Process for the preparation of vinylaromatic polymers with a...

Details Process for the preparation of vinylaromatic polymers with a... Process for the preparation of vinylaromatic polymers with a...

1999-10-12

2001-01-16

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S153000, C526S157000, C526S160000, C526S346000, C526S347200, C502S132000, C502S153000

Reexamination Certificate

active

06174973

ABSTRACT:

The present invention relates to a process for the preparation of vinylaromatic polymers with a high degree of syndiotaxy.
More specifically, the present invention relates to a process for the preparation of crystalline poly-styrene in which the polymeric chains have an essentially syndiotactic configuration and the catalyst suitable for the purpose.
Polystyrene is a thermoplastic polymer obtained by the radicalic polymerization of styrene having an atactic stereo structure. It is an amorphous polymer, with excellent insulating properties and reasonable thermal resistance and is used in the production of moulded articles, films, materials for household appliances, packaging, etc. For numerous applications however, it is preferable to use crystalline materials with a high thermal resistance and resistance to solvents, characteristics which atactic polystyrene does not have.
European patent 210.615 describes a polystyrene having a structure characterized by a high degree of stereoregularity, in which the phenyl substituents are arranged, in the polymeric chain, in such a way as to give a syndiotactic polymer. This material does not have the above disadvantages of atactic polystyrene as it is crystalline and consequently, once transformed, can be subjected to orientation processes; it is insoluble in almost all organic solvents and has a melting point within the range of 260-280° C., giving it a high thermal resistance, comparable to or higher than that of condensation thermoplastic polymers (polyesters, polyamides, polyimides, etc.).
Syndiotactic polystyrene can be prepared according to what is described in literature, for example according to what is disclosed in European patent EP 272.584 or in U.S. Pat. No. 4,978,730, by polymerization catalyzed by compounds of Ti, Zr, in the presence of a cocatalyst represented by MAO, methylaluminoxane (a mixture of cyclic and linear oligomers containing the repetitive unit —AlCH
3
O—), or, as described in published European patent application 421.659, by derivatives of boron containing fluorinated groups.
Both the cocatalyst MAO and fluorinated boranes are costly high-quality products and it would therefore be preferable to reduce the amount in the catalytic formulation.
Scientific literature cites studies for finding alternatives to MAO or fluorinated boranes. For example, it is known that the addition of certain aluminum alkyls, such as aluminum triisobutyl, to titanium/MAO systems can increase, under particular experimental conditions, the catalytic activity even if this result is not verified under all conditions such as, for example, with low molar ratios aluminum/titanium. Apart from this, a non-correct concentration of aluminum alkyl may cause a considerable reduction in the molecular weight of the polymer or, in the case of aluminum trimethyl, act in the completely opposite way causing a lowering in the polymerization yields (“Polymer”, 39, 959, 1998).
It has also been observed that zinc diphenyl increases the catalytic activity of titanium metallocenes activated with MAO (“Polymer Bulletin”, Berlin, 39, 693, 1997). However, zinc diphenyl is also an expensive high-quality product and also causes the formation of fractions of atactic polymer to a greater degree than MAO.
By treating hydrated salts such as Ti(SO
4
)
2
.4H
2
O or K
2
TiO(C
2
O
4
)
2
.2H
2
O with aluminum trimethyl, catalysts are obtained with a reasonable activity (“Makromoleculare Chemie, Rapid Communications”, 9, 351, 1988). These however are systems based on MAO which is formed in situ by the reaction of aluminum alkyl with the hydration water of the salt.
Catalytic systems containing hemimetallocene derivatives of titanium, activated with aluminum trimethyl and hexaalkyldistannoxanes or dialkyltin oxides, are described in U.S. Pat. No. 5,326,837. Also in this case however the activities are lower than those of the systems based on MAO and additionally contain toxic derivatives of tin.
