TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The block (A) is styrene or tert-butylstyrene. The block (D0) is para-methylstyrene. The inert solvent comprises an aprotic hydrocarbon solvent, or halogenated hydrocarbon solvent, preferably cyclohexane and optionally heptane. The halogenating agent is bromine or chlorine, preferably N-bromo-succinimide (NBS), N-chloro-succinimide, N-bromo-tert- butylamine, N-bromo-hydantoin or N,N-dibromo hydantoin.    TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The functionalized block copolymer is of formula: A-D'-A, A-D'-A-D'-A, (A-D'-A)nX, (A-D')nX, A-B'-D'-B'-A, A-D'-B'-D'-A, (A-B'-D')nX, (A-D'-B')nX. The selectively halogenated block copolymer is of formula: A-Dasterisk-A, A-Dasterisk-A-Dasterisk-A, (A-Dasterisk-A)nX, (A-Dasterisk)nX, A-B'-Dasterisk-B'-A, A-Dasterisk-B'-Dasterisk-A, (A-B'-Dasterisk)nX, (A-Dasterisk-B')nX. The precursor block copolymer is of formula: A-D0-A, A-D0-A-D0-A, (A-D0-A)nX, (A-D0)nX, A-B'-D0-B'-A, A-D0-B'-D0-A, (A-B'-D0)nX, (A-D0-B')nX. Each block (A) is chosen from group consisting of polymerized ethylene monomer (i), propylene monomer (ii), styrene and alpha -alkyl styrene monomer (iii) having phenyl ring which is optionally substituted by one or more alkyl groups, (meth)acrylate ester monomer (iv), and mixture (v) of monomers chosen from (i)-(iv). Each block (D) is chosen from group consisting of segment (i) derived from homo- and copolymers of styrene, segment (ii) derived from homo- and copolymers of styrene having a phenyl ring which is substituted by a primary alkyl group, segment (iii) derived from homo- and copolymer of alpha -alkyl styrene, and segment (iv) derived from homo- and copolymers of alpha -alkyl styrene having phenyl ring which is substituted by a primary alkyl group. 25-100% Styrene or alpha -alkyl styrene derived polymer units of each block (D), block (Dasterisk) and block (D0) are polymer units (I) or corresponding onium salt. Each block (D), block (Dasterisk) and block (D0) consist of polymer units (I), or the corresponding onium salt, and optionally polymer units derived from more monomer(s) chosen from group consisting of styrene and alpha -alkyl styrene having a phenyl ring which is optionally substituted by a primary alkyl group. The functionalized block copolymer further comprises at least one non-functionalized interior block (B') having number average molecular weight of 1000-100000 and glass transition temperature of 20 degrees C or less. Each block (B') is chosen from group consisting of polymerized ethylene monomer (i), 3-8C alpha -olefin monomer (ii), isobutylene monomer (iii), conjugated diene monomer (iv), (meth)acrylate ester monomer (v), silicon polymer (vi), and mixture (vii) of monomers chosen from (i)-(v). The segments containing polymerized conjugated diene monomers are optionally hydrogenated. At least one of membranes of electro-deionization assembly is a cation-exchange membrane. The cation-exchange membrane comprises a sulfonated block copolymer comprising at least two polymer end blocks (E) and at least one polymer interior block (F). Each block (E) contains essentially no sulfonic acid or sulfonated ester functional groups. Each (F) block comprises sulfonation susceptible polymer units based on the number of the sulfonation susceptible polymer units, and 10-100 mol% sulfonic acid or sulfonate ester functional groups. The halogenated block copolymer is prepared using radical initiator chosen from tert-butyl hypochlorite, peroxides, benzoyl peroxide, azo compounds and azo-bis-isobutyronitrile. The amino- or phosphino- functionalized block copolymer is prepared by reacting halogenating agent with precursor block copolymer in presence of an inert solvent. 10-100% Functional groups of the block(s) (D) are in form of onium salt. The halogenated block copolymer is reacted with heterocyclic amine or tertiary amine, preferably 1,4-diazabicyclo(2,2,2)octane or trimethylamine.n=2-30;andX=coupling agent residue. Preferred Definitions:    two R+Z=pyrrolidine, piperidine, piperazine, 1-azabicyclo(2,2,2)nonane, 1,4-diazabicyclo (2,2,2) octane, morpholine, pyrrole, pyrazole, imidazole, pyridine, pyridazine, pyrimidine, pyrazine, indole, isoindole, indazole, purine, carbazole, phenoxazine, or azepine.