DHBCs

Amphiphilic block copolymers containing a cationic block may carry as the second block a hydrophilic or a hydrophobic one. Those containing a hydrophilic second block belong to the so called double-hydrophilic block copolymers (DHBC), which have recently been reviewed [60]. This promising class of polymers has potential applications in drug-carrier systems, gene therapy, desalination membranes, as switchable amphiphiles, and others [45, 60, 77]. 

A novel route to DHBC is outlined in Scheme 11. Reaction of TEMPO-ter-minated poly(VBC) with PEG-monomethyl ether results in reactive block copolymers 47, which can be converted readily to polymers 48 and 49 containing a cationic or a betainic block besides a PEG block [84]. 

Kitaura et al. obtained a new lamellar compound [Cu(dhbc)2(4,4 -bpy)] H20 from a diethyl ether solution of copper nitrate, 4,4 -bipyridine, and 2,5-dihydroxybenzoic acid (dhbc).[202] There are strong n-n interactions between the layers of the compound, and because of these interactions the compound behaves as a 3-D framework structure with channels containing guest water molecules that are removable reversibly. After dehydration, the host material exhibits adsorption properties, but its adsorption capacity for nitrogen is affected by the gas pressure. 

Double hydrophilic block copolymers (DHBCs) are a class of polymers that combine the self assembly ability of block copolymers with the water solubility of hydrophilic macromolecular chains. Numerous sophisticated works have been already described in the literature, indicating the potential of this class of copolymers in emerging technologies. The synthesis of novel DHBCs, using either new monomers or post polymerization functionalization schemes, is the subject of intense investigation during the current years. 

Moreover their self-assembly in aqueous media has been also studied in considerable detail. Emphasis has been given to the stimuli responsive character of the DHBCs upon environmental changes, like solution pH, ionic strength or temperature. Moreover, the potential utilization of DHBCs in a wide range of applications has been demonstrated in a number of publications. 

In this chapter recent developments in the field of DHBCs are described. Excellent review articles have already outlined the major features of DHBCs [1,2]. A number of exeellent reviews summarize speeific aspects of DHBCs in detail [3,4,5]. This chapter is focused on recent advances in the particular research area. The synthetic strategies followed for the preparation of DHBCs, important investigations on their aqueous solution behavior and a number of potential applications are presented. The field is growing rapidly. Therefore, the creation of a complete list of works, concerning DHBCs, is practically impossible and beyond the goal of this chapter. The works presented here have been selected in order to representatively describe the current developments in DHBC research. 

The synthesis of a variety of linear diblock DHBCs structures has been realized by the vast majority of the so called controlled polymerization techniques. Herein, we describe recent achievements in the synthesis of DHBCs categorized according to the followed polymerization mechanism. 

Anionic polymerization is known to give model block copolymers with controlled molecular weights, narrow molecular weight distributions and versatile architecture. Anionic polymerization has been used for the synthesis of DHBCs in several eases although this type of polymerization teehnique is relatively intolerant to the presence of polar functionalities on the monomers utilized. A recent example has been described by Hadjichristidis and coworkers [6]. They have presented the synthesis of a series of poly (2-vinylpyridine-b-... 

Combination of anionic polymerization and post polymerization reactions has been used for the synthesis of poly(acrylic acid-b-N,N-diethylacrylamide) (PAA-PDEA) copolymers [9]. Initially the synthesis of a precursor poly(tert-butylacrylate-b- N,N-diethylacrylamide) (PtBMA-PDEAAm) block copolymer was realized via sequential anionic polymerization of the tert-butyl acrylate and diethylacrylamide monomers. However, an amount of PtBMA homopolymer was detected in the crude reaction product. In order to remove the vast majority of the homopolymer, the authors proposed the precipitation of the crude product in hexane, where the homopolymer is highly soluble, in contrast to the block copolymer. The piuified block copolymer was subjected to deprotection of the tert-butyl group in acidic media, leading to the desirable DHBC. The final block copolymer showed pH and thermosensitive solution aggregation. 

