Effect of Spacer Arm Length

Polymeric anion exchangers are normally prepared by chloromethylation of the benzene ring followed by reaction with a tertiary amine to give a quaternary ammonium group. Thus the N is connected to the benzene ring by a single - 

CH2 group. Suppose the was connected to the benzene ring by a longer series of -CH2 groups, sometimes called the spacer arm. What effect would a longer spacer arm have on ion-exchange selectivity  

Warth and Fritz synthesized a series of resins with spacer arms of varying lengths [17]. The benzene ring of a macroporous resin (Rohm Haas XAD-1) was reacted under controlled conditions with a bromoalkene with CF3SO3H as catalyst and was then quaternized by reaction with trimethylarrime. This gave a resin of the following structure  

Conditions were adjusted so that all of the resins had almost identical exchange capacities. The spacer arm length varied from one to six methylene groups. 

Chromatographic retention times of a number of anions were compared, with 6.0 mM nicotinic acid and 2.0 mM phthalate (pH 6) as the eluents. In many cases, the length of the spacer arm had very little effect on the relative retention times (relative to Cl ). However, the selected data in Table 3.4 show that the relative retention times of bromide, nitrate, chlorate and iodide decreased, while that of sulfate increased slightly. 

Chromatographic retention times of a number of anions were compared with 

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