as catalyst for the preparation of a copolymer of ethylene and 1,2- polybutadiene. A concentration of the catalyst of approx. 20 μmol per litre was needed for attaining 95% ethylene conversion. Dimethylaniliniumtetrakispentafluorophenylborate was applied as activator. A concentration of the activator of approx. 40 μmol per litre was needed for attaining 95% ethylene conversion. The concentration of triethylaluminium applied in the autoclave amounted to approx. 40 μmol per litre. In a seperate vessel a mixture of 1,2-polybutadiene (1,2-polybutadiene) with a Mn of approx. 3000 g/mol, containing approx. 50 vinyl groups per molecule chain (grade B-3000 from Messrs NISSOH IWAI) and special boiling point spirit was prepared. A certain amount of 1,2-polybutadiene solution was continuously pumped from this vessel to the autoclave. The desired rate at which 1,2-polybutadiene was added was adjusted by suitably choosing the concentration of 1,2-polybutadiene in the solution. Here, 1 g/h of 1,2-polybutadiene corresponded to 0.088 wt.% referred to converted ethylene. The properties of the copolymers obtained with various rates of addition of 1,2-polybutadiene are shown in Table 1.

TABLE 1

example addition of MI MFR Mw/Mn D23

1,2- polybutadiene g/h

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

These results indicate that the MFR can be controlled by means of the rate at which 1,2-polybutadiene is added.

Examples V-IX

Terpolymerizations of ethylene, 1-octene and 1,2- polybutadiene:

Analogously to Examples I-IV, polymerizations were carried out in which 0.2 kg/hour of octene-1 was added to the autoclave as extra monomer.

The properties of the copolymers obtained with various rates of addition of 1,2-polybutadiene are shown in Table

2. The polymers contained 15 wt.% octene.

TABLE 2

example addition of MI MFR Mw/Mn D23

1,2- polybutadiene g/h

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 0.6 56.7 5.0 916.2 8

IX 25 0.1 79.4 8.2 917.7 6

10.

The process of the invention proves suitable for producing terpolymers also. The MFR shows a distinct increase when as little as 1 g/h of 1,2-polybutadiene, corresponding to 0.088 wt.% referred to the converted ethylene, is added.

15

Examples X-XIII

Analogously to the previous examples, terpolymers of ethylene, 1-octene and 1,2-polybutadiene were produced using two different 1,2-polybutadiene grades

20 in order to study the effect of the molar mass of the 1,2- polybutadiene on the MFR in the terpolymerization of ethylene, octene and 1,2-polybutadiene. Use was made of grade B-3000 and grade B-2000 already mentioned in Examples V-IX, from the same supplier, with a Mn of 2000

25 g/mol, containing approx. 33 vinyl unsaturations per molecule chain. The results are shown in Table 3. TABLE 3

Examples XIV-XVIII Polymerizations were carried out analogously to

Examples I-IV except that diphenylmethylene-fluorenyl- cyclopentadienyl-hafnium-dimethyl {[ (C₆H₅)₂C]FluCpZrMe₂} was used as catalyst. As 1,2-polybutadiene a grade supplied by Messrs Aldrich with a Mn of approx. 1300 g/mol, approx. 99% unsaturated and with a vinyl:trans unsaturations of 40:30 (designated grade A) and a grade from the same company with a Mn of 1800 g/mol, approx. 60% unsaturations and with the distribution of the unsaturations among the various possibilities given by vinyl:trans:cis = 45:10:5

(designated grade B) were used. The results are shown in Table 4.

TABLE 4

n.d. : not determined

Claims

C L I M S

1. Thermoplastic polyolefin which is a copolymer of at least one olefinic monomer and from 0.005 to 10 wt.% 1,2-polybutadiene referred to the copolymer.

2. Thermoplastic polymer according to claim 1 in which the olefinic monomer is ethylene.

3. Thermoplastic polymer according to claim 1, in which the olefinic monomer contains ethylene and 0-50 wt% of a C₄-C₂₀ α-olefin with respect to the total amount of olefinic monomer present.

4. Thermoplastic polymer according to claim 3, in which the α-olefin is a C₄-C₁₀ α-olefin.

5. Polyolefin according to any one of claims 1 - 4, in which the chain length of the 1,2-polybutadiene, expressed as the number of polymerized butadiene units, is not more than 5000.

6. Polyolefin according to any one of claims 1-5, which contains from 0.01 to 5 wt.% polybutadiene.

7. Polyolefin according to any one of claims 1-6 in which the chain length of the 1,2-polybutadiene is at least 10.

8. Polyolefin according to any one of claims 1-7 in which the number of 1,2-vinyl unsaturations per polybutadiene chain is at least 3.

9. Process for producing a thermoplastic polyolefin of at least one olefinic monomer comprising contacting ethylene and optionally one or more C -C₂₀ α-olefins with a cyclopentadienyl-containing transition metal complex as catalyst under conditions at which the monomers polymerize in the presence of the catalyst, characterized in that polymerization takes place in the presence of 1,2-polybutadiene.

10. Process according to claim 9 in which a cyclopentadienyl-containing transition metal complex with the formula R⁴MX¹X² is used as catalyst, where M is a transition metal from Group 4 of the Periodic system, not of the highest valency, preferably Ti³⁺, where X¹ and X² may be the same or different and are chosen from the group of:

- halogens

- aliphatic or aromatic substituents

- substituents which contain an element from group 15 or 16 of the Periodic System such as OR³, NR³ and the like, where R³ may be an aliphatic or aromatic substituent which may optionally contain silicon, where R⁴ is equal to

[Cp'-Y-Z(R⁵)_n]^~

where Cp'is a cyclopentadienyl derivative substituted with an aliphatic or aromatic groups or groups containing a heteroatom, Y is an aliphatic or aromatic group or a silicon-containing or heteroatom-containing group, Z is an element from group 15 or 16 of the Periodic System, preferably N or P, R^s is an aliphatic, aromatic or silicon- containing group and n is equal to the valency of Z minus 1.

11. Process according to any one of claims 9-10 in which polymerization takes place in the presence of 0.01-10 wt.% 1,2-polybutadiene.

12. Process according to any one of claims 9-11 in which polymerization takes place in the presence of 0.01-5 wt.% 1,2-polybutadiene.