Process for the preparation of crosslinked spherical polymers

Details Process for the preparation of crosslinked spherical polymers Process for the preparation of crosslinked spherical polymers

1999-06-04

2001-04-24

Nutter, Nathan M. (Department: 1711)

Plastic and nonmetallic article shaping or treating: processes

Encapsulating normally liquid material

Liquid encapsulation utilizing an emulsion or dispersion to...

C427S213340, C428S402210, C428S402220, C428S402240

Reexamination Certificate

active

06221287

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for the preparation of crosslinked spherical polymers having high swellability and low contents of soluble fractions using peroxyesters as polymerization initiators.
Ion exchangers having as uniform a particle size as possible (referred to below as “monodisperse”) have very recently become increasingly important because economic advantages can be obtained in many applications due to the more advantageous hydrodynamic properties of an exchanger bed of monodisperse ion exchangers. Monodisperse ion exchangers can be obtained by functionalization of monodisperse bead polymers.
One possible method for preparing monodisperse bead polymers is the so-called seed/feed process, in which a monodisperse polymer (“seed”) is swollen in the monomer and then polymerized. Seed/feed processes are described, for example, in European Patent Applications 98,130 and 101,943
Seed polymers should have a high swelling index so that they are capable of absorbing a large amount of the added monomer in the seed/feed process. The swelling index (“SI”) is defined as the quotient of the volume of the swollen polymer and the volume of the polymer that is not swollen. The swelling index can be controlled in a known manner by the content of crosslinking agent: low contents of crosslinking agent lead to high swelling indices and vice versa. Thus, for example, styrene polymers that are crosslinked with 0.8 to 2.0% by weight of divinylbenzene have swelling indices of 2.5 to 8 in toluene. However, seed polymers having a low degree of crosslinking have a very high fraction of uncrosslinked, soluble polymers. This fraction of uncrosslinked soluble polymers in the seed polymer is undesired in many respects:
1. The polymerization of the swollen seed can be impaired because the polymer fractions dissolved from the seed by the added monomer cause the particles to adhere to one another.
2. The functionalization for the preparation of the ion exchangers can be complicated because the dissolved-out polymer fractions accumulate in the reaction solutions used for the functionalization.
3. The end products (i.e., ion exchangers) may contain large amounts of soluble polymers, which may lead to undesired bleaching of the ion exchangers.
The object of the present invention is to provide crosslinked spherical polymers having high swellability and low contents of soluble fractions.
It has now been found that polymers having low contents of crosslinking agent and correspondingly high swellability have particularly low contents of soluble polymer fractions when peroxyesters are used as initiators in their preparation.
SUMMARY OF THE INVENTION
The present invention relates to a process for the preparation of crosslinked spherical polymers comprising suspension polymerizing a mixture of
(a) 96.5 to 99.0% by weight (preferably 97.3 to 99.0% by weight) of a monomer having one C═C double bond capable of free radical polymerization per molecule,
(b) more than 0.8% by weight to not more than 2.5% by weight (preferably more than 0.8% by weight to 1.5% by weight, particularly preferably 0.801% by weight to 1.5% by weight, very particularly preferably 0.801% by weight to 1.0% by weight, especially preferably 1.0% by weight) of a crosslinking agent containing two or more (preferably two to four) double bonds capable of free radical polymerization per molecule, and
(c) 0.2 to 1.0% by weight of at least one aliphatic peroxyester as polymerization initiator,
the percentages being based on the sum of the components (a) to (c).
DETAILED DESCRIPTION OF THE INVENTION
For the purposes of the invention, monomers (a) are compounds having one C═C double bond capable of free radical polymerization per molecule. Preferred compounds of this type include aromatic monomers, such as vinyl and vinylidene derivatives of benzene and of naphthalene (for example, vinylnaphthalene, vinyltoluene, ethylstyrene, &agr;-methyl-styrene, chlorostyrenes, and, preferably, styrene) and nonaromatic vinyl and vinylidene compounds, such as acrylic acid, methacrylic acid, C
1
-C
8
alkyl acrylates, C
1
-C
8
alkyl methacrylates, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl chloride, vinylidene chloride, and vinyl acetate. The nonaromatic monomers are preferably used in amounts of 0.1 to 50% by weight (particularly 0.5 to 20% by weight) based on aromatic monomers. In most cases, however, exclusively aromatic monomers are used.
Suitable crosslinking agents (b) are compounds that contain two or more (preferably two to four) double bonds capable of free radical polymerization per molecule. Examples of suitable crosslinking agents (b) include, for example, divinylbenzene, divinyltoluene, trivinylbenzene, divinylnaphthalene, trivinylnaphthalene, diethylene glycol divinyl ether, 1,7-octadiene, 1,5-hexadiene, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, allyl methacrylate, and methylene-N,N′-bisacrylamide. Divinylbenzene is preferably used as the crosslinking agent. For most applications, commercial divinylbenzene qualities which also contain ethylvinylbenzene in addition to the isomers of divinylbenzene, are sufficient.
Aliphatic peroxyesters (c) correspond to the formulas I, II, or III
in which
R
1
denotes an alkyl radical having 2 to 20 carbon atoms or a cycloalkyl radical having up to 20 carbon atoms,
R
2
denotes a branched alkyl radical having 4 to 12 carbon atoms, and
L denotes an alkylene radical having 2 to 20 carbon atoms or a cycloalkylene radical having up to 20 carbon atoms.
Preferred aliphatic peroxyesters according to the formula I include, for example, tert-butyl peroxyacetate, tert-butyl peroxyisobutyrate, tert-butyl peroxypivalate, tert-butyl peroxyoctanoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyneodecanoate, tert-amyl peroxypivalate, tert-amyl peroxyoctanoate, tert-amyl peroxy-2-ethylhexanoate, and tert-amyl peroxyneodecanoate.
Preferred aliphatic peroxyesters according to the formula II include, for example, 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane, 2,5-dipivaloyl-2,5-dimethylhexane, and 2,5-bis(2-neodecanoylperoxy)-2,5-dimethylhexane.
Preferred aliphatic peroxyesters according to the formula III include, for example, di-tert-butyl peroxyazelate and di-tert-amyl peroxyazelate.
The term suspension polymerization is understood to mean a process in which a monomer phase, which contains an initiator that is soluble in the monomer, is dispersed in the form of droplets in a phase that is essentially immiscible with the monomer, and is cured by increasing the temperature while stirring. Further details of the suspension polymerization are described, for example, in the publication
Polymer Processes,
edited by C. E. Schildknecht, published in 1956 by Interscience Publishers, Inc. New York, in the chapter “Polymerization in Suspension” at pages 69 to 109. In the present invention, the phase that is essentially immiscible with the monomer is preferably an aqueous phase.
In a preferred embodiment of the present invention, the mixture of vinylaromatic monomer (a), crosslinking agent (b), and aliphatic peroxyester (c) is microencapsulated.
The materials known for this intended use, particularly polyesters, natural and synthetic polyamides, polyurethanes, and polyureas, are suitable for the microencapsulation. Gelatin is particularly suitable as a natural polyamide and is used in particular as a coacervate and complex coacervate. For the purposes of the invention, gelatin-containing complex coacervates are understood to mean in particular combinations of gelatin and synthetic polyelectrolytes. Suitable synthetic polyelectrolytes are copolymers having incorporated units of, for example, maleic acid, acrylic acid, methacrylic acid, acrylamide, and methacrylamide. Gelatin-containing capsules can be hardened using conventional curing agents, such as, for example, formaldehyde or glutardialdehyde. The encapsulation of monomer droplets, for example, with gelatin, gelatin-containing coacervates, and ge

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