Polymerization of siloxanes

Details Polymerization of siloxanes Polymerization of siloxanes

1999-12-09

2001-09-04

Dawson, Robert (Department: 1712)

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

Synthetic resins

From silicon reactant having at least one...

C528S021000, C528S037000, C528S012000, C556S459000, C556S460000

Reexamination Certificate

active

06284859

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the condensation polymerization of siloxanes catalyzed by certain phosphazene bases.
BACKGROUND OF THE INVENTION
In EP0860461-A, there is described a process for the ring-opening polymerization of cyclosiloxanes, which comprises contacting a cyclosiloxane with 1 to 500 ppm of a phosphazene base, by weight of cyclosiloxane, in the presence of water. In GB 2311994, there is described a method of effecting polycondensing which comprises contacting at a temperature of from 0 to 200° C. and a pressure up to 350 torr, a silanol-containing organosiloxane with an amount of a peralkylated phosphazene base which is effective for polycondensation of said organosiloxane. The preferred peralkylated phosphazene base has the formula
wherein R# is a C
1-4
alkyl radical, R* is a C
1-10
alkyl radical and n is 2 or 3. EP0879838-A describes a process of preparing a polymer which comprises conducting ring-opening polymerization of a 4- to 10-membered cyclic monomer in the presence of a phosphazenium salt catalyst of an active hydrogen compound represented by the formula
SUMMARY OF THE INVENTION
This invention is a process for polymerizing a siloxane having silicon-bonded groups R′, wherein R′ denotes a hydroxyl group or a hydrocarbonoxy group having up to 8 carbon atoms, comprising mixing the siloxane with an ionic phosphazene base catalyst and allowing the siloxane to polymerize by condensation via reaction of Si—R′ groups with the formation of a Si—O—Si linkage. We have surprisingly found that ionic phosphazene base materials are at least as effective as catalysts for polymerization of siloxanes which comprises the condensation of silanol groups. We have furthermore found that these ionic phosphazene base materials are useful in the combined polymerization via condensation and polymerization by equilibration, when carried out simultaneously. This is unexpected as there is usually a substantial difference in catalytic rate between both reactions.
DETAILED DESCRIPTION OF THE INVENTION
A polymerization process according to the invention comprises mixing siloxanes having silicon-bonded groups R′ with ionic phosphazene base catalysts and allowing condensation via reaction of Si—R′ groups with the formation of a Si—O—Si linkage, R′ denoting a hydroxyl group or a hydrocarbonoxy group having up to 8 carbon atoms.
Numerous phosphazene bases, some ionic phosphazene bases and routes for their synthesis have been described by Schwesinger et al., Liebigs Ann. 1996, 1055-1081. Ionic phosphazene base materials have the formula A
+
B
−
, wherein A
+
is the phosphazene base cation, and B
−
is an anion, which is preferably a strong anion such as fluoride or hydroxide, which is active in initiating polymerization.
The ionic phosphazene base is found to be a very powerful catalyst for the polymerization, and can therefore be present in a relatively low proportion, for example from 2 to 1000 ppm by weight, preferably 10 to 500 ppm, based on the weight of siloxanes having Si—R′ groups. The proportion of catalyst actually used will be selected depending on the speed of polymerization that is sought.
A proportion of water may be present in the reaction. Where this is the case, it is preferably at least 0.5, more preferably from 0.5-10 mols per mol of the ionic phosphazene base, most preferably from 1 to 10 mols per mol of ionic phosphazene base. It is possible to allow higher proportions of water, and this can have the benefit of enabling greater control over the polymerization reaction, as described in more detail below.
