Hydrophilic A blocks

Other. Diblock copolyme result from a combmation of a hydrophilic (A) block... 

Only one membrane conformation is possible for AB copolymers, where the B blocks constitute the hydrophobic layer and the A blocks extend outwards on both sides of the membrane. In the case of the ABA triblocks, the hydrophobic B block either can form a loop so as the hydrophilic A chains are on the same side of the membrane (U shape) or can stretch forming a monolayer with the two A blocks at the opposite sides of the membrane (I shape), while for the BAB type only, the U-shaped conformation is possible since the hydrophobic B chain ends must assemble into a membrane and the hydrophilic A blocks must form a loop. Similar conformations can be anticipated for ABABA pentablock copolymers. The same general rules apply when a third chemically different block is added, and although the overall geometry is the same, multiblock copolymers have an extra level of control over the polymersome structure introduced by the extra interaction between the blocks. Hence, ABC triblock copolymers assemble into asymmetric or Janus membranes, and the resulting vesicles are characterised by different external and internal surface chemical properties. Even more interestingly, ABCA tetrablocks (where A is hydrophilic and B and C are both hydrophobic) have been shown to... 

The above analysis indicates that adding AB diblock copolymers to a melt of B homopolymers would also promote the exfoliation of the clay particles. Here, a short hydrophilic A block will anchor the chain to the bare (non-modified) clay sheets. The large organophilic B block will extend away from the surface, and could drive the separation and dispersion of these sh ts. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

PSt or PMMA, respectively, was coupled with polymethacrylate having a PEG side chain or methylammo-niumchloride side chain to prepare a block copolymer for giving a hydrophilic surface [55]. Also, PSt-b-PVP [36,37], PSt-b-(hydrophilic vinyl copolymer) [56], PSt-b-po y(sodium acrylate) (PNaA) [57], and PSt-b-PNaA-b-(polyperfluoroacrylate) (PFA) [58] were synthesized for the same application. 

The synthesis of block copolymers by macromonotner RAFT polymeriza tion has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thioearbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block formation is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared.476 638 The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

In 2000, the first example of ELP diblock copolymers for reversible stimulus-responsive self-assembly of nanoparticles was reported and their potential use in controlled delivery and release was suggested [87]. Later, these type of diblock copolypeptides were also covalently crossUnked through disulfide bond formation after self-assembly into micellar nanoparticles. In addition, the encapsulation of l-anilinonaphthalene-8-sulfonic acid, a hydrophobic fluorescent dye that fluoresces in hydrophobic enviromnent, was used to investigate the capacity of the micelle for hydrophobic drugs [88]. Fujita et al. replaced the hydrophilic ELP block by a polyaspartic acid chain (D ). They created a set of block copolymers with varying... 

In their efforts to construct stimuli-responsive, supramolecular amphiphiles, Frechet et al. [126-129] reported the synthesis of a novel series of AB and ABA block copolymers via the Williamson ether synthesis (e.g., 47, Fig. 21). Polyethylene glycols (PEGs) of different lengths were used as the linear hydrophilic B block while polyaryl ether dendrons of different sizes were used as the hydro-phobic A block. These copolymers were characterized by optical microscopy,... 

Double hydrophilic star-block (PEO-fo-PAA)3 copolymers were prepared by a combination of anionic and ATRP of EO and fBuA [150]. Three-arm PEO stars, with terminal - OH groups were prepared by anionic polymerization, using l,l,l-tris(hydroxymethyl)ethane, activated with DPMK as a trifunctional initiator. The hydroxyl functions were subsequently transformed to three bromo-ester groups, which were utilized to initiate the polymerization of f-butyl acrylate by ATRP in the presence of CuBr/PMDETA. Subsequent hydrolysis of the f-butyl groups yielded the desired products (Scheme 74). 

Polysaccharides are hydrophilic natural polymers that can be degraded enzymatically. Block copolymers containing polysaccharide as a block were reviewed recently... 

Many kinds of nonbiodegradable vinyl-type hydrophilic polymers were also used in combination with aliphatic polyesters to prepare amphiphilic block copolymers. Two typical examples of the vinyl-polymers used are poly(/V-isopropylacrylamide) (PNIPAAm) [149-152] and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) [153]. PNIPAAm is well known as a temperature-responsive polymer and has been used in biomedicine to provide smart materials. Temperature-responsive nanoparticles or polymer micelles could be prepared using PNIPAAm-6-PLA block copolymers [149-152]. PMPC is also a well-known biocompatible polymer that suppresses protein adsorption and platelet adhesion, and has been used as the hydrophilic outer shell of polymer micelles consisting of a block copolymer of PMPC -co-PLA [153]. Many other vinyl-type polymers used for PLA-based amphiphilic block copolymers were also introduced in a recent review [16]. 

