Organocatalytic synthesis and postpolymerization functionalization of allyl-functional poly(carbonate)s

Article abstract of DOI:10.1021/ma102882v

Well-defined allyl-functional poly(carbonate)s were synthesized via the organocatalytic ring-opening polymerization of 5-methyl-5-allyloxycarbonyl-1,3- dioxan-2-one using the dual 1-(3,5-bis(trifluoromethyl)phenyl)-3- cyclohexylthiourea and (-)-sparteine catalyst system. The resulting allyl-functional poly(carbonate)s obtained showed low polydispersities and high end-group fidelity, with the versatility of the system being demonstrated by the synthesis of block copolymers and telechelic polymers. Further functionalization of homopolymers with degrees of polymerization of 11 and 100 were realized via the radical addition of thiols to the pendant allyl functional groups, resulting in a range of functional aliphatic poly(carbonate)s.

Full text of DOI:10.1021/ma102882v

ARTICLE
pubs.acs.org/Macromolecules
Organocatalytic Synthesis and Postpolymerization Functionalization
of Allyl-Functional Poly(carbonate)s
Sarah Tempelaar,† Laetitia Mespouille,‡ Philippe Dubois,‡ and Andrew P. Dove*,†
†Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K.
‡Laboratory of Polymeric and Composite Materials (LPCM), Center of Innovation and Research in Materials and Polymers (CIRMAP),
University of Mons (UMONS), 20, Place du Parc, 7000 Mons, Belgium
S Supporting Information
b
ABSTRACT: Well-defined allyl-functional poly(carbonate)s
were synthesized via the organocatalytic ring-opening polym-
erization of 5-methyl-5-allyloxycarbonyl-1,3-dioxan-2-one
using the dual 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohex-
ylthiourea and (-)-sparteine catalyst system. The resulting
allyl-functional poly(carbonate)s obtained showed low poly-
dispersities and high end-group fidelity, with the versatility of the system being demonstrated by the synthesis of block copolymers
and telechelic polymers. Further functionalization of homopolymers with degrees of polymerization of 11 and 100 were realized via
the radical addition of thiols to the pendant allyl functional groups, resulting in a range of functional aliphatic poly(carbonate)s.
’ INTRODUCTION
offers great versatility to tune the polymer functionality, the
preparation and ROP of a functional monomer that can be
functionalized postpolymerization in an efficient manner would
provide an opportunity to yield a wide range of poly(carbonate)s
with pendant functionalities including those that may not be
compatible with the ROP process.12-15 Notably, there are several
reports in which degradable polymers bearing alkyne/azide16-19 or
(meth)acrylate20-23 groups have been functionalized utilizing
efficient chemistries. Chen et al. recently reported the synthesis
and copolymerization of (meth)acrylate-functional cyclic carbonate
monomers with either L-lactide or ε-caprolactone using tin-based
catalysts at high temperatures.23 Michael addition of a range of thiol-
containing molecules (in excess) occurred readily to the acrylate-
functional polymers with the conversion of these groups ranging
between 30 and 100% depending on the substrate.
As a consequence of their unsaturation, allyl groups provide a
versatile option for ready functionalization via radical addition of
thiols, epoxidation, halogenation, and hydroboration, among
others.13,14,24-28 The introduction of a pendant carbon-carbon
double bond in a poly(carbonate) backbone has previously been
reported via the ring-opening polymerization of an allyl ester
functional cyclic carbonate, 5-methyl-5-allyloxycarbonyl-1,3-
dioxan-2-one (MAC).29-32 Previously reported homopolymeriza-
tions of MAC were either performed without added catalyst at
elevated temperature (at 115 °C; Mn = 13 700 g mol-1; PDI =
1.8),31 in toluene at 95 °C using tin(II)-based poly(lactide)
macroinitiator ([M]/[I] = 40; Mn = 9200 g mol-1; PDI = 2.5),31 or
at 110 °C catalyzed by diethylzinc ([M]/[I] = 200; Mn = 19 000 g
mol-1; PDI = 1.61);29 however, in all cases polymerizations showed
Aliphatic poly(carbonate)s are excellent candidates for phar-
maceutical applications as a consequence of their low toxicity,
biocompatibility, and biodegradability. Tailoring of the poly-
(carbonate) structure by manipulation of pendant functionality
on the polymer backbone enables precise control over the
physical properties of the polymer or the addition of biologically
active molecules. Poly(carbonate) synthesis is commonly rea-
lized either by the copolymerization of epoxides with carbon
dioxide or by the ring-opening polymerization (ROP) of cyclic
carbonate monomers.1-6 The synthesis of poly(carbonate)s via
CO2/epoxide copolymerization is an exciting method of utilizing
CO2; however, the formation of five-membered cyclic carbonate
byproduct, the presence of ether linkages, and the use of air-
sensitive coordination compounds make its application synthe-
tically challenging. Despite the requirement to synthesize the
monomers, the ROP of cyclic carbonate monomers has received
significantly more attention and consequently provides an
attractive and versatile methodology for the synthesis of functio-
nalized poly(carbonate)s that has been studied by many groups.