In principle, any ionic phosphazene base is suitable for use in the present invention. Phosphazene bases generally include the following core structure P═N—P═N, in which free N valencies are linked to hydrogen, hydrocarbon, —P═N or ═P—N, and free P valencies are linked to —N or ═N. Some ionic phosphazene bases are commercially available e.g. from Fluka Chemie AG, Switzerland. The ionic phosphazene bases preferably have at least 3 P-atoms. Some preferred phosphazene bases are of the following general formulae:
{((R
1
2
N)
3
P═N—)
x
(R
1
2
N)
3−x
P—N(H)R
2
}
+
{A
−
} or
{((R
1
2
N)
3
P═N—)
y
(R
1
2
N)
4−y
P}
+
{A}
−
in which R
1
, which may be the same or different in each position, is hydrogen or an optionally substituted hydrocarbon group, preferably a C
1
-C
4
alkyl group, or in which two R
1
groups bonded to the same N atom may be linked to complete a heterocyclic ring, preferably a 5- or 6-membered ring; R
2
is hydrogen or an optionally substituted hydrocarbon group, preferably a C
1
-C
20
alkyl group, more preferably a C
1
-C
10
alkyl group; x is 1, 2 or 3, preferably 2 or 3; y is 1, 2, 3 or 4, preferably 2, 3 or 4; and A is an anion, preferably fluoride, hydroxide, silanolate, alkoxide, or bicarbonate.
Particularly suitable compounds are those where R
1
is methyl, R
2
is tertiary butyl or tertiary octyl, x is 3, y is 4 and A is fluoride or hydroxide.
Alternative phosphazene base catalysts have the general formula
wherein R denotes a hydrocarbon having 1 to 10 carbon atoms, e.g. methyl, or wherein the two R groups on one N atom form with the N atom a heterocyclic group, e.g. pyrrolidine, X denotes an anion and n is from 1 to 10. These can be prepared by reacting a linear phosphonitrile halide compound with a secondary amine or a salt of a secondary amine or a metal amide thereof to form an aminated phosphazene material, followed by an ion exchange reaction replacing the anion with a nucleophilic anion A—.
The polymerization can be carried out in bulk or in the presence of a solvent. Suitable solvents are liquid hydrocarbons or silicone fluids. The ionic phosphazene base catalyst can be diluted in a polar solvent, for example dichloromethane or an alcohol, or dispersed in a silicone fluid such as polydiorganosiloxanes. Where the ionic phosphazene base catalyst is initially in a solvent, the solvent can be removed by evaporation under vacuum, and the catalyst dispersed in a silicone fluid to give a stable clear solution. When this silicone dissolved catalyst is used for polymerization reactions, the catalyst disperses evenly and gives reproducible results. The catalyst can in some cases be dissolved in water, and this has the advantage of moderating and enabling greater control over the polymerization reaction, as described below.
The polymerization reaction can be carried out at ambient temperature or under heating at a temperature as high as 250° C. or 300° C. or even higher. Heating, for example to 100° C. or higher, is appropriate when the catalyst activity has been moderated as described below. The preferred temperature range may be from 50 to 170° C. The time taken for polymerization will depend on the activity of the catalyst in the chosen system, and on the desired polymer product. In the absence of moderation, the ionic phosphazene base catalysts are sufficiently active to convert siloxanes to high molecular weight polysiloxane gums within a short time frame.
Starting materials for the condensation reaction of silanol containing siloxanes are organosiloxanes having silicon-bonded hydroxyl groups or hydrolyzable groups such as alkoxy or aryloxy groups, which may form silanol groups in situ. These include, for example, organosiloxanes having the general formula (3):
In formula (3), R
3
is a hydrogen or an alkyl or aryl group having up to 8 carbon atoms, each R
4
is the same or different and denotes a monovalent hydrocarbon group preferably having 1 to 18 carbon atoms or halogenated hydrocarbon group preferably having 1 to 18 carbon atoms and t is an integer having a value of from at least 2. Preferably R
4
denotes an alkyl group having from 1 to 6 carbon atoms and more preferably a methyl group. The value of t is preferably such that the average viscosity of the polyorganosiloxanes does not exceed 200 mm
2
/s at 25° C.
Suitable organosiloxanes may have

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