Diblock copolymers using ATRP

Figure 3 Reaction scheme for the synthesis of a semi-branched OEGMA-PPO diblock copolymer by ATRP using a PPO-based macro-initiator...

Triblock copolymers can be prepared from diblock copolymers by a third monomer addition. They can also be prepared using a bis-funetional NMP or ATRP initiator or a bis-RAFT agent (for examples, see Table 9.13). Symmetrical trithiocarbonates (Table 9.15) should also be considered as bis-RAFT agents in... 

Diblock copolymers PEO-fo-PS have been prepared using PEO macroinitiator and ATRP techniques [125]. The macroinitiator was synthesized by the reaction of monohydroxy-functionalized PEO with 2-chloro-2-phenylacetyl-chloride. MALDI-TOF revealed the successful synthesis of the macroinitiators. The ATRP of styrene was conducted in bulk at 130 °C with CuCl as the catalyst and 2,2 bipyridine, bipy, as the ligand. Yields higher than 80% and rather narrow molecular weight distributions (Mw/Mn < 1.3) were obtained. The surface morphology of these samples was investigated by atomic force microscopy, AFM. 

The properties of the hybrid diblock structures can be altered drastically by simply taking advantage of the high terminal functionality of the dendritic block. For example unusual diblock structures useful for the modification of surfaces have been prepared by ATRP of polystyrene (PS) initiated from the benzylic halide focal point of Frechet-type dendrons with terminal isophthalate ester groups [9b], Well-defined copolymers with narrow molecular weight distributions were obtained and excellent agreement was observed between calculated... 

In this review, synthesis of block copolymer brushes will be Hmited to the grafting-from method. Hussemann and coworkers [35] were one of the first groups to report copolymer brushes. They prepared the brushes on siUcate substrates using surface-initiated TEMPO-mediated radical polymerization. However, the copolymer brushes were not diblock copolymer brushes in a strict definition. The first block was PS, while the second block was a 1 1 random copolymer of styrene/MMA. Another early report was that of Maty-jaszewski and coworkers [36] who reported the synthesis of poly(styrene-h-ferf-butyl acrylate) brushes by atom transfer radical polymerization (ATRP). 

This was the first report using ATRP and sequential monomer addition. Hydrolysis of these diblock copolymer brushes yielded poly(styrene-fc-acrylic acid) brushes. 

The first diblock copolymer brushes synthesized in our group were made by a combination of carbocationic polymerization and ATRP (Scheme 1) [46]. Zhao and co-workers [47] synthesized diblock copolymer brushes consisting of a tethered chlorine-terminated PS block, produced using carbocationic polymerization, on top of which was added a block of either PMMA, poly(methyl acrylate) (PMA) or poly((Ar,M -dimethylamino)ethyl methacrylate) (PDMAEMA), synthesized using ATRP. The thickness of the outer poly(meth)acrylate block was controlled by adding varying amounts of free initiator to the ATRP media. It has been reported that the addition of free initiator is required to provide a sufficiently high concentration of deactivator, which is necessary for controlled polymerizations from the sur-... 

Scheme 2 Synthesis of surface-immobilized diblock copolymer brush (Si/Si02//PS-fc-PMMA) using reverse atom transfer radical polymerization and ATRP...

Exploiting ATRP as an enabling technology, we have recently synthesised a wide range of new, controlled-structure copolymers. These include (1) branched analogues of Pluronic non-ionic surfactants (2) schizophrenic polymeric surfactants which can form two types of micelles in aqueous solution (3) novel sulfate-based copolymers for use as crystal habit modifiers (4) zwitterionic diblock copolymers, which may prove to be interesting pigment dispersants. Each of these systems is discussed in turn below. 

Poly(alkylene oxide)-based (PEO-PPO-PEO) triblock and diblock copolymers are commercially successful, linear non-ionic surfactants which are manufactured by BASF and ICI. Over the last four decades, these block copolymers have been used as stabilisers, emulsifiers and dispersants in a wide range of applications. With the development of ATRP, it is now possible to synthesise semi-branched analogues of these polymeric surfactants. In this approach, the hydro-phobic PPO block remains linear and the terminal hydroxyl group(s) are esteri-fied using an excess of 2-bromoisobutyryl bromide to produce either a monofunctional or a bifunctional macro-initiator. These macro-initiators are then used to polymerise OEGMA, which acts as the branched analogue of the PEO block (see Figures 2 and 3). 

The synthesis of these block copolymers is based on the reductive amination of dextran using a bromoisobutyrate containing primary amine, followed by silylation of the oligosaccharide hydroxyls before polymerization of styrene by ATRP. ° Three diblock copolymers were thus prepared with a DP of the PS block ranging from 5 to 775, tlat is from 7% to 92% w/w, using the same dextran-based precursor (Mn = 6600 g/mol, PDI -1.5 corresponding to a DP = 40). Finally, these (silylated dextran)-Z)-PS block copolymers were readily... 

It is of obvious interest to explore the use of other polymerization techniques that, being more tolerant to the experimental conditions and monomers, can produce amphiphilie azobenzene BCPs with no need for post reactions. Notably, Su et al. have reeently reported the synthesis of such an amphiphilic diblock copolymer with PAA as the hydrophilic block using reversible addition-fragmentation transfer (RAFT) polymerization (structure d in Fig. 6.2) (Su et al., 2007). Using RAFT, they prepared PAA capped with dithiobenzoate and used it as the macro-RAFT transfer agent to polymerize the hydrophobic azobenzene polymer successfully. It ean be expected that more amphiphilic azobenzene BCPs will be synthesized using the eontrolled radical polymerization techniques (ATRP and RAFT) because of their simplicity, versatility, and efficiency. 

Block Type The preparation of molecular brushes with block copolymer backbones has been reported [39, 58, 90, 110-112]. These examples are mostly brush-coil block copolymers, in which one block is a cylindrical brush while the other is composed of a Hnear polymer chain. As an example, comonomers of octadecyl methacrylate (ODMA) and TMS-HEMA were polymerized sequentially via ATRP to afford a PODMA-b-P(TMS-HEMA) (PODMA = poly(octadecyl methacrylate)) diblock copolymer main chain. The poly(HEMA-TMS) block was converted into PBIEM polyinitiator, which was used for the polymerization of nBA this formed a PnBA block brush with a PODMA coil at the end of the main chain [28]. Owing to the crystalline nature of the PODMA segments, the self-assembly of the brush-coil block molecular bmshes was observed using AFM. This type of material gives rise to a new class of supersoft thermoplastic elastomers [95,113]. 

The emergence of controlled living radical polymerization techniques has opened a novel era for the synthesis of a vast variety of diblock copolymers consisting of blocks that could not be linked together by using the ionic polymerization techniques. The control of chain transfer reactions and the suppression of unwanted termination are now possible in free radical polymerization techniques through ATRP, RAFT, and NMRP. 

Monodisperse terfluorene segments were used by our group for the syntheses of diblock copolymers 19 and triblock copolymers 20 utilizing a CRP technique. " The rods were synthesized via classical organic synthesis and were then used as macroinitiators for the ATRP of styrene, tert-butyl acrylate (tBA), and 2-hydroxyethyl methacrylate (HEMA). 

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