8010 Macromolecules, Vol. 43, No. 19, 2010
Brignou et al.
After being cooled to room temperature, a saturated Na2SO4
aqueous solution was added, the mixture was filtered, and the
organic phase was separated. Removal of volatiles and distilla-
tion of the residue under a vacuum at 100 °C afforded (()-2-
methyl-1,4-butanediol as a colorless oil (5.8 g, 55.7 mmol, 74%).
1H NMR (200 MHz, CDCl3): δ 3.78-3.43 (m, J = 7 Hz, 4H,
CH2OH), 3.34 (s, 2H, OH), 1.81 (m, 1H, CHMe), 1.60 (m, J = 6
Hz, 2n H, CH2CH2CH), 0.94 (d, J = 7 Hz, 3H, CH3). 13C{1H}
NMR (125 MHz, CDCl3): δ 67.8 (CHMe), 60.6 (CHMe), 37.2
(CHMeCH2OH), 33.7 (CH2CH2CHMe), 17.2 (CH2CH2OH).
(()-5-Methyl-1,3-dioxepan-2-one (β-Me7CC). Pyridine (40.0
mL, 400 mmol, 6.0 equiv) was added dropwise over 1 h to a
solution of (()-2-methyl-1,4-butanediol (7.0 g, 67 mmol, 1.0
equiv) and triphosgene (10.0 g, 34 mmol, 0.5 equiv) in CH2Cl2
(500 mL) at -90 °C. After the mixture was stirred at -78 °C over
2 h, a saturated NH4Cl aqueous solution (200 mL) was added
quickly (the desired product is soluble in water) to extract the
product. The organic phase was dried over MgSO4, volatiles
were removed under a vacuum, and the resulting residue was
distilled quickly (to avoid polymerization by residual basic
impurities) under a vacuum (reduced pressure) at 100 °C, to
give β-Me7CC as a colorless oil (4.9 g, 57%). The carbonate was
stored under inert atmosphere at -30 °C. Lowering the amount
of pyridine (3 equiv) added over 30 min to a THF solution of the
diol, and reacting over 6 h from 0 to 23 °C gave a comparable
yield (57%). Attempts to exchange pyridine by antipyrine
(2 equiv) with its addition carried out at 45-50 °C, as initially
reported for the synthesis of R-substituted 7CCs,4 led in our
hands to maximal yields of 30%. 1H NMR (500 MHz, CDCl3):
δ 4.30 (m, J = 7 Hz, 1H, CH2CHHO), 4.16 (m, J = 7 Hz, 2H,
CH2CHHO, CHCHHO), 3.86 (m, J = 7 Hz, 1H, CHCHHO),
2.13 (m, J = 7 Hz, 1H, CH), 2.00 (m, J = 7 Hz, 1H,
CH2CHHCH), 1.66 (m, J = 7 Hz, 1H, CH2CHHCH), 1.01
(d, J = 7 Hz, 3H, CH3) (Figure S3, Supporting Information).
13C{1H} NMR (125 MHz, CDCl3): δ 155.0 (CdO), 75.1
(CHCH2O), 69.0 (CH2CH2O), 34.9 (CH2CH2CHMe), 32.1
(CHMe), 15.6 (CH3) (Figure S4, Supporting Information).
