One of the authors discovered helix-sense-selective polymerization (HSSP) of an achiral substituted phenylacetylene which has two hydroxymethyl groups and a relatively hydrophobic substituent by ...using a chiral catalytic system in 2003. The one-handed helicity is stable and static in nonpolar solvents because it is maintained by intramolecular hydrogen bonds. Since the resulting polymer has extremely tight helicity in its main chain, it shows many interesting and unusual properties including highly selective photocyclic aromatization (SCAT). In this review, the discovery and development of the HSSP and SCAT reactions, and the properties and application of the resulting polymers prepared by HSSP and the resulting supramolecular polymers prepared by SCAT will be examined.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The living cationic polymerizations of sulfide-containing vinyl ethers, namely, alkyl-sulfide-containing 2-(ethylthio)ethyl vinyl ether
(ESEVE)
and arylsulfide-containing 2-(phenylthio)ethyl and ...2-4-(methylthio)phenoxyethyl vinyl ethers (
PSEVE
and
MSPEVE
, respectively), were examined using HCl/ZnCl
2
, 1-(isobutoxy)ethyl acetate (IBEA)/Et
1.5
AlCl
1.5
, and IBEA /Et
1.5
AlCl
1.5
/SnCl
4
as initiators. The polymerization of
ESEVE
terminated at a 35% monomer conversion, and a high-molecular-weight polymer was not obtained; instead, the nucleophilicity of the sulfur moiety in
ESEVE
led to cyclization, forming five-membered ring sulfonium species. In contrast, the side reactions were suppressed when
PSEVE
was used, which has a less-nucleophilic sulfur moiety on its benzene ring, and polymerization proceeded to an 88% conversion, affording a high-molecular-weight polymer with a narrow molecular weight distribution (
M
w
= 12,500,
M
w
/
M
n
= 1.18). Furthermore, cyclic sulfonium was not produced during the polymerization of
MSPEVE
, in which the sulfur atom was farther from the generated carbocation; the polymerization proceeded quantitatively producing a high-molecular-weight polymer with a narrow molecular weight distribution, despite the short lifetime of the growing species. In particular, a high-molecular-weight polymer with the narrowest distribution (
M
w
= 13,500,
M
w
/
M
n
= 1.08) was obtained when SnCl
4
was used as the Lewis acid.
To develop new phenylacetylene monomers more suitable for helix-sense-selective polymerization (HSSP) we reported previously and to improve the efficiency of the HSSP and membrane performance of the ...resulting polymers, novel phenylacetylenes having a flexible oligosiloxanyl group (SnBDHPA) together with the other related three series of monomers were synthesized and polymerized by using a chiral catalytic system and enantioselectivity in permeation of the membranes from the resulting chiral polymers were examined. SnBDHPA was the most suitable for the HSSP and the CD absorptions (G values) of poly(SnBDHPA) were stronger and more stable than those of the corresponding polymers having rigid alkyl groups. The polymers could be fabricated to flexible self-supporting membranes by using solvent-casting method. In addition, enantioselectivity in permeation of one of poly(SnBDHPA) membranes was much higher than that of a poly(phenylacetylene) membrane having alkyl groups. This was because the polymers having oligosiloxane groups had high regularity of structures, i.e., chemical structures of the macromolecules such as one handedness and high order structures such as columnar contents in the membranes, and the membranes were flexible and had almost no defects. These good properties as optical resolution membrane materials were caused by flexibility, hydrophobicity, and bulkiness of the oligosiloxane chains. S3BDHPA having a trisiloxanyl group was found to be the best monomer for the HSSP and for obtaining good optical resolution membrane materials.
The Ru‐catalyzed dehydrogenative silylation of 2‐alkyloxazolines with 1,1,1,3,5,5,5‐heptamethyltrisiloxane took place site‐selectively at methyl C(sp3)−H bonds located γ to the nitrogen atom of the ...oxazolyl groups. Pyridine and pyrazole rings could also be used as a directing group. A catalytic mechanism based upon successive σ‐bond metathesis is proposed.
Chelate‐assisted C(sp3)−H bond activation: The dehydrogenative silylation of 2‐alkyloxazolines with 1,1,1,3,5,5,5‐heptamethyltrisiloxane took place site‐selectively at methyl C(sp3)−H bonds located γ to the nitrogen atom of the oxazolyl groups.
Three novel chiral phenylacetylenes having an octyloxyethanolamine residue derived from a l-aminoalcohol and two hydroxymethyl groups were synthesized and polymerized by two achiral catalysts ...((nbd)Rh+η6-(C6H5)B−(C6H5)3 and Rh(nbd)Cl2/triethylamine (TEA)) and a chiral catalytic system (Rh(nbd)Cl2/(S)- or (R)-phenylethylamine ((S)- or (R)-PEA)). All of the resulting polymers showed Cotton effects at wavelengths around 430 nm. This observation indicated that they had an excess of one-handed helical backbones. Positive and negative Cotton effects were observed for the polymers having an l-valinol residue produced by using (S)- and (R)-PEA as a cocatalyst, respectively, although the monomers had the same chirality. The two polymers having an l-alaninol or l-phenylalaninol residues obtained by using (S)- and (R)-PEA as a cocatalyst showed CD absorptions with identical signs. Therefore, we found that the chiral monomer having an l-valinol residue was suitable for both modes of asymmetric polymerization, that is, helix-sense-selective polymerization (HSSP) with the chiral catalytic system and asymmetric-induced polymerization ( AIP ) with the achiral catalysts. However, the other two monomers having an l-alaninol or l-phenylalaninol residue were not suitable for HSSP because the helix sense could not be controlled by the chirality of PEA. To explain the unexpected behaviors in the asymmetric polymerizations of the two chiral monomers having a chiral bidentate ligand, a novel third mechanism of asymmetric polymerization, that is, self-helix-sense-selective polymerization (SHSSP), is proposed in this Article. This Article discusses the contribution of the three mechanisms (AIP, HSSP, and SHSSP) in asymmetric polymerizations of the three monomers.
By using a new achiral phenylacetylene having bulky substituents as a monomer, living-like helix-sense-selective polymerization (HSSP) has been achieved. This is the first example of the HSSP of ...substituted acetylenes where the degree of polymerization was controlled. The resulting one-handed helical living polymer initiated polymerization of a second monomer to successfully yield a block copolymer.
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