Macromolecules
Article
To characterize stereoregularities of the polymers in detail,
new parameters, tacticity entropy and coherence number
(ncoh), have been introduced. In terms of the coherence
number, the reason why only trans-PCHCs of Pmeso ≥ 0.90 are
crystallizable has been elucidated. Inasmuch as the ncoh value
depends on Pmeso and increases sharply over Pmeso ≈ 0.9, if one
wishes to acquire highly crystalline trans-PCHCs, one must
raise Pmeso up to as close to unity as possible. Syndio-rich trans-
PCHC would be amorphous and possibly behave like an
elastomer if properly cross-linked.
METHODS
■
Synthesis of trans-DMCC. Methyl chloroformate (1.0 mL, 13
mmol) was added dropwise under dry nitrogen gas flow to trans-1,2-
cyclohexenediol (0.23 g, 2.0 mmol) and dry pyridine (5.0 mL) placed
in a three-necked flask dipped in an ice bath, and the mixture was
stirred at 0 °C for 1 h and at room temperature for 3 h.44
Diethyl ether (15 mL) was added to the reaction mixture and
rinsed thrice with water (10 mL), and the organic layer was extracted
and condensed at normal pressure at 40 °C to remove diethyl ether.
The remaining viscous liquid was dissolved in a mixture of n-hexane
(10 mL) and water (10 mL) and evaporated at reduced pressure, and
the residue was white powder, which was rinsed with water and dried
in vacuo at 40 °C for 2 h to yield trans-DMCC (yield, 0.24 g, 50%).
To avoid the formation of mono-substituted trans-methoxycarbony-
loxycyclohexanol, the molar ratio of methyl chloroformate to trans-
1,2-cyclohexanediol was set equal to 6.5:1. The byproducts of this
reaction were dimethyl carbonate and pyridinium chloride, which
were removed by evaporation at reduced pressure and by water
rinsing, respectively.
Figure 15. Dimer models with (a) meso and (b) racemo linkages in
the most stable conformation. The arrow expresses the end-to-end
vector, r. The dotted lines express intramolecular C−H···OC
hydrogen bonds.
SUMMARY
■
1H NMR (500 MHz, CDCl3, δ): 1.29−1.54 (m, 4H), 1.67−1.82
(m, 2H), 2.09−2.22 (m, 2H), 3.78 (s, 6H), 4.62−4.72 (m, 2H); 13C
NMR (126 MHz, CDCl3, δ): 23.0, 29.7, 54.7, 74.8, 155.0.
Conformational characteristics of PCHCs were investigated
with the two model compounds, trans-DMCC and cis-DMCC:
their conformers generated under the RIS approximation
underwent the structural optimization by the B3LYP-DFT
method, Gibbs free energies were derived from the MP2
Synthesis of cis-DMCC. cis-DMCC was synthesized in the same
manner as that employed for trans-DMCC with the exception of
usage of cis-1,2-cyclohexenediol in place of trans-1,2-cyclohexenediol
and a longer reaction time of 12 h. The product was yellowish powder
(yield, 25%).
calculations, and NMR vicinal H −1H and 13C −1H coupling
1
constants observed therefrom were analyzed. The bond
1H NMR (500 MHz, CDCl3, δ): 1.30−1.50 (m, 2H), 1.60−1.74
(m, 4H), 1.87−1.99 (m, 2H), 3.78 (s, 6H), 4.86−4.95 (m, 2H); 13C
NMR (126 MHz, CDCl3, δ): 21.3, 27.4, 54.7, 74.8, 155.1.
conformations of the CH−CH bond of trans-DMCC, being
1
determined from the H −1H vicinal coupling constants, were
in good agreement with those from the MO calculations. The
Synthesis of trans-DCHCC. Pyridine (4.38 mL), dissolved in
toluene (30 mL), was added dropwise under dry nitrogen flow to
bis(trichloromethyl)carbonate (4.39 g, 14.8 mmol) and toluene (52
mL) stirred by a magnetic stirrer in a four-necked flask dipped in an
ice bath, and then, dry cyclohexanol (3.60 g, 36.0 mmol) dissolved in
toluene (20 mL) was added dropwise. The mixture was stirred at
room temperature for 15 h, added to distilled water (60 mL), and
extracted twice with toluene (40 mL). The toluene solution was
rinsed twice with water (40 mL) and, furthermore, with saturated
saline. The organic layer was dried over anhydrous sodium carbonate
overnight, filtrated, and dried in vacuo at 25 °C for 3 h to yield
cyclohexane chloroformate (yield, 2.3 g, 39%)
3
Karplus equation for JCH of the 13C−O−C−1H bond
sequence of the carbonate group was formulated by DFT
3
calculations and used to calculate JCH values from the free
energies and dihedral angles that the MO computations
3
yielded. Consequently, the calculated JCH values agreed well
with the experimental observations.
The MO free energies thus established were applied to the
RIS calculations. The stereosequences of atactic PCHC chains
were arranged by Bernoulli trials, and isotactic, syndiotactic,
and in-between tacticities were generated according to the
Markov stochastic process. trans-PCHC strongly prefers a tg+t
conformation [(R,R)-monomer] in the O−CH−CH−O
bonds; the equatorial−equatorial orientation of the cyclo-
hexane ring is more stable by about −1 to −2 kcal mol−1 than
the axial−axial one. The unperturbed chain dimension, ⟨r2⟩0/
nl2, of trans-PCHC depends considerably on tacticity: isotactic,
28.95; syndiotactic 0.83 (in chloroform at 25 °C). cis-PCHC
exclusively adopts either g−g+g+ or g−g−g+ [(R,S)-monomer] in
the O−CH−CH−O bonds probably owing to intramolecular
C−H···OC hydrogen bonds. The tacticity dependence of
the configurational properties of cis-PCHC would be
completely opposite to that of trans-PCHC; the ⟨r2⟩0/nl2
values of isotactic and syndiotactic cis-PCHCs in chloroform
at 25 °C were evaluated to be 0.59 and 24.86, respectively.
Cyclohexane chloroformate (2.3 g) prepared as above was added
via a syringe to trans-1,2,-cyclohexanediol (0.46 g, 4.0 mmol) and
pyridine (5 mL) placed in a three-necked flask dipped in an ice bath.
The mixture was stirred at 0 °C for 1 h and then at room temperature
for 3 h. Diethyl ether (30 mL) was added to the reaction mixture and
rinsed thrice with water (20 mL), and the organic layer was dried over
anhydrous sodium carbonate overnight, filtrated, and condensed at
reduced pressure to remove diethyl ether. The remaining yellowish
liquid was dried in vacuo at 25 °C for 3 h, dissolved in a mixed solvent
of n-hexane and ethyl acetate (20:1 in volume), and subjected to
column chromatography on silica gel. The fraction of Rf = 0.19 was
collected and evaporated, and the residue was dried in vacuo at 25 °C
for 3 h to yield trans-DCHCC (yield, 0.47 g, 32%).
1H NMR (500 MHz, CDCl3, δ): 1.18−1.64 (m, 16H), 1.66−1.83
(m, 6H), 1.84−1.98 (m, 4H), 2.08−2.19 (m, 2H), 4.55−4.63 (sept,
K
Macromolecules XXXX, XXX, XXX−XXX
Yoshida, Naofumi
