Alkoxyamines, TEMPO-derived

Matyjaszewski ° reported a simple and versatile method for the preparation of several TEMPO-derivative alkoxyamines based on atom-transfer radical addition (ATRA). This reaction involves halogen transfer from organic halides to Gu(I)com-plexes to produce the alkyl radical, which is then trapped by the nitroxide to afford the targeted alkoxyamine. Matyjaszewski et al and Matyjaszewski and Greszta have shown that 1.05 equiv. of Gu(0) with respect to the alkyl halide is necessary for a quantitative reaction and a slight excess of nitroxide (1.2 equiv.) is needed to minimize alkyl radical termination reactions (Figure 11(a)). 

LB lithography for spatially controlled attachment of initiator to form regular stripes of PS and poly(n-butyl-acrylate) (PBA) brushes was reported by Studer and coworkers [16]. Mixed monolayers of L-adipalmytoyl-phosphatidylcholine (DPPC) and TEMPO-derived alkoxyamines 4 or 5 (Scheme 5) were transferred by the LB technique onto oxidized silicon wafer in regular stripes with submicrometer lateral dimensions. Physisorbed DPPC was removed by washing, and NMP of styrene from the surface of silicon wafer covalently bonded to 4 was performed at 125°C for 24 h in the presence of 6 as sacrificial polymerization regulator. NMP of n-butyl acrylate was conducted with a surface modified with 5 in the presence of 7 at 105°C for 24 h. Polymer stripe width was controlled by the concentration of alkoxyamine in the mixed phase and adjusted from about 0.2 to 1.3 pm. The height of the stripes increased to 8 0.2 nm for PS and 4.7 0.2 nm for PBA (Scheme 6). 

Figure 4.4 Rate constants log(k

Tetramethylpiperidine-l-oxy (TEMPO)-containing alkoxyamine derivatives are widely used as unimolecular initiators for living radical polymerization [5], The key step of the presently accepted mechanism of polymerization is the reversible capping of the polymer chain by the nitroxide radical. In 2002, Otsuka and Takahara applied the reversible... 

Figure8.1 Comparison of PS chains having initiating versus terminating fragment derived from TEMPO-based alkoxyamine initiators. Polymerizations were carried out at 120 °C...

In total, alkoxyamine systems with large cleavage (activation) rate constants tend to show small coupling (deactivation) rate constants. This provides large equilibrium constants that increase the conversion rates. It must not deteriorate the control since this depends on kd and the product k kc. In comparison, the more recently introduced nitroxides 6, 8, and 9 provide larger equilibrium constants than e.g. 3 (TEMPO). For acrylate-derived radicals, the equilibrium constants are usually smaller than for styryl type radicals, and this may, at least in part, explain the failure of TEMPO-regulated acrylate polymerizations. However, judging from model studies,62-63 this reason does not apply for methacrylates. 

NMP can be achieved in either of two ways (1) by adding a stable nitroxyl radical to the polymerization reaction initiated by traditional initiators (peroxides or azo compounds) to generate an alkoxyamine in situ, or (2) by preparing an alkoxyamine to be used as the initiator. Two widely stndied examples of stable nitroxyl radicals are 2,2,6,6-tetramethylpiperidinyl-l-oxyl [2564-83-2] (4, TEMPO) or 4-substitnted derivatives thereof and N-tert-bntyl-iV-[l-(diethylphosphono)-2,2-dimethylpropyl]nitroxide [188526-94-5] (5) (165-172). 

As for the other methods mentioned above, NMP and ATRP living polymerizations of NIPAM have also been reported very recently. NMP has been carried out with initiator 6 (Fig. 3), cK-hydrogen alkoxyamine derivatives, instead of the TEMPO-based systems [53-55]. Although the living polymerization behavior has not been reported in detail, the synthesis of homopolymer of NIPAM with a narrow MWD, its block copolymers with polystyrene (PS) [53], and well-controlled star block copolymers having PNIPAM and PS segments was achieved by Hawker and Fr6chet et al. [54]. On the other hand, the successful ATRP of NIPAM has been reported by Masci et al. [56] and StOver et al. [57]. In each case, alkyl 2-chloropropionate (7), copper(I)... 

Whereas diblock copolymers were mainly prepared by sequential polymerizations of two different monomers, triblock copolymers were generally obtained by divergent chain growth from a difunctional alkoxyamine (Table 6). However, a few examples report consecutive polymerization of three different monomers from monofunctional initiators (Table 7). TEMPO and TEMPO-like nitroxides were almost exclusively employed for the polymerization of styrenic derivatives. The versatility of TIPNO and SGI nitroxides allowed greater flexibility regarding the range of monomer that can be controlled. Consequently, they have been intensively employed for the design of a myriad of block copolymers. 

LB reactive deposition was also employed for anchoring of triethoxysilyl alkoxyamine derivatives of TEMPO 8, SGI 9, and TIPNO 10 nitroxides to silicon substrates (Scheme 7) [17]. The nitroxide polarity and, therefore, the behavior of the corresponding alkoxyamine at the air-water interface of the LB trough had a... 

Scheme 7 Triethoxysilyl alkoxyamines derivatives of TEMPO 8, SGI 9, and TIPNO 10...

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