Monomer synthesis can be achieved in a number of ways
although commonly involves multistep syntheses using toxic
phosgene derivatives.5,6 Recent examples from Hedrick and co-
workers have demonstrated versatile routes to cyclic carbonate
monomers via efficient routes from a single core scaffold based
around 2,2-bis(hydroxymethyl)propionic acid (bisMPA) with
the most recently reported approach being achieved in just two
steps while avoiding the use of phosgene.7,8 These routes have
been used to prepare and subsequently polymerize a wide range
of functional monomers including carbohydrate-,9 RAFT CTA-,10
and poly(ethylene glycol)-functional monomers,11 with the
latter being applied in hydrogel synthesis. While the synthesis of
functional monomers from a single tunable monomer scaffold
Received: December 22, 2010
Revised:
February 7, 2011
Published: March 15, 2011
r
2011 American Chemical Society
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Scheme 1. (a) Synthesis and (b) Ring-Opening Polymeriza-
tion of 5-Methyl-5-allyloxycarbonyl-1,3-dioxan-2-one
(MAC)a
Table 1. Catalyst Screening for the Ring-Opening Polymer-
ization of MACa
catalyst(s)
time (h)
monomer conv (%)b
Mnc (g mol-1
)
PDIc
1d
4
51.5
768
70
90
92
94
88
92
10700
4600
1.80
1.12
1.13
1.12
1.49
2e þ 3f
2e þ 4f
2e þ 5f
1d þ 3f
4200
3530
4
10470
a Reactions were performed in CDCl3 at 25 °C, [MAC] = 2.0 M,
[M]/[I] = 20 using benzyl alcohol as the initiator. b Measured by H
1
NMR spectroscopy. c Determined by GPC analysis in THF. d Neopen-
tanol was used as initiator with 5 mol % DBU. e 5 mol % (-)-sparteine, 2.
f 10 mol % cocatalyst (3-5).
stored under inert atmosphere. CDCl3, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), (-)-sparteine, R,R,R0,R0-tetrakis(trifluoromethyl)-1,3-benzenedi-
methanol (5), and dithiodiethanol were dried over CaH2, distilled,
degassed, and stored under inert atmosphere. Benzyl alcohol and 1,3-
propanediol were dried and stored over 4 Å molecular sieves under inert
atmosphere. 2-Hydroxyethyl disulfide was purchased from Aldrich and
purified by distillation from CaH2. Methylene chloride was purified over an
Innovative Technology SPS alumina solvent column and degassed before
use. 5-Methyl-5-allyloxycarbonyl-1,3-dioxan-2-one (MAC) was synthesized
as reported,32 recrystallized several times before use, and dried over CaH2 in
dry THF at 50-60 °C. Thiourea catalysts 3 and 4 were synthesized as
previously reported45 and then dried over calcium hydride in dry tetra-
hydrofuran and recrystallized from dry methylene chloride. Silica gel (pore
size = 40 Å) was obtained from Fisher Scientific and used as received. All
other solvents and chemicals were obtained from Sigma-Aldrich or Fisher
Scientific and used as received.
a Conditions: (i) allyl bromide, KOH, DMF, 100 °C for 1 h and then
45 °C for 16 h; (ii) ethyl chloroformate, THF, NEt3, 0 °C for 16 h.
