Copolymer synthesis/linking approaches

In most of the studies discussed above, except for the meta-linked diamines, when the aromatic content (dianhydride and diamine chain extender), of the copolymers were increased above a certain level, the materials became insoluble and infusible 153, i79, lsi) solution to this problem with minimum sacrifice in the thermal properties of the products has been the synthesis of siloxane-amide-imides183). In this approach pyromellitic acid chloride has been utilized instead of PMDA or BTDA and the copolymers were synthesized in two steps. The first step, which involved the formation of (siloxane-amide-amic acid) intermediate was conducted at low temperatures (0-25 °C) in THF/DMAC solution. After purification of this intermediate thin films were cast on stainless steel or glass plates and imidization was obtained in high temperature ovens between 100 and 300 °C following a similar procedure that was discussed for siloxane-imide copolymers. Copolymers obtained showed good solubility in various polar solvents. DSC studies indicated the formation of two-phase morphologies. Thermogravimetric analysis showed that the thermal stability of these siloxane-amide-imide systems were comparable to those of siloxane-imide copolymers 183>. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

Amphiphilic block copolymers with covalentiy linked hydrophilic and hydrophobic blocks capable of forming micelles can be conveniently prepared by ATRP techniques [86-88]. A representative example is the synthesis of polymethacrylate based pentablock copolymers (PBPs) via consecutive ATRP technique reported by Xu et al. [89] for non-viral gene delivery applications. In their approach, PBPs of poly(HEMA)-Z7-poly(DMAEMA)-fc-PEG-fc-poly(DMAEMA)-fo-poly(HEMA) were prepared via consecutive ATRPs from di-2-bromoisobutyryl-terminated PEG (Br-PEG-Br) (Fig. 2.24). The very low in vitro cytotoxicity and excellent gene transfer efficacies exhibited by these PBPs demonstrated the versatility of the ATRP technique to tailor the polymer structural-property characteristics for such applications. 

The other possibility, at first examined by Wooley and co-workers [231,232] is to crosslink the corona of the micelles. These kinds of nanoparticles are designated by shell cross-linked knedel-Hke (SCK) micelles by these authors. Wooley et al. have applied this concept to a large variety of block copolymers, mainly hydrophobic-hydrophilic copolymers with PAA or quaternized PVP as the water-soluble block, which can be chemically cross-linked in their micellar form. A similar approach has been described by Armes and co-workers [233] for the synthesis of shell cross-linked micelles where core and shell are both hydrophilic. 

The second general approach for the covalent cross-linking was the use of side double bond functionalized copolymers [24]. In this case the synthesis of aromatic polyethers containing polar pyridine units in the main chain and side cross-linkable propenyl groups was the key step. These polymers were subjected to thermal cross-linking with the use of a cross-linker. The effect of the cross-linking on the thermal and mechanical... 

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