By varying all the parameters of the process the authors prepared a set of copolyesters containing units and blocks with a tertiary amino group, which, in turn, could be transformed into a hydrophilic hydrochloride salt thus imparting water-compatibility to the initially organosoluble macromolecules. The principle involved was the formation of a block-type structure of chains in order to facilitate their further protein-like folding in an aqueous medium. It was shown that the main factors responsible for the blockiness were the relative reactivities of the (B) and (C) components (NMDEA and bisphenol) and the order of their addition to the reaction. 

Nonionic hydrophilic PEO blocks have also been combined with a variety of other hydrophobic blocks, including PI [135], poly(amino acids) [136], aliphatic polyesters [137], etc. It is not possible to review all the published works on these copolymers due to space limitations. We will therefore only present a selected example. 

In a very recent set of papers [48,54,59,60,131,132,324-328], the synthesis and characterization of metallosupramolecular amphiphilic block copolymers containing a hydrophilic PEO block linked to a hydrophobic PS or PEB block through a fozs-2,2/ 6/,2/terpyridine-ruthenium(II) complex have been described. These copolymers form the so-called metallosupramolecular micelles . 

In a very recent investigation, hydrophobic PFS (Sect. 7.1) was attached to a hydrophilic PEO block to form an amphiphilic PFSi2-[Ru]-PE07o block copolymer [331]. Rodlike micelles were observed in water for this copolymer (Fig. 24). These micelles have a constant diameter but are rather polydisperse in length, and DLS measurements indicate that they are flexible. Crystallization of the PFS in these micelles was observed and is thought to be the key behind the formation of rodlike structures. The cylindrical micelles can be cleaved into smaller rods whenever the temperature of the solution is increased or whenever they are exposed to ultrasound. 

It is well known that AB diblock copolymers form micelles in solvents that are selective for one of the blocks. By varying the nature of the solvent, it is also possible to form micelles with the A block in the core or with the B block in the core. However, we have recently demonstrated that certain hydrophilic AB diblock copolymers can form either A-core micelles or B-core micelles in aqueous media. In the original example, both blocks were based on tertiary amine methacrylates and the diblock copolymer was prepared by group transfer polymerisation, a special type of anionic polymerisation which is particularly... 

A polymer is considered to be a copolymer when more than one type of repeat unit is present within the chain. There are a variety of copolymers, depending on the relative placement of the different types of repeat units. These are broadly classified as random, block, graft, and alternating copolymers (see Fig. 2.1 for structural details Cheremisinoff 1997 Ravve 2000 Odian 2004). Among these stmctures, block copolymers have attracted particular attention, because of their versatility to form well-defined supramolecular assemblies. When a block copolymer contains two blocks (hydrophobic and hydrophilic), it is called an amphiphilic diblock copolymer. The immiscibility of the hydrophilic and lipophilic blocks in the polymers provides the ability to form a variety of assemblies, the stmctures and morphologies of which can be controlled by tuning the overall molecular weight and molar ratios of the different blocks (Alexandridis et al. 2000). 

To understand whether the preorganization in a block copolymer assembly indeed provides any advantages in presenting multivalent hgand copies, one could envision the presentation of a single copy of a specific ligand at the hydrophilic chain terminus... 

The addition of large linear blocks to dendrons with opposite polarity creates a desymmetrized structure predisposed to sequester insoluble components by aggregation rather than intramolecular hydrogen-bonding. Amphiphilic, linear-dendritic diblock (AB) and triblock (ABA) copolymers self-assemble into multimolecular micelles with CMC values that are well below those of low molecular weight surfactants. Typically, a hydrophilic linear block such as PEG is attached to the focal point... 

In ionic block copolymers, micellization occurs in a solvent that is selective for one of the blocks, as for non-ionic block copolymers. However, the ionic character of the copolymer introduces a new parameter governing the structure and properties of micellar structures. In particular, the ionic strength plays an important role in the conformation of the copolymer, and the presence of a high charge density leads to some specific properties unique to ionic block copolymers. Many of the studies on ionic block copolymers have been undertaken with solvents selective for the ionic polyelectrolyte block, generally water or related solvents, such as water-methanol mixtures. However, it has been observed that it is often difficult to dissolve ionic hydrophilic-hydrophobic block copolymers in water. These dissolution problems are far more pronounced than for block copolymers in non-aqueous selective solvents, although they do not always reflect real insolubility. In many cases, dissolution can be achieved if a better solvent is used first and examples of the use of cosolvents are listed by Selb and Gallot (1985). 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

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