Nitroxide-Mediated Living Polymerizations

X-ray diffractograms of filler before grafting (I) and filler after grafting using conventional free radical polymerization (II) and nitroxide-mediated living polymerization (III). (Reproduced from Mittal, V., /. Colloid Interface Sci., 314,141, 2007.)... 

The process is called NMLP (from nitroxide mediated living polymerization). 

The brominated regjoregular P3HT 35 was reacted with the boronic ester of TEMPO (36). The macroinitiator 37 was then used for the polymerization of styrene and chloromethylstyrene by nitroxide-mediated living polymerization to afford the comb-like polymer 38. FinaUy, Qo vvas grafted via ATRA reaction to yield polymer 39 with a fuUerene content of 47 wt96. 

Most of the LFRP research ia the 1990s is focused on the use of nitroxides as the stable free radical. The main problems associated with nitroxide-mediated styrene polymerizations are slow polymerization rate and the iaability to make high molecular weight narrow-polydispersity PS. This iaability is likely to be the result of side reactions of the living end lea ding to termination rather than propagation (183). The polymerization rate can be accelerated by the addition of acids to the process (184). The mechanism of the accelerative effect of the acid is not certain. 

Hawker, C. J. Bosman, A. W. Harth, E. New polymer synthesis by nitroxide mediated living radical polymerizations. Chem. Rev. 2001,101, 3661-3688. 

Hawker CJ, Barclay GG, Orellana A et al. (1996) Initiating systems for nitroxide-mediated living free radical polymerizations synthesis and evaluation. Macromolecules 29 5245-5254... 

Lohmeijer BGG, Schubert US (2004) Expanding the supramolecular polymer LEGO system nitroxide mediated living free radical polymerization for metallo-supramolecular block copolymers with a polystyrene block. J Polym Sci Part A Polym Chem 42 4016-4027... 

C.J. Hawker, A.W. Bosman, E. Harth, C/iem. Rev. 101,3661-3688 (2001) (New Polymer Synthesis by Nitroxide Mediated Living Radical Polymerization)... 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

Scheme 1.33 Preparation of PS by nitroxide-mediated living free-radical polymerization and its utilization for the functionalization of shortened SWCNTs.

Homopolymer PS and block copolymer poly(tert-butyl acrylate)-b-styrene, prepared by nitroxide-mediated living free-radical polymerization, were utilized for the functionalization of shortened SWCNTs through a radical coupling reaction (Scheme 1.33) [194]. 

Bergbreiter, D. E., Waichuk, B. Meisenheimer Rearrangement of Aiiyi N-Oxides as a Route to Initiators for Nitroxide-Mediated "Living" Free Radical Polymerizations. Macromolecules 1998, 31, 6380-6382. 

New Polymer Synthesis by Nitroxide Mediated Living Radical Polymerizations... 

III. Nitroxide-Mediated Living Free Radical Polymerizations 3663... 

It is obvious from the above discussion that under the correct conditions and with the appropriate mediating nitroxide free radical, living polymerization conditions can be achieved. On the basis of this realization, numerous groups have demonstrated that the degree of structural control normally associated with more traditional living processes, such as anionic procedures, can be equally applied to nitroxide-mediated living free radical polymerizations. 

The potential for surface-initiated polymerizations using nitroxide-mediated living free radical procedures is perhaps best illustrated by the direct synthesis of dispersed nanocomposities by Sogah and Giannelis.212 In this approach the synthetic versatility of the alkoxyamine group is again exploited to... 

Scheme 1.33. Nitroxide-mediated living radical polymerization of styrene using TEMPO. After Hawker et al. (2001).

A similar approach was followed with the eROP of 4-MeCL, followed by nitroxide mediated living free radical polymersation (NMP) of styrene using a bifunctional catalyst (Scheme 11.18) [62]. Styrene, the monomer for the NMP, was added already at the beginning since it proved to be a good solvent for the eROP of lactones. At low temperatures, no radical polymerization occurs thus the two polymerization mechanisms are thermally separated. When the eROP reached a conversion of 50%, a lipase inhibitor, paraoxon, was added to the reaction mixture to prevent further incorporation of the undesired enantiomer. Increasing the temperature to 95 °C started the nitroxide mediated LFRP, to afford block copolymers. After precipitation, the chiral block copolymers obtained showed two Tg s at-51 °C and 106 °C. The specific rotation [a]D25 of the block copolymer was -2.6°. 

Figure 12.9 Block copolymers by one-pot enzymatic ROP and nitroxide-mediated living free-radical cascade polymerization [24].

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