Helix-Sense-Selective and Enantiomer-Selective Polymerization of a Chiral Methacrylate by Anionic and Free-Radical Mechanisms

Abstract

ABSTRACT:( ± )-1 O.ll-0-lsopropylidene-lrans-1 0.11-dihydroxy-5-phcnyl-1 O.ll-dihydro-5H-dibcnzo[a.d]cyclohcptcnc-5-ylmcthacrylate (IDPDMA) was synthesized and resolved into an optically pure form by chiral HPLC. Anionic polymerization of lDPDMA was carried out using the complexes of N.N'-diphcnylcthylcnediamine monolithium amide (DPEDA-Li) with 1 + )-1-(2-pyrrolidinylmethyl)pyrrolidine. ( + )-2.3-dimethoxy-1.4-bis(dimcthylamino )butane. and (-)-sparteine in toluene at 7R Cor using DPEDA-Li in tetrahydrofuran at -78 C. The polymers obtained from the racemic monomer using the chiral initi<Jtors and the polymer obtained from (+)-monomer under the achiral condition appeared to have a helical conformation with excess hclicity. The polymerizations of ( ± )-IDPDMA using the chiral initiators were enantiomcr-selectivc: (+)-monomer was preferentially polymerized. Free-radical polymerization of optically active IDPDMA having various enantiomeric excesses using in toluene at 60 C gave almost completely isotaetic polymers which probably had a helical conformation with excess hclicity. Enantiomer selection was also confirmed in the radical polymerization.KEY WORDS Methacrylate Radical Polymerization Helix Enantiomer Selection .Bulky methacrylates including triphenylmethyl methacrylate (TrMA) and 1-phenyldibenzosuberyl methacrylate (PDBSMA) afford single-handed helical. highly isotactic polymers by asymmetric anionic polymerization using chiral organolithium complexes as initiators (helixsense-selective polymerization), 1 · 2 and the polymers show chiral recognition ability toward a wide range of racemic compounds. · 1 · 4 PDBSMA is an especially interesting one among such bulky monomers because it gives an almost completely isotactic polymer (an equimolar mixture of right-and left-handed helices) even by free-radical polymerization regardless of reaction conditions'-" as well as by anionic polymerization 8 while free-radical polymerization of common vinyl monomers in solution generally has poor control of stereochemistry.9·1 ° Taking advantage of this high stereospecificity of the PDBSMA polymerization, we have succeeded in obtaining poly(PDBSMA)s with excess right-or left-handed helicity by free-radical mechanism using optically active initiators. chain transfer agents, solvents, or a free-radical metallic species. 7 · 11 In the present work. we synthesized racemic ( ± )-, which is a chiral analogue of PDBSMA. resolved it into an optically pure form by chiral HPLC, and investigated the stereochemistry of anionic and free-radical polymerizations of the racemic and the optically active monomers. In the polymerization PDBSMA IDPDMA 464 of a chiral bulky monomer, helix-sense selection based on the chirality of monomer itself and enantiomer selection can take place.u IDPDMA was designed based on the PDBSMA structure in search of monomers which enables helix-sense-selective and enantiomer-selective polymerization not only by anionic mechanism but also by free-radical mechanism.EXPERIMENTAL M atcria!s PDBSMA was available from...

Stereocontrol in the free-radical polymerization of methacrylates with fluoroalcohols

“…It is known that very bulky monomers such as triphenylmethyl methacrylate and 1-phenyldibenzosuberyl methacrylate afford highly isotactic polymers by radical polymerization because of the helix formation of the growing chain. 9 In the IPMA and tBuMA systems, some of the growing species probably take a partial, temporary helical conformation because of the apparently very bulky side group bound by the fluoroalcohols, and an isotactic control mechanism may play a role in the polymerization along with the syndiotactic control assumed for the MMA and EMA systems. The balance of the two mechanisms may be determined by the sizes of the monomers and additives.…”

Chiral Polyolefins

“…Further work included the preparation and polymerization of enantiopure and racemic 10,11-O-isopropylidene-trans-10,11-dihydroxy-5-phenyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5-yl methacrylate (ID-PMA) (5b) with anionic and radical initiators. [85] The polymerization was stereoelective when anionically polymerized, and demonstrated stereoelective tendencies for radical polymerization. This was an impressive finding, because the only other polymer previously reported to be stereoelective for radical polymerization was phenyl-2-pyridyl-o-tolylmethyl methacrylate (PPyoTMA) (5c).…”

“…[84,85] The polymerization of (±)-trans-4,5-bis-((methacryloyloxy)diphenylmethyl)-2,2-dimethyl-1,3-dioxacyclopentane (5a), which is based on the chiral molecule (±)-trans-4,5-bis(hydroxydiphenylmethyl)-2,2-diemthyl-1,3-dioxacyclopentaneÐan efficient ligand for asymmetric synthesis [86±88] Ðyielded a highly isotactic polymer that had higher optical activity than the monomer with a different circular dichroism (CD) spectrum. [84] The changes in CD spectra were due to helical formation, which is in agreement with the findings discussed in Section 2 of this review.…”

Asymmetric Polymerization

“…Enantiomer selection was achieved by free -radical polymerization of 250 [433] or by radical cyclopolymerization of racemic 2,4 -pentanediyl dimethacrylates [434 -436] although the selectivity was much lower than that observed in the anionic polymerization. …”