Living Free Radical Polymerization of Styrene

The 1,3,5-triphenylverdazyl coupling product with the 2-cyano-2-propyl radical was examined as a unimolecular initiator of the living free radical polymerization of styrene <1998PB539>. 

Knoop CA, Studer A (2003) Hydroxy- and silyloxy-substituted TEMPO derivatives for the living free-radical polymerization of styrene and n-butyl acrylate synthesis, kinetics, and mechanistic studies. J Am Chem Soc 125 16327-16333... 

Figure 2. Monomer Conversion for the Living Free Radical Polymerization of Styrene vs t-Butyl acrylate...

While the living free-radical polymerization of styrene from an alkoxyamine can lead to narrow polydispersity polymers, these having the nitroxyl radical at the terminals can be expected to behave as polymeric counter radicals for propagating polymeric radicals. The irse of such... 

He also prepared a poly(styrene-g-styrene) polymer by this technique [114], The lack of crosslinking in these systems is indeed proof of the control achieved with this technique. An eight-arm star polystyrene has also been prepared starting from a calixarene derivative under ATRP conditions [115]. On the other hand, Sawamoto and his coworkers used multifunctional chloroacetate initiator sites and mediation with Ru2+ complexes for the living free-radical polymerization of star poly(methylmethacrylate) [116,117]. More recent work by Hedrick et al. [84] has demonstrated major progress in the use of dendritic initiators [98] in combination with ATRP and other methodologies to produce a variety of structure controlled, starlike poly(methylmethacrylate). 

Living free-radical polymerization represents a promising technique to produce polymers with highly controlled structures. Different possible systems known from bulk polymerizations have been used in miniemulsions. The living free radical polymerization of, e.g., styrene via the miniemulsion approach allows one to eliminate the drawback of the bulk system where an increase in polydis-persity was found at high conversions due to the very high viscosity of the reaction medium [90]. 

Significant improvement in controlled polymerizations of a variety monomers, including styrene, acrylates, acrylamide, acrylonitrile, 1,3-dienes, and maleic anhydride has been achieved when alkoxyamines have been used as initiators for living, free radical polymerization.(696c, 697) Alkoxyamines can be easily synthesized in situ by the double addition of free radicals, generated by thermal decomposition of an azo-initiator, such as 2,2 -azo-h/.s-/.so-butyronitrile (AIBN), to nitrones (Scheme 2.206). 

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. 

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]. 

Frechet and coworkers recently described how living free radical polymerization can be used to make dendrigrafts. Either 2,2,6,6-tetramethylpiperidine oxide (TEMPO) modified polymerization or atom transfer radical polymerization (ATRP) can be used [96] (see Scheme 10). The method requires two alternating steps. In each polymerization step a copolymer is formed that contains some benzyl chloride functionality introduced by copolymerization with a small amount of p-(4-chloromethylbenzyloxymethyl) styrene. This unit is transformed into a TEMPO derivative. The TEMPO derivative initiates the polymerization of the next generation monomer or comonomer mixture. Alternatively, the chloromethyl groups on the polymer initiate an ATRP polymerization in the presence of CulCl or CuICl-4,4T dipyridyl complex. This was shown to be the case for styrene and n-butylmethacrylate. SEC shows clearly the increase in molecu-... 

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