Weak Base Anion Exchangers

Commercial weak base anion exchangers are prepared by the condensation of m-phenylenediamine

Figure 8 Preparation of strong base anion exchangers based on 4-vinylpyridine-DVB copolymer.

with formaldehyde. The chemical structure of this exchanger is illustrated in Figure 10. In this structure, the amine groups directly attached to benzene rings have a very low basicity.

Polycondensation weak base exchangers with higher basicity were later obtained by the condensation of other reagents. An example is the epoxy structures formed by the condensation of aliphatic polyamines with epichlorohydrin. This halo-epoxy compound can react even with tertiary amine groups, thus anion exchangers containing amine and quaternary ammonium groups can be obtained as shown in Figure 11 .

The most readily available commercial weak base exchangers are the polymerization structures based on polystyrene or acrylic matrices containing primary, secondary or tertiary amine groups, or all these groups together.

The polystyrene-based weak base exchangers are obtained by the same reaction scheme as the strong base ones with polystyrene matrices, but with the difference that dimethylamine is used in the amination step instead of trimethylamine or dimethylethanolamine. When using the secondary amine, in contrast with the tertiary amines, besides the main amination reaction which leads to the tertiary amine groups, an undesirable side reaction can also take place. This is the quaternization reaction between the pre-attached tertiary amine and -CH2Cl groups. It can take place intra- or inter-chain but both situations can occur.

The chemical structures of aminated units are shown in Figure 12. Both quaternization types (inter-and intra-chain) lead to anion exchangers with mixed

where /

Figure 9 Preparation methods of phosphonium-type strong base anion exchangers.

where /

Figure 9 Preparation methods of phosphonium-type strong base anion exchangers.

functional groups and the inter-chain ones also control the degree of crosslinking.

Amination takes place quantitatively under mild conditions (30-40°C; 4-6 h) in aqueous or organic media. By using a large excess of amine, the side quaternization reaction is greatly reduced.

The most common acrylic weak base exchangers are made by the acylation of primary or secondary amines. This reaction with esters, so-called ester aminolysis, is the most frequently used method to produce acrylic weak base exchangers.

The aminolysis of macroporous or gel-type ethyl acrylate-DVB copolymers, in bead form, with 3-dimethylamino-1-propylamine, ethylendiamine or other aliphatic polyamines, is illustrated in Figure 13.

The same structures can also arise from amino-lysis-hydrolysis reactions of acrylonitrile-DVB copolymers. An example is shown in Figure 14.

nh2 nh2

ch2 nh

Figure 10 Chemical structure of m-phenylenediamine-formaldehyde weak base anion exchanger.

Figure 11 Chemical structure of an epoxy-type base anion exchanger.

The aminolysis and aminolysis-hydrolysis reactions take place under more stringent reaction conditions (temperature over 100°C and reaction time over 10 h) than the amination of the chloro-methylated styrene-DVB copolymers.

Acrylic weak base exchangers synthesized from aliphatic polyamines have much higher exchange capacities than polystyrene-based structures.

The weak base exchangers cannot be regarded as typical ionic polymers since their amine groups are ionized only under certain conditions. Thus, an amine group in an alkaline medium is in the free-base form, but in a neutral medium it can exist in a partial ionization level, which depends upon amine basicity.

In an acid medium a high level of ionization is present and because of this, the weak base exchangers are usually used to retain strong acids in water treatment. They also can be used as insoluble acceptors of acids in different chemical reactions such as the preparation of esters from acid chlorides and alcohols, etc. For the latter aim, the best known are the 4-vinyl-pyridine-DVB copolymers with a low degree of crosslinking.

Scheme 2 shows some more distinctive ion exchange reactions for the four above-mentioned types of exchangers.

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