Table 2 Random copolymerizations of TMC with 1-derived mono-
mers
c
Conversion Conversion
Mn
/
Mw/
c
Comonomera Time/h TMCb (%) comonomerb (%) g mol21 Mn
1b
1g
1h
1i
a
1
4
1
4
1
4
1
4
61
95
46
91
49
92
60
91
.95
.95
.95
.95
.95
.95
.95
.95
4700
6500
3900
5700
5300
7800
6100
8600
1.10
1.11
1.07
1.07
1.09
1.11
1.05
1.05
Conditions: 1 M monomer in CH2Cl2, 80 : 20 TMC : comonomer
(mol : mol), 0.02 M PyBuOH, 0.05 M TU, 0.05 M DBU, 20 uC.
b
c
By 1H NMR spectroscopy. By GPC vs. polystyrene standards,
uncorrected.
for the homopolymerizations of TMC and 1a described above
(Table 2). Monitoring experiments (1H NMR spectroscopy) reveal
that all of the 1b–1i comonomer was incorporated into polymer
within 1 h, while the conversion of TMC lagged and did not reach
.90% conversions until after 3 h. The relative reactivities match
the higher reactivity found for 1a vs. TMC in homopolymeriza-
tion, and suggest that gradient copolymers are formed. The
observation that these polymerizations continue on after complete
consumption of the faster-reacting monomer contrasts with
behavior we have observed for random copolymerizations of
lactones, in which the faster reacting monomer reacts exclusively
under organocatalytic conditions.12 Block copolymers of different
carbonate repeat units can also be constructed by sequential
polymerization: for instance, following in situ formation of a
PTMC macroinitiator (conditions as per Table 1; [M]0/[I]0 = 37.5,
TU–DBU, 3 h; 86% conversion, Mn = 5900, PDI = 1.03),
monomer 1a was added to the reaction solution, and after 30 min
the chain-extended polymer was obtained (conversion = 92%
(TMC), 88% (1a); Mn = 9700, PDI = 1.08).
Scheme 3 Monomers synthesized via procedures in Scheme 2.
sic catalyst 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) showed high
conversions of 1a to polymer in relatively short times. Good
control over the molecular weight and polydispersity was achieved
with the TU–DBU co-catalyst, while TBD is more active and its
use leads to broadening of the polydispersity via transcarbonation
of the polymer chains. No scrambling of the pendant benzyl ester
into the poly(1a) chains could be observed using 1H-NMR
spectroscopy. While excessively bulky substituents (e.g. 2,2-
diphenyl) impede the ring-opening of six-membered cyclic
carbonates,8a,11 the increased rate of polymerization for 1a when
compared to TMC indicates that the methyl and carboxylate
substituents are well-suited for polymerization. The location of
steric bulk distant from the polymerizing carbonate also avoids
interference with the organocatalysts; substituents in the a-position
of cyclic ester monomers dramatically reduce the rates of
polymerization when organocatalysts are used, making them
incompatible with effective derivatization strategies using a-chloro
and a-azido groups.
In summary, a general synthetic route to incorporate a broad
range of functional groups into cyclic carbonate monomers has
been developed. These monomers can be polymerized under mild,
one-pot conditions to create random or block copolymers. Further
studies are underway to complete procedures for full deprotection
and derivatization of polar sidegroups once they are incorporated
into polymers.
Random copolymerizations of the cyclic carbonates with TMC
were conducted using organocatalytic procedures similar to those
Notes and references
Table 1 Organocatalytic polymerizations of 1a vs. TMC
1 T. Mathisen, K. Masus and A.-C. Albertsson, Macromolecules, 1989,
22, 3842; K. Stridsberg and A.-C. Albertsson, J. Polym. Sci., Part A:
Polym. Chem., 1999, 37, 3407; R. K. Srivastava and A.-C. Albertsson,
J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 4206.
2 M. Trollsa˚s, V. Y. Lee, D. Mecerreyes, P. Lo¨wenhielm, M. Mo¨ller,
R. D. Miller and J. L. Hedrick, Macromolecules, 2000, 33, 4619.
3 B. Parrish, J. K. Quansah and T. Emrick, J. Polym. Sci., Part A: Polym.
Chem., 2002, 40, 1983; B. Parrish and T. Emrick, Macromolecules, 2004,
37, 5863; B. Parrish, R. B. Breitenkamp and T. Emrick, J. Am. Chem.
Soc., 2005, 127, 7404; B. Parrish and T. Emrick, Bioconjugate Chem.,
2007, 18, 263.
4 R. Riva, S. Schmeits, F. Stoffelbach, C. Jerome, R. Jerome and
P. Lecomte, Chem. Commun., 2005, 5334; P. Lecomte, R. Riva,
S. Schmeits, J. Rieger, K. Van Butsele, C. Jerome and R. Jerome,
Macromol. Symp., 2006, 240, 157; R. Riva, S. Schmeits, C. Jerome,
R. Jerome and P. Lecomte, Macromolecules, 2007, 40, 796;
H. Li, R. Riva, R. Jerome and P. Lecomte, Macromolecules, 2007,
40, 824.
Monomera Catalystb Time/h Conv.c (%) Mnd/g mol21 Mw/Mn
d
1a
TU–DBU 0.5
93
94
95
11 600
11 500
12 900
13 200
8000
1.12
1.15
1.20
1.52
1.76
TU–DBU
TU–DBU
TBD
1
2
5 min 95
1
TBD
96
TMC
TU–DBU
TU–DBU
TU–DBU
TBD
1
2
3
45
74
90
4400
7300
8600
8900
11 000
1.03
1.03
1.03
1.08
1.31
5 min 98
98
TBD
1
a
Conditions: 1 M monomer in CH2Cl2 with 0.02 M PyBuOH, 20 uC.
TU–DBU: both 0.05 M; TBD: 0.01 M.
b
c
By 1H NMR
d
spectroscopy. By GPC vs. polystyrene standards, uncorrected.
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Chem. Commun., 2008, 114–116 | 115