HRMS-ESIþ (m/z): 130.063 ([M]þ, calc: 130.0630). IR υC(O):
68.8 (OCH2Ph), 67.7 (OCH2(CH2)2CH(CH3)OH), 67.1 (CH2-
OC(O)O), 67.0 (OCH(CH3)CH2CH2CH2OH), 66.9 (OC(O)-
OCH2), 66.8 (O(CH2)3CH(CH3)OH), 35.2 (CH2CH2CH2CH-
(CH3)OH, CH(CH3)CH2CH2CH2OH), 32.1 (CH2CH2CH2-
CH(CH3)), 25.2 (CH2CH2CH2CH(CH3)OH, CH2CH2OH), 24.6
(CH2CH2CH2CH(CH3), CH(CH3)CH2CH2CH2O), 23.5 (CH-
(CH3)OH,), 19.6 (CH2CH2CH2CH(CH3)O) (Figure S7, Sup-
porting Information). BnO-[poly(β-Me7CC)]-H. 1H NMR (500
MHz, CDCl3): δ 7.38 (m, 5H, C6H5), 5.17 (s, 2H, CH2Ph), 4.19
(m, J = 6 Hz, 4nH, CH3CHCH2OC(O)O), 4.00 (m, J = 7 Hz,
4nH, OC(O)OCH2CH2CH(CH3)), 3.84 (t, J = 5 Hz, CH3-
CHCH2OH), 3.71 (m, J = 7 Hz, CH2CH2OH), 2.00 (m, J =
5 Hz, 2nH, CH3CHCH2OC(O)O), 1.84, 1.55 (m, J = 7 Hz, 4nH,
OC(O)OCH2CH2CH(CH3)CH2O), 1.02 (d, J = 6 Hz, 6nH,
CHCH3O) (Figure 7). 13C{1H} NMR (100 MHz, C6D6): δ 155.8
(CdO), 128.0 (C6H5), 72.3 (OC(O)OCH2CH(CH3)(CH2)2OH),
71.8 (CH(CH3)CH2OC(O)O), 67.2 (OCH2Ph), 66.0 (CH(CH3)-
CH2OH); 65.4 (CH(CH3)CH2CH2OC(O)O); 59.9 (OC(O)-
OCH2CH(CH3)CH2CH2OH), 36.0(OC(O)OCH2CH2CH(CH3)-
CH2OH; 32.6 (OC(O)O(CH2)2CH(CH3)CH2OH, OC(O)OCH2-
CH(CH3)(CH2)2OH); 32.2 (OC(O)OCH2CH(CH3)CH2CH2OH,
OC(O)O(CH2)2CH(CH3)CH2OH); 31.9 (OC(O)OCH2CH2-
CH(CH3)CH2O, OC(O)OCH2CH(CH3)CH2CH2O); 29.7 (OC-
(O)O(CH2)2CH(CH3)CH2O, OC(O)OCH2CH(CH3)(CH2)2O);
16.4, 16.2 (OC(O)O(CH2)2CH(CH3)CH2OH, OC(O)OCH2-
CH(CH3)(CH2)2OH) ; 16.0 (OC(O)OCH2CH(CH3)(CH2)2O,
OC(O)O(CH2)2CH(CH3)CH2O) (Figure S8, Supporting Infor-
mation).
Results and Discussion
Synthesis of the Monomers. R- and β-Methyl-tetramethy-
lene carbonates (R-Me7CC and β-Me7CC) were synthesized
from 1,4-pentanediol and 2-methyl-1,4-butanediol, respectively,
using regular academic (not green) routes, upon cyclization
with triphosgene in the presence of pyridine at -78 °C, as
initially reported by Burk and Roof for the synthesis of cyclic
carbonates from 1,2- and 1,3-diols (Scheme 3).11 Treatment
of the diols at such a temperature, lower than that reported
in the original preparation of R-Me7CC (45-50 °C),4 allowed
higher yields to be reached (at best 57%) upon minimizing
the side formation of oligocarbonates which is favored at
higher temperatures. β-Me7CC, first synthesized in the pres-
ent work, was isolated in slightly higher yields (up to 70%),
possibly because of the different position of the methyl group
which may sterically disfavor the formation of oligomers.
The use of antipyrine as the base in place of pyridine, under
the reaction conditions previously reported (45-50 °C, THF
or CHCl3), did not give satisfactory yields of pure 7CCs.
Note also that the chloroformate route commonly employed
for the synthesis of six-membered cyclic carbonates3 proved
to be inefficient in this case (see the Experimental Section).
Like other seven-membered tetramethylene carbonates,3
both monomers R-Me7CC and β-Me7CC are rather tricky
to manipulate and especially to purify because of their strong
ability to polymerize.