The Applicant has now found that it is possible to obtain crystalline vinylaromatic polymers, in particular crystalline polystyrene, having a configuration with a high degree of syndiotaxy and with high yields, by adding to the traditional catalytic systems based on titanium and MAO, aluminum trifluoride also in partial substitution of the MAO itself.
The present invention therefore relates to a process for the preparation of crystalline vinylaromatic polymers with a high degree of syndiotaxy which comprises polymerizing vinylaromatic monomers, alone or mixed with at least another copolymerizable ethylenically unsaturated monomer, in the presence of a catalytic system essentially consisting of:
a) a complex of titanium having the general formula:
CpTiX
1
X
2
X
3
  (I)
wherein Cp represents a cyclopentadienyl ligand optionally substituted with C
1
-C
10
alkyl radicals, whereas X
1
, X
2
, X
3
, the same or different, are selected from a halogen such as chlorine, or from alkyl groups, alkoxides, carboxylates, di(alkyl)amides having from 1 to 10 carbon atoms;
b) a polyalkylaluminoxane in which the alkyl group contains from 1 to 8 carbon atoms, preferably MAO;
c) aluminum trifluoride.
The use of component (c) in the catalytic mixture of the present invention allows, as shown hereafter, higher conversions to be obtained (even higher than 100%) under the same reactions conditions, with respect to the mixture without this component, or a reduction in the absolute quantity of component (b) to reach a specific conversion.
The compounds having general formula (I) are products known in literature and described in “Progress in Polymer Science” vol. 21, page 47, 1996.
Typical examples of titanium complexes having formula (I), particularly suitable for the present invention are: CpTiCl
3
; CpTi(OCH
3
)
3
; CpTi(OC
2
H
5
)
3
; CpTi(OC
3
H
7
)
3
; CpTi(Oi—C
3
H
7
)
3
; CpTi(OC
4
H
9
)
3
; [Cp(CH
3
)
5
]TiCl
3
; [Cp(CH
3
)
5
]TiBr
3
; [Cp(CH
3
)
5
]TiF
3
; CpTi(OC
5
H
6
)
3
; [Cp(CH
3
)
5
]Ti(OCH
3
)
3
; [Cp(CH
3
)
5
]Ti(OC
2
H
5
)
3
; [Cp(CH
3
)
5
]Ti(OC
3
H
7
)
3
; [Cp(CH
3
)
5
]Ti(OC
4
H
9
)
3
; [Cp(CH
3
)
5
]Ti(OC
5
H
6
)
3
; CpTiH
3
; CpTi(CH
3
)
3
; CpTi(C
2
H
5
); CpTi(CH
2
C
5
H
6
)
3
; [Cp(CH
3
)
5
]TiH
3
; [Cp(CH
3
)
5
]Ti(CH
3
)
3
; [Cp(CH
3
)
5
]Ti(C
2
H
5
)
3
; [Cp(CH
3
)
5
]Ti(CH
2
C
5
H
6
)
3
; CpTi(OPh)
3
; [Cp(CH
3
)
5
]Ti(OPh)
3
. Particularly preferred are the complexes in which the cyclopentadienyl group is pentamethyl substituted.
The compounds of titanium having general formula (I) are added to the polymerization mixture in such a quantity that the molar ratio vinylaromatic monomer/Ti is between 10,000 and 1,000,000, preferably between 100,000 and 500,000.
The polyalkylaluminoxane (cocatalyst) essentially consists of mixtures of products having a linear, cyclic or caged structure. In the first case the structure is represented by general formula (II):
whereas in the second case by general formula (III):
wherein m represents an integer between 1 and 40 and R′ is a C
1
-C
8
alkyl radical, for example it is preferably a methyl. The caged structure is described in Molecular Symposium, Vol 97, 1995.
Polyalkylaluminoxanes are known in literature and described, for example, in published European patent applications 272.584 and 421.659 or in U.S. Pat. No. 4,978,730.
The cocatalyst can be treated at a temperature ranging from 20 to 80° C. under vacuum for at least an hour, and can then be added to the polymerization mixture as such or in the form of a solution in a suitable solvent, for example toluene.
The cocatalyst is generally used in such quantities that the molar ratio aluminum/Ti is between 50 and 5,000.
According to the process of the present invention, the catalytic system described above may optionally also comprise an aluminum alkyl in which the alkyl group contains from 1 to 6 carbon atoms, for example aluminum trimethyl, aluminum triethyl, aluminum triisobutyl, etc. so as to substitute a part of cocatalyst corresponding to a p

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