DHBCs of the type poly(p-hydroxystyrene-b-methaciylic acid) (PHOS-PMAA) were also synthesized via anionic polymerization followed by acidic hydrolysis [10]. Both blocks of the precursor poly(p-tert-butoxystyrene-b-tert-butylmethacrylate) (PtBOS-PtBMA) copolymers, formed by sequential addition of the protected monomers, could be deprotected in a single step giving the desired pH-responsive block copolymers. Figure 2. Hydrolysis was found to be nearly quantitative and resulted in a series of copolymers with well defined molecular characteristics and of variable composition. 

Figure 1. Synthesis of P2VP-PEO DHBCs by anionic polymerization. Reproduced from [6] by permission of Elsevier.

Different series of DHBCs were prepared from anionically synthesized poly(p-tert-butoxystyrene-b-ethylene oxide) (PtBOS-PEO) precursors [12], Post polynnerization acidic hydrolysis of the PtBOS block resulted in poly(p-hydroxystyrene-b-ethylene oxide) (PHOS-PEO) copolymers. Further functionalization of the PHOS block via a Mannich type aminomethylation reaction gave the poly[3,5-bis(dimethylaminomethylene)hydroxystyrene-b-ethylene oxide] (PNHOS-PEO) copolymers, as testified by FT-IR and NMR experiments. In these copolymers the PNHOS block carries two dimethylamino groups per monomeric imit that can be protonated in acidic media and weakly acidic phenolic groups that have their own pH sensitivity. The PNHOS-PEO block copolymers were further quatemized with... 

The synthesis of an interesting DHBC, namely poly (4-vinylbenzoic acid-block-2-(diethylamino)ethyl methacrylate) (PVBA-PDEAEMA), has been presented by Armes and Liu [13], The synthesis was performed by ATRP using protecting group chemistry in three steps. Initially the polymerization of a tert-butyl protected PVBA macro-initiator was performed followed by the polymerization of the second monomer, DEAEMA. Finally the hydrolysis of the tert-butyl protected block was realized giving rather monodisperse block copolymers. 

Figure 4. Synthesis of a PVim containing DHBC by RAFT. Reproduced from [23] by permission of the American Chemical Society.

The synthesis of a novel, primary amine containing, DHBC has been described by Liu and coworkers [31]. The synthesis of the amine-containing monomer was performed by a two step click reaction. The preparation of the block copolymer, namely poly(N-isopropylacrylamide)-b-1 -(3 -aminopropyl)-4-acrylamido-1,2,3 -triazole hydrochloride)... 

The synthesis of a well defined poly(vinyl alcohol)-b-poly(aciylic acid) (PVA-PAA) DHBC has been recently reported in the literature [32] by a two step synthetic scheme. First the synthesis of a poly(acrylonitrile) (PAN) block was realized via cobalt-mediated radical polymerization, using a poly(vinyl acetate) (PVAc) macroinitiator, followed by hydrolysis of both blocks. The polymerization was performed in DMF, a very good solvent for PAN, and at low temperature, where block copolymers with low polydispersity were obtained. The polymerization procedure led to well defined macromolecules with relatively high molecular weights. The obtained copolymers were transformed to the desired DHBCs by hydrolysis, using large excess of potassium hydroxide in a water/ethanol mixture. The successful completion of the hydrolysis reaction was monitored by NMR and IR spectroscopy. An additional macroscopical indication of the DHBC formation was the aqueous solubility of the reaction product. 

Figure 6. Schematic illustration of DHBCs formation through inclusion complexation. Reproduced from [33] by permission of The Royal Society of Chemistry.

A similar approach has been also used for the synthesis of covalently linked DHBCs. Polysaccharide based DHBCs were prepared by end-to-end eoupling of two readily available biocompatible water-soluble homopolymers [35]. The synthesis was a two step reaction where a) a terminal aldehyde group of a dextran homopolymer was oxidized and b) a monoamine end functionalized PEG reaeted with the oxidized dextran, via a lactone aminolysis reaction. Interestingly, the obtained polymer could be chemically modified, in a controlled way, in order to produce neutral-cationic or neutral-anionic DHBCs. 

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