Enantiomer-Selective Radical Polymerization of rac-2,4-Pentanediyl Dimethacrylate by 2,2’-Azobisisobutyronitrile/Copper(II) Trifluoromethanesulfonate/Chiral Diamine as Asymmetric Reverse Atom Transfer Radical Polymerization Initiating System

Abstract

KEY WORDSAtom Transfer Radical Polymerization (ATRP) / Enantiomer-Selective Polymerization / Cyclopolymerization / Asymmetric polymerization, such as asymmetric synthesis polymerization, helix-sense selective polymerization, and enantiomer-selective polymerization, is of significant interest from the viewpoint of the precise control of a polymerization system. In general, ionic and coordination polymerization systems were easily modified into chiral ones, so that they were used for preparing various types of asymmetric polymerizations, e.g., high enantiomer-selectivity was achieved for the anionic polymerization of the racemic α-monosubustitued benzyl methacrylate using the (-)-sparteine/Grignard reagent 1-3 and the ringopening polymerization of propylene sulfide with (-)-binaphthol/ZnEt 2 . 4,5 Although the radical polymerization is also available for asymmetric polymerization, little is known about the enantiomer-selective radical polymerization. 6,7 Recently, we reported the enantiomer-selective radical cyclopolymerization of rac-2,4-pentanediyl dimethacrylate (rac-1) using the atom transfer radical polymerization (ATRP) initiating system modified with chiral ligands, i.e., the methyl 2-bromoisobutyrate/CuBr/chiral amine ligand. 8 Matyjaszewski has expanded the ATRP initiating system into the "reverse" ATRP initiating system such as the 2,2 -azobisisobutyronitrile (AIBN)/copper(II) trifluoromethanesulfonate (Cu(OTf) 2 )/amine ligand. 9 Therefore the significant interest is to examine the "reverse" ATRP initiating system as a chiral one for the asymmetric polymerization. Here, we report the enantiomer-selective radical cyclopolymerization of rac-1 using the AIBN/Cu(OTf) 2 /chiral diamine ligand such as 6,6 -bis{(1R,4R)-hydroxy-1,7,7-trimethylbicyclo[2.2.1]heptan-2-endo-yl}-2,2 -bipyridine (3), (-)-sparteine (4), 3-bis[4 -(S )-isopropyloxazoline-2 -yl]pyridine (S -5), and 3-bis[4 -(R)-isopropyloxazoline-2 -yl]pyridine (R-5) as the asymmetric reverse ATRP initiating system, as shown in Scheme 1. Table I lists the results for the polymerization of rac-1 using the initiating system consisting of AIBN, Cu(OTf) 2 , and 3, 4, S -5, and R-5 as chiral ligands. All polymerizations homogeneously proceeded to yield gel-free polymers. The characteristic resonance due to the methacrylic groups was not observed in the 1 H and 13 C NMR spectra of the obtained polymers, indicating that the polymerization of rac-1 proceeded through a cyclopolymerization mechanism to afford the polymer essentially consisting of the cyclic repeating units, i.e., the extent of cyclization was ca. 100%. The number average molecular weights (M n s) of the resulting polymers ranged from 47900 to 59100 and the polydispersity indices (M w /M n s) were 1.41-2.53. RESULTS AND DISCUSSIONWhen 3 and S-5 were used as the chiral ligands, (2S ,4S )-2,4-pentanediyl dimethacrylate (SS-1) was predominantly polymerized, resulting in the fact that the enantiomeric excess (e.e.) of the recovered monomer for 3 was 4.2% for a monomer conversion of 22.8% and that for S...

Stereospecific Living Radical Polymerization: Dual Control of Chain Length and Tacticity for Precision Polymer Synthesis

“…In contrast to no effects of the chiral ligands on the tacticity, interesting results were reported for the enantiomer-selective radical cyclopolymerization of racemic dimethacrylate by the metal catalysts with chiral ligands or additives (Figure ). − A racemic mixture of ( R , R )- and ( S , S )-2,4-pentanediyl dimethacrylate (CM- 1 ) was polymerized with CuBr in the presence of chiral bi- or tridentate nitrogen-based ligands in anisole to give optically active polymers with controlled molecular weights via cyclopolymerization, in which the enantiomeric excess of the recovered monomer was 6.8−15.3%. A similar enantiomer-selective radical cyclopolymerization was also achieved by RuCl 2 (PPh 3 ) 3 in the presence of chiral additives to provide an enantiomeric excess of 16.9−22.6%, in which the growing chain ends of the ( R , R )- and ( S , S )-monomer units preferentially reacted with the ( R , R )-monomer.…”

Asymmetric Polymerization

“…[ α ] 435 [39] , lithium ( S ) -(+) -1 -(2 -pyrrolidinylmethyl)pyrrolidinylamide [39] , BuLi/bisoxazolines ( 6 , 7b ) [36,40 -44] , Et 2 Zn/bisoxazolines [41 -43,45,46] , lithium oxazolidinylamide ( 8 ) [42,47] , and Co(II) -salen complex ( 9 ) [48 -50] have been reported to promote the asymmetric polymerization of 5 . The polymer from N -1 -naphthylmaleimides showed a remarkably high optical rotation presumably due to not only confi gurational chirality in the main chain but also a conformational one based on higher -order structures [41,43,45] .…”

Enantiomer-Selective Radical Polymerization of Bis(4-vinylbenzoate)s with Chiral Atom Transfer Radical Polymerization Initiating Systems

“…Wulff and co-workers , and Kakuchi and co-workers , have established a methodology to provide main-chain regulated polymers through the radical cyclopolymerization of a bifunctional monomer comprised of a chiral auxiliary. We have demonstrated that a racemic mixture of bifunctional methacrylates was polymerized by atom transfer radical polymerization (ATRP) using chiral ligands, and the system resulted in significant enantiomer selectivities. − Thus, the combination of cyclopolymerization and chiral initiating systems can provide the opportunity for the enantiomer-selective radical polymerizations due to the integrity and guidance of cyclization and chiral initiators. Progress toward increasingly stereospecific radical polymerizations through cyclopolymerization can be underscored by the design of monomers and ATRP ligands as well as the fundamental insight into the cyclopolymerization effects on the enantiomer selectivity.…”