Process for preparing monodisperse crosslinked bead polymers

Details Process for preparing monodisperse crosslinked bead polymers Process for preparing monodisperse crosslinked bead polymers

2000-12-14

2002-04-02

Zitomer, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S243000, C526S078000, C526S218100, C526S227000, C526S336000, C526S342000, C526S347000, C521S025000

Reexamination Certificate

active

06365683

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for preparing substantially monodisperse crosslinked bead polymers useful as precursors for ion exchangers.
In recent times increasing importance has been placed on ion exchangers with very uniform particle size (hereinafter termed “mono-disperse”), since the more advantageous hydrodynamic properties of an exchanger bed made of monodisperse ion exchangers can provide cost advantages in many applications. Monodisperse ion exchangers can be obtained by functionalizing monodisperse crosslinked bead polymers.
One way of preparing monodisperse crosslinked bead polymers is known as the seed/feed process. In this process, monodisperse polymer particles (“seed”) are swollen in the monomer, which is then polymerized. These seed/feed processes are described in EP 98,130 B1 and EP 101,943 B1, for example. EP-A 826,704 and DE-A 19,852,667 disclose seed/feed processes using microencapsulated polymer particles as seed. Compared with conventional, directly synthesized bead polymers, the bead polymers obtained by the processes described above have an increased content of uncrosslinked soluble polymer. This content of uncrosslinked soluble polymer is undesirable during the conversion to ion exchangers, since the polymer fractions dissolved out can become concentrated in the reaction solutions used for the functionalization. In addition, the relatively large amounts of soluble polymer can cause undesirable leaching of the ion exchangers.
U.S. Pat. No. 5,068,255 describes a seed/feed process in which a first monomer mixture is polymerized to a conversion of from 10 to 80% and is then mixed with a second monomer mixture essentially free from free-radical initiator as feed under polymerizing conditions. However, this process cannot prepare monodisperse particles.
The object of the present invention is to provide monodisperse crosslinked bead polymers with a low content of soluble polymer. It has now been found that monodisperse crosslinked bead polymers with a low content of soluble polymer can be obtained by a seed-feed process in which the seed used comprises incompletely polymerized, monodisperse microencapsulated monomer droplets.
SUMMARY OF THE INVENTION
The present invention relates to a process for preparing mono-disperse crosslinked bead polymers as precursors for ion exchangers comprising
(a) preparing monodisperse monomer droplets in aqueous suspension from a monomer mixture
1
comprising styrene, divinylbenzene, and a free-radical generator,
(b) microencapsulating the resultant monomer droplets,
(c) polymerizing the microencapsulated monomer droplets to a conversion of from 10 to 75%,
(d) adding a monomer mixture
2
comprising styrene and divinyl-benzene at a temperature at which the free-radical generator from monomer mixture
1
is active, whereupon the monomer mixture penetrates into the microencapsulated monomer droplets that have begun to polymerize, and
(e) completing the polymerization of the monomer mixtures.
One preferred embodiment of the present invention relates to a process in which monomer mixture
2
also comprises acrylonitrile and/or a free-radical generator and in which at least one of the free-radical generators from monomer mixture
1
or
2
is active in step (d).
One particular embodiment of the present invention relates to a process for preparing monodisperse crosslinked bead polymers as precursors for ion exchangers comprising
(a) producing monodisperse monomer droplets in aqueous suspension from a monomer mixture
1
comprising from 87.5 to 99.7% by weight of styrene, from 0.2 to 10% by weight of divinylbenzene, and from 0.1 to 2.5% by weight of a free-radical generator,
(b) microencapsulating the resultant monomer droplets,
(c) polymerizing the microencapsulated monomer droplets to a conversion of from 10 to 75%,
(d) adding a monomer mixture
