Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Reexamination Certificate
2000-12-14
2004-02-24
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Ion-exchange polymer or process of preparing
C521S031000
Reexamination Certificate
active
06696503
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for preparing crosslinked ion exchangers with a homogeneous network structure based on unsaturated aliphatic nitrites in the presence of film-forming protective colloids, where the process carried out is not a seed/feed process.
Copolymerization of two or more monomers generally gives rise to polymers whose composition changes to some extent as conversion increases. Depending on the type of polymerization, two types of heterogeneity can be distinguished.
If the lifetime of the active group on the polymer chain is the same as the duration of the polymerization, as in the case of anionic polymerization, the composition changes along the chain. At any given juncture during the polymerization all of the polymer chains have the same overall composition.
If the lifetime of the active group on the polymer chain is significantly shorter than the overall reaction time, the composition of the polymer chains changes as the conversion proceeds in the polymerization. This second type of heterogeneity is typical of free-radical polymerization reactions in which the lifetime of the growing polymer radicals is in the region of seconds and the polymerization time in the region of hours.
This heterogeneity is associated with disadvantages for many applications. In the case of adsorber resins and ion exchangers, which are crosslinked bead polymers, non-crosslinked or very weakly crosslinked fractions are highly undesirable. These always arise if there is more than proportional incorporation of crosslinking agent, the concentration of which decreases markedly as the polymerization proceeds. A typical example of this is the acrylonitrile/divinylbenzene combination. Bead polymers and/or weakly acidic ion exchangers prepared from this combination contain considerable amounts of non-crosslinked polymers, which can exude and give products which are mechanically and osmotically unstable. Attempts are made to compensate for the drop in concentration of the crosslinking agent during the polymerization by either using a second crosslinking agent which is incorporated only slowly or feeding further amounts of the crosslinking agent whose concentration drops rapidly (in this case divinylbenzene). The second crosslinking agents used in industry are di- or triallyl compounds, such as 1,7-octadiene or trivinylcyclohexane. These substances react only incompletely and have to be carefully removed so that the resin does not cause difficulties in downstream applications. Feeding further amounts of the crosslinking agent is difficult and requires a sophisticated feed strategy, and there is also a limitation on the selection of the suspension stabilizers. Protective colloids frequently used in bead polymerization, such as gelatins, polyvinyl alcohol, or cellulose derivatives, are film-forming substances which are unsuitable since they inhibit the diffusion of the crosslinking agent into the polymerizing beads (EP-A 98,130).
There is therefore great interest in any process which does not have the disadvantages described above. Surprisingly, it has been found that unsaturated aliphatic nitrites, such as acrylonitrile, in combination with di- and polyvinyl ethers of alkanediols or of glycols build up a very homogeneous network and give products which do not have the disadvantages described above. It is possible to dispense with feeding of further amounts of the crosslinking agent and with the use of a second crosslinking agent.
The use of di- or polyvinyl ethers for crosslinking polymers is prior art.
EP-A 10,265 describes the preparation of synthetic resins based on crosslinked copolymers of mono- and polyvinyl compounds. A substantive feature of the invention is the joint use of two crosslinking agents, as stated in the Abstract: methacrylates of polyhydric alcohols and/or aromatic polyvinyl compounds and unsaturated hydrocarbons which have at least two allyl groups in the molecule and/or polyvinyl ethers of polyhydric alcohols. There is also an indication of the use of acrylonitrile in the bead polymers, but susceptibility to hydrolysis is described in the text as a serious disadvantage of known synthetic resins based on acrylonitrile.
U.S. Pat. No. 3,586,646 describes the use of divinyl ethers for preparing spongy cation exchangers having groups selected from the class consisting of sulfonic acid groups, carboxylic acid groups, phosphoric acid groups, and phosphonic acid groups, in the presence of a porogen (an organic solvent which dissolves the monomer but is a precipitant for the polymer). However, U.S. Pat. No. 3,586,646 does not describe the use of unsaturated nitrites in the bead polymers.
EP-A 98,130 describes the preparation of crosslinked copolymer beads by a seed/feed process, and describes the copolymer beads themselves and their use as adsorbers or, after introduction or addition of functional groups, as ion-exchange resins. Monomers mentioned for the seed and/or the feed include nitriles of acrylic and/or methacrylic acid, and crosslinking agents mentioned include polyvinyl ethers of glycol, glycerol, pentaerythritol, resorcinol or monothio- or dithio derivatives of glycols. The preparation of the seed/feed particles requires the absence of protective colloids, or a drastic reduction in the amount of these, as they prevent, or dramatically delay, the absorption of the feed component by the seed particles. The particles prepared according to EP-A 98,130 also show multistage swelling behaviour in toluene and birefringence in the form of a maltese cross under polarized light.
None of the patent applications/patents cited gives any indication as to how monomers and crosslinking agents have to be selected in order to obtain crosslinked bead polymers with a homogeneous network structure, particularly in the presence of film-forming protective colloids.
The object of the present invention was to prepare ion exchangers, preferably weakly acidic cation exchangers, having a homogeneous network structure and based on unsaturated aliphatic nitrites in the presence of film-forming protective colloids.
SUMMARY OF THE INVENTION
The present invention achieves this object in providing a process for preparing crosslinked ion exchangers comprising
(a) polymerizing unsaturated aliphatic nitrites with di- or polyvinyl ethers as crosslinking agents and also with initiators in suspension in the presence of protective colloids, but not by a seed/feed process, to give bead polymers, and
(b) functionalizing the resultant bead polymers to give ion exchangers.
If desired, other monovinyl compounds and/or other crosslinking agents and/or porogen may be added to the polymerization reaction mixture.
The bead polymers prepared by step (a) are likewise provided by the present invention. They have a homogeneous network structure and can be used as adsorber resins.
DETAILED DESCRIPTION OF THE INVENTION
In step (b) the bead polymers are functionalized by customary methods known to those skilled in the art to give ion exchangers, particularly weakly acidic cation exchangers. To prepare weakly acidic cation exchangers, the bead polymers obtained in step (a) are hydrolysed under alkaline conditions, subjected to ion-exchange if desired, and purified. Alkaline hydrolysis has proven to be a particularly effective and economic practical process. The weakly acidic cation exchangers preferably obtained by the novel process exhibit a particularly high capacity.
For the purposes of step (a) of the present invention, unsaturated aliphatic nitrites are defined by the general formula (I)
wherein each of A, B, and C, independently of the others, represents hydrogen, alkyl, or halogen.
For the purposes of the present invention, the term “alkyl” refers to straight-chain or branched alkyl having from 1 to 8 carbon atoms (preferably from 1 to 4 carbon atoms). For the purposes of the present invention, the term “halogen” refers to chlorine, fluorine, or bromine.
For the purposes of the present invention, preferred nitriles are acrylonitrile and methacrylonitrile, and the use of acrylo
Born Ralf-Jürgen
Happ Michael
Klipper Reinhold
Schnegg Ulrich
Soest Hans-Karl
Akorli Godfried R.
Bayer Aktiengesellschaft
Eyl Diderico van
Lipman Bernard
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