Process for preparing polymers from vinylaromatic compounds...

Details Process for preparing polymers from vinylaromatic compounds... Process for preparing polymers from vinylaromatic compounds...

1999-07-08

2001-05-22

Boykin, Terressa M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

Reexamination Certificate

active

06235838

ABSTRACT:

The invention relates to a process for preparing polymers of vinylaromatic compounds in dispersion in the presence of metallocene catalyst systems.
The polymers thereby obtainable can be used to produce fibers, films and moldings.
Polymerizing styrene in the presence of metallocene catalyst systems leads to polymers of high stereoregularity and is described at length, for example, in EP-A 0 210 615. Because of its high crystallinity, syndiotactic polystyrene has a very high melting point of about 270° C., high rigidity and tensile strength, dimensional stability, a low dielectric constant and high chemical stability. The profile of mechanical properties is retained even above the glass transition temperature.
In the metallocene-catalyzed polymerization of styrene, there is frequently crystallization of the resulting syndiotactic polystyrene starting at a level of only about 10% conversion. This leads firstly to the formation of deposits on the walls and secondly to an extreme viscosity rise during the polymerization, which makes handling and dissipation of the heat of reaction more difficult, especially on the industrial scale.
To solve this problem a variety of techniques using special reactors or extruders have been tried out. EP-A-0 535 582 describes a process for preparing syndiotactic polystyrene in a stirred bed of solids, which is able to reduce the wall deposits but not prevent them. The reactor has to be equipped with a special stirrer in order to produce a homogeneous fluidized bed. Temperature control is by way of partial evaporation of styrene by reduced pressure, using a complex vacuum control system.
EP-A 0 584 646 and EP-A 0 389 939 describe the preparation of syndiotactic polystyrene in self-cleaning twin-screw extruders or compounders with no dead spaces. In both processes, owing to the sudden rise in frictional forces at higher levels of conversion, and to the motor output required for continued operation, polymerization is carried out not to complete conversion but only to a level where the polymer powder, soaked with monomers, no longer agglomerates in the course of subsequent processing steps.
In the case of anionic initiation, the technique of dispersion polymerization is known. It is employed specifically to prepare small polystyrene particles, as described for example in Journal of Polymer Science, Part A, Polymer Chemistry, Vol. 34 (1996), pages 2633-2649. Of critical importance is the selection of the dispersing auxiliary for stabilizing the dispersion.
DE-A 43 30 969 describes a process for preparing polystyrene mixtures by polymerizing styrene in an organic liquid in the presence of a styrene-butadiene block copolymer and of a metallocene catalyst system. For the preferred embodiment, however, pressures of from 5 to 20 bar are required; otherwise the resulting polymers have a very low molecular weight of around 30,000 g/mol.
It is an object of the present invention to provide a process for preparing syndiotactic vinylaromatic polymers using metallocene catalysts, which does not have the above disadvantages and can be carried out in customary stirred reactors at low viscosities.
We have found that this object is achieved by conducting the metallocene-catalyzed polymerization of vinylaromatic monomers in dispersion using styrene/diphenylethylene-diene block copolymers as dispersing auxiliaries.
Particularly suitable vinylaromatic compounds are those of the formula I
where
R
1
is hydrogen or C
1
-C
4
-alkyl,
R
2
to R
6
independently are hydrogen, C
1
-C
12
-alkyl, C
6
-Cl
8
-aryl or halogen, or two adjacent radicals together are cyclic groups having 4 to 15 carbons, for example C
4
-C
8
-cyclo-alkyl, or fused ring systems.
It is preferred to employ vinylaromatic compounds of the formula I in which
R
1
is hydrogen.
Particularly suitable substituents R
2
to R
6
are hydrogen, C
1
-C
4