2
comprising from 80 to 99% by weight of styrene, from 1 to 12% by weight of divinylbenzene, from 0 to 8% by weight of acrylonitrile, and, optionally, a free-radical generator at a temperature at which at least one of the free-radical generators from monomer mixture
1
or monomer mixture
2
is active, whereupon the monomer mixture penetrates into the microencapsulated monomer droplets that have begun to polymerize, and
(e) completing the polymerization of the monomer mixtures.
DETAILED DESCRIPTION OF THE INVENTION
The monomer mixture
1
preferably comprises from 89.5 to 99.4% by weight of styrene, from 0.5 to 8% by weight of divinylbenzene, and from 0.1 to 2.5% by weight of free-radical generator, particularly preferably from 92.5 to 98.7% by weight of styrene, from 1 to 6% by weight of divinylbenzene, and from 0.3 to 1.5% by weight of free-radical generator. The percentages given for divinylbenzene are based on pure divinylbenzene. It is, of course, also possible to use commercial qualities of divinylbenzene which contain ethylvinylbenzene in addition to isomers of divinylbenzene.
Free-radical generators that may be used are conventional initiators such as azo compounds and/or peroxo compounds, for example:
dibenzoyl peroxide
dilauroyl peroxide
bis(p-chlorobenzoyl) peroxide
dicyclohexyl percarbonate
2,2′-azobisisobutyronitrile
2,2′-azobis(2-methylbutyronitrile)
Preferred free-radical generators are aliphatic peroxy esters corresponding to the formulas (I), (II), or (III):
wherein
R
1
represents an alkyl radical having from 2 to 20 carbon atoms or a cycloalkyl radical having up to 20 carbon atoms,
R
2
represents a branched alkyl radical having from 4 to 12 carbon atoms, and
L represents an alkylene radical having from 2 to 20 carbon atoms or a cycloalkylene radical having up to 20 carbon atoms.
Examples of aliphatic peroxy esters according to formula (I) are tert-butyl peroxyacetate, tert-butyl peroxyisobutyrate, tert-butyl peroxypivalate, tert-butyl peroxyoctoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyneodecanoate, tert-amyl peroxyneodecanoate, tert-amyl peroxypivalate, tert-amyl peroxyoctoate, and tert-amyl peroxy-2-ethylhexanoate.
Examples of aliphatic peroxy esters according to formula (II) are 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane, 2,5-dipivaloyl-2,5-dimethylhexane, and 2,5-bis(2-neodecanoylperoxy)-2,5-dimethylhexane.
Examples of aliphatic peroxyesters according to formula (III) are di-tert-butyl peroxyazelate and di-tert-amyl peroxyazelate.
It can be advantageous to use mixtures of different initiators, in particular mixtures of initiators with different half-lives.
The conversion of the monomer mixture
1
into monodisperse monomer droplets in step (a) takes place by way of known spraying techniques, by which means the monomer mixture is dispersed in water. Particularly suitable spraying techniques are those that are combined with vibrational excitation. A process of this type is described in detail in EP-A 173,518 and U.S. Pat. No. 3,922,255, for example. The ratio of monomer mixture to water is generally from 1:1 to 1:10, preferably from 1:1.5 to 1:5.
The particle sizes for the monomer droplets are from 10 to 500 &mgr;m, preferably from 20 to 400 &mgr;m, particularly preferably from 100 to 300 &mgr;m. Conventional methods, such as image analysis, are suitable for determining the average particle size and the particle size distribution. The ratio between the 90% value (Ø(90)) and the 10% value (Ø(10)) for the, volume distribution gives a measure of the breadth of the particle size distribution of the novel bead polymers. The 90% value (Ø(90)) is the diameter that exceeds that of 90% of the particles. Correspondingly, the 10%(Ø(10)) diameter value exceeds that of 10% of the particles. For the purposes of the present invention, monodisperse particle size distributions have Ø(90)/Ø(10)≦1.5, preferably Ø(90)/Ø(10)≦1.25.
Possible materials for the microencapsulation in step (b) are those known for this purpose, particularly polyesters, naturally occurring or synthetic polyamides, polyurethanes, or polyureas. A particularly suitable naturally occurring polyamide is gelatin, used in particular as coacervat

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