Polymerization of r-Me7CC and β-Me7CC. The ROP of
R-Me7CC and β-Me7CC was investigated with some well-
established metal-based catalysts, namely, the Lewis acid
Al(OTf)3,10f,g and the organometallic complexes [(BDIiPr)-
Zn(N(SiMe3)2)]10a-e,12 ((BDIiPr) = 2-((2,6-diisopropylphenyl)-
amido)-4-((2,6-diisopropylphenyl)-imino)-2-pentene] and
[(ONOOtBu)Y(N(SiHMe2)2)(THF)]13 (ONOOtBu = amino-
alkoxy-bis(phenolate)) (Scheme 4). These zinc and yttrium
catalysts were selected because of their high performances
in the stereoselective ROP of racemic lactide,12,13 and of
racemic β-butyrolactone.14 Representative results obtained
with these catalysts are reported in Tables 1 and 2. The more
recently unveiled organocatalysts such as amines, guanidines,
1750 cm-1
.
Typical Polymerization Procedure. [(BDIiPr)Zn(N(SiMe3)2)]
(10.0 mg, 15.5 μmol) was added to benzyl alcohol (1.6 μL, 15.5
μmol, 1 equiv) placed in toluene (0.1 mL) and stirred over 15 min
just prior to the addition of the monomer via a syringe (0.60 g,
4.6 mmol). The mixture was then stirred at the required tem-
perature over the appropriate time (reaction times were not
systematically optimized). The reaction was quenched with an
excess of an acetic acid solution (ca. 2 mL of a 1.74 mol/L solu-
tion in toluene). The resulting mixture was concentrated under a
vacuum and the conversion was determined by 1H NMR
analysis of the residue. This crude polymer was then dissolved
in CH2Cl2 and purified upon precipitation in cold methanol,
filtered, and dried under a vacuum. The final colorless polymer
was then analyzed by NMR and SEC. BnO-[poly(r-Me7CC)]-
H. 1H NMR (300 MHz, CDCl3): δ 7.40 (s, 10H, C6H5), 5.15 (s,
4H, CH2Ph), 4.79 (m, J = 6 Hz, (2nþ1)H, CH2CH(CH3)O),
4.14 (m, J = 6 Hz, (4nþ2)H, C(O)OCH2), 3.84 (m, 1H, CH-
(CH3)OH), 3.66 (t, J = 6 Hz, 2H, CH2OH), 1.71 (m, J = 6 Hz,
(8nþ6)H, CH2CH2CH2CH(CH3)), 1.52 (m, 2H, CH2CH2CH2-
CH(CH3)OH); 1.30 (d, J = 6 Hz, (3nþ3)H, CH3CHO), 1.23 (d,
J = 6 Hz, 3H, CH3CHOH) (Figure 5). 13C{1H} NMR (100 MHz,
C6D6): δ 160.8 (CdO), 128.1 (C6H5), 74.4 (OC(O)OCH(CH3)-
(CH2)3OH), 74.0 (CH2CH(CH3)OC(O)O), 73.8 (OC(O)OCH-
(CH3)(CH2)3), 68.9 (OCH2Ph), 67.6 (OC(O)OCH2(CH2)2CH-
(CH3)OH), 67.1 (CH2OC(O)O), 67.0 (CH2OH), 66.9 (OC(O)-
OCH2), 66.8 (CH(CH3)OH), 35.1 (CH2CH2CH2CH(CH3)OH),
32.0 (CH2CH2CH2CH(CH3)), 25.1 (CH2CH2CH2CH(CH3)OH,
CH2CH2OH), 24.6 (CH2CH2CH2CH(CH3)), 23.4 (CH(CH3)OH,),
19.6 (OC(O)OCH(CH3)). 13C{1H} NMR (125 MHz, CDCl3): δ
160.8 (CdO), 128.1 (C6H5), 74.4 (OC(O)OCH(CH3)(CH2)3OH),
74.0 (CH2CH(CH3)OC(O)O), 73.8 (O(CO)OCH(CH3)CH2),