-alkyl, chlorine or phenyl, biphenyl, naphthalene or anthracene. Two adjacent radicals may also together be cyclic groups having 4 to 12 carbons, so that compounds of the formula I may also, for example, be naphthalene derivatives or anthracene derivatives.
Examples of such preferred compounds are:
styrene, p-methylstyrene, p-chlorostyrene, 2,4-dimethylstyrene, 4-vinylbiphenyl, 2-vinylnaphthalene or 9-vinylanthracene.
It is also possible to employ mixtures of different vinylaromatic compounds, in which case one component may also carry further hydrocarbon radicals, such as vinyl, allyl, methallyl, butenyl or pentenyl groups, preferably vinyl groups, on the phenyl ring. It is preferred, however, to use only one vinylaromatic compound.
Particularly preferred vinylaromatic compounds are styrene and p-methylstyrene.
The preparation of vinylaromatic compounds of the formula I is known per se and is described, for example, in Beilstein 5, 367, 474, 485.
Suitable dispersion auxiliaries are block copolymers having at least one diene block B and at least one block S comprising a copolymer of a vinylaromatic monomer of the formula (I) and 1,1-diphenylethylene or its aromatic ring-substituted derivatives, including those substituted with alkyl of up to 22 carbons, as are described, for example, in DE-A 44 20 917.
Suitable examples are block copolymers with blocks S and B, of the general structures (S—B)
n
, S—B—S, B—S—B, X[(S—B)
n
]
m
, X[(BS)
n
]
m
, X(S—B—S)
m
and X(B—S—B)
m
, where X is the radical of an m-functional coupling agent or of an m-functional initiator, n is an integer from 1 to 5 and m is an integer from 2 to 20.
All dienes are suitable in principle as the diene component for the block B, although preference is given to those having conjugated double bonds, such as butadiene, isoprene, dimethylbutadiene and phenylbutadiene. The diene block may be partially or completely hydrogenated or unhydrogenated. The molecular weights Mw of the blocks B are generally from 10,000 to 500,000, preferably from 50,000 to 350,000 and, with particular preference, from 70,000 to 250,000, g/mol.
The blocks S consist of a copolymer of a vinylaromatic monomer of the formula (I) and 1,1-diphenylethylene or its ring-substituted derivatives, including those substituted with alkyl of up to 22 carbons, preferably of 1 to 4 carbons, such as methyl, ethyl, isopropyl, n-propyl and n-, iso- or tert-butyl. Particular preference, however, is given to the use of unsubstituted 1,1-diphenylethylene itself. The proportion of diphenylethylene in the block S is from 15 to 65% by weight, preferably from 25 to 60% by weight. The molar ratio of the units derived from the vinylaromatic monomer to units derived from 1,1-diphenylethylene is generally in the range from 1:1 to 1:25, preferably from 1:1.05 to 1:15 and, with particular preference, in the range from 1:1.1 to 1:10.
The copolymer block S is preferably random in composition and has a molecular weight Mw of in general from 20,000 to 500,000, preferably from 50,000 to 300,000. Particular preference is given to a copolymer block S of styrene and 1,1-diphenylethylene.
The block ratio S to B is generally in the range from 90:10 to 20:80, particularly preferably from 90:15 to 65:35. The block transitions can be either clean-cut or tapered. A tapered transition is one where the adjacent blocks B and S may, in the transition region, also contain monomers of the other block.
The block copolymers can be prepared by customary methods of anionic polymerization, as described for-example in M. Morton, Anionic Polymerisation, Principles and Practice, Academic Press, New York 1983. The anionic polymerization is initiated by means of organometallic compounds. Preference is given to compounds of the alkali metals, especially of lithium. Examples of initiators are lithium alkyls such as methyllithium, ethyllithium, isopropyllithium, n-, sec- or tert-butyllithium. It is particularly preferred to employ n- or s-butyllithium. Suitable solvents are those which are inert toward the organometallic initiator. Aliphatic or aromatic hydrocarbons are judiciously used. Examples of suitable solvents are cyclohexane, methylcyclohexane, benzene, toluene, ethylb

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