Chapter 35
Controlled Radical Polymerization of Alkyl Methacrylates in the Presence of NO/N 0 Mixtures Downloaded by UNIV OF GUELPH LIBRARY on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch035
2
1,2
1
1,2
Christophe Detrembleur , Michael Claes , and Robert Jerome 1
Center for Educational and Research on Macromolecules (CERM), University of Liege, Sart-Tilman, B6,4000 Liege, Belgium Current address: Bayer A G , Leverkusen, Germany 2
Radical polymerization of alkyl methacrylates initiated by A I B N is controlled when conducted in the presence of a mixture o f N O / N O . Reaction of alkyl methacrylates with N O / N O leads indeed to the monomer adduct and parent α -nitro,ω-nitroso oligomers, which are precursors of nitroxides, known to control the radical polymerization of alkyl methacrylates, according to a "Nitroxide-Mediated Polymerization" (NMP) mechanism. Although some side reactions may occur with time (increasing polydispersity), polymerization of MMA initiated by AIBN at low temperature (60°C) after bubbling of N O / N O is relatively fast, and the molecular weight is dictated by the amount of N O / N O . Finally, for the first time, copolymerization o f MMA with HEMA (10/1; v/v) has been controlled by this mixture of N O and N O although the reaction remains very fast (ca. 65% monomer conversion after 5 h at 60°C), which is of prime importance for coating applications. 2
2
2
2
2
496
© 2003 American Chemical Society
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
497
Downloaded by UNIV OF GUELPH LIBRARY on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch035
Introduction
Nitroxide-Mediated Polymerization (NMP) is one of the three main strategies commonly used to control the radical polymerization (CRP) of a large range of monomers (1-3). The basic mechanism consists of the capture of the propagating radicals by nitroxides with formation of thermally labile alkoxyamines (Scheme 1). The dormant species are fragmented at high temperature (T > 100°C), and the released polymeric radicals add a limited number of monomers before recombination with nitroxides. Repetition of this homolysis-monomer addition-recombination cycle allows Macromolecules to grow without significant interruption by irreversible termination.
P
#
+
*0—N^
P—O—
R2
where
R2
is the propagating chain Scheme 1
Instead of using preformed nitroxides and alkoxyamines, that may be expensive, several research groups have contemplated the "in situ" formation of nitroxides from readily available and cheap precursors. In 1995, Matyjaszweski et al. (4) observed that radicals were trapped by nitroso compounds with formation of alkoxyamines. Recently, Nesvadba et al. patented the use of nitrones and nitroso compounds as control agents for the radical polymerization of vinyl monomers (5). The addition of these compounds to initiating and/or propagating radicals (Scheme 2) actually forms nitroxides able to mediate radical polymerization. Only one monomer, i.e., η-butyl acrylate (n-BuA), was considered by the authors, and low molecular weight poly(n-BuA) (M„ < 10000) with polydispersity ranging from 1.4 to 2.0 was formed. Later on, Grishin et al. showed that the polymerization of methyl methacrylate ( M M A ) initiated by azobisisobutyronitrile (AIBN) was controlled by the addition of either a nitrone, the C-phenyl-N-tert-butylnitrone (PBN) (6), or a nitroso compound, the 2methyl-2-nitrosopropane (7). The polydispersity was ca. 1.6 at low monomer conversion (< 10%) and increased up to 2.0 to 2.5 with the monomer conversion. Very high molecular weight (Mw) was also reported (10 to 3 10 ). In 2001, Catala at al. reported on the C R P of styrene in the presence of nitroso-tert-octane (8). In all these investigations, only one monomer was tested and no experiment of block copolymerization was considered. Finally, Detrembleur et al. successfully used an easily available nitrone (N-tert-butyl-aisopropylnitrone) for the controlled radical polymerization of styrene, styrene/acrylonitrile mixtures, and dienes (9). For the first time, well-defined 5
6
In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
498 0° R—Ν R" Nitroxide
Nitrone
Downloaded by UNIV OF GUELPH LIBRARY on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch035
R-N R* Nitroxide
poly(styrene)-b-poly(styrene-co-acrylonitrile), poly(styrene)-b-poly(n-butyl acrylate), and poly(styrene)-b-poly(isoprene) copolymers were synthesized in the presence of this nitrone used as a nitroxide precursor. In an effort to make the N M P process still more attractive, some of us proposed to use sodium nitrite as precursor of nitroxides (10,11). The radical polymerization of tert-butylmethacrylate (tBMA) in water was controlled by combining sodium nitrite with a reducing agent, e.g. iron(II) sulfate and ascorbic acid, at a relatively low temperature (80°C). Because t B M A is not miscible with water, droplets of the organic phase (tBMA, growing polymer chains and initiator) were dispersed in the water phase under vigorous stirring, and no gross precipitation of P t B M A was observed during polymerization. Nitroxides are thought to be formed by a three-step process (scheme 3): (i) formation of nitric oxide (NO) by reduction of sodium nitrite by iron(II) sulfate or ascorbic acid; (ii) trapping of radicals (initiating or propagating species) by N O with formation of the parent nitroso compounds (R-NO); (iii) these compounds are efficient spintraps, that react with radicals to form nitroxides (R -NO°). 2
FeS0
R