General Considerations. Polymerizations were performed under
inert atmosphere in a glovebox. Polymer functionalizations were carried
out under oxygen-free conditions using standard Schlenk-line techni-
ques. 1H NMR and 13C NMR spectra were recorded on a Bruker DPX-
300, DPX-400, DRX-500, or AV II-700 spectrometer at 293 K. Chemical
shifts are reported as δ in parts per million (ppm) and referenced to the
residual solvent signal (CDCl3: 1H, δ = 7.26 ppm; 13C, δ = 77.16 ppm;
(CD3)2SO: 1H, δ = 2.50; 13C, δ = 39.52). Mass spectra were acquired by
MALDI-TOF (matrix-assisted laser desorption ionization-time of
flight) mass spectrometry using a Bruker Daltonics Ultraflex II MAL-
DI-ToF mass spectrometer, equipped with a nitrogen laser delivering
2 ns laser pulses at 337 nm with positive ion ToF detection performed
using an accelerating voltage of 25 kV. Solutions of trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propylidene]malonitrile (DCTB) as a matrix
(0.3 μL of a 10 g L-1 solution in methylene chloride), sodium
trifluoroacetate as a cationization agent (0.3 μL of a 10 g L-1 solution
in methylene chloride), and analyte (0.3 μL of a 1 g L-1 solution in
methylene chloride) were applied sequentially to the target followed by
solvent evaporation to prepare a thin matrix/analyte film. The samples
were measured in linear and reflectron ion mode and calibrated by
comparison to 2 ꢀ 103 and 5 ꢀ 103 g mol-1 poly(ethylene oxide)
monomethyl ether standards. Gel-permeation chromatography (GPC)
was used to determine the molecular weights and polydispersities of the
synthesized polymers. GPC in THF was conducted on a system
composed of a Varian 390-LC-Multi detector suite fitted with differ-
ential refractive index (DRI), light scattering (LS), and ultraviolet (UV)
detectors equipped with a guard column (Varian Polymer Laboratories
PLGel 5 μM, 50 ꢀ 7.5 mm) and two mixed D columns (Varian Polymer
Laboratories PLGel 5 μM, 300 ꢀ 7.5 mm). The mobile phase was either
tetrahydrofuran eluent or tetrahydrofuran with 5% triethylamine eluent
at a flow rate of 1.0 mL min-1, and samples were calibrated against
Varian Polymer Laboratories Easi-Vials linear poly(styrene) standards
(162-3.7 ꢀ 105 g mol-1) using Cirrus v3.3. GPC in DMF was
Figure 1. Organic catalysts screened for the ring-opening polymeriza-
tion of MAC.
relatively poor control and resulted in branched polymers.
Studies involving MAC or similar monomers have mostly
focused on its copolymerization with lactide and include amphi-
philic block copolymers of PMAC-co-PLA and poly(ethylene
oxide) to form micelles and polycations from poly(ethylenimine)-
grafted polycarbonate.29-37
A wide range of catalytic species have been studied in ROP
reactions including cationic, anionic, coordination-insertion,
enzymatic, and organocatalytic methods.38-40 Recent advances,
especially in organic catalysis, have led to marked improvements
in selectivity and polymerization control.40-46 Herein, we report
the utilization of these highly specific organocatalysts for the
controlled ROP of the allyl ester-functional cyclic carbonate,
MAC, and describe its postpolymerization functionalization via
radical addition of thiols to the pendant allyl groups, giving rise to
a range of functional poly(carbonate)s through mild, versatile,
and selective routes.
’ EXPERIMENTAL SECTION
Materials. rac-Lactide was purified from dry methylene chloride and
sublimed twice before use and stored under inert atmosphere. Poly-
(ethylene oxide) methyl ether was dried in a desiccator over P2O5 and
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phenyl)-3-cyclohexylthiourea (10 mol % to monomer) catalyst were
weighed and dissolved in dry CDCl3 or dry methylene chloride. MAC
was dissolved separately in the same solvent and added to the initiator/
catalyst solution. After the desired time, the polymerizations were
precipitated directly into hexanes. 1-(3,5-Bis(trifluoromethyl)phenyl)-
3-cyclohexylthiourea and (-)-sparteine impurities were removed by
column chromatography on silica gel in hexanes/ethyl acetate (4:1).
The polymer containing fractions were concentrated then dissolved in a
minimal amount of methylene chloride and precipitated into hexanes.
1H NMR (700 MHz, CDCl3) δ 7.37 (m, OBn-ArH), 5.87 (m, CHvinyl),
5.37-5.20 (m, CH2-vinyl), 5.15 (s, OBn-CH2), 4.63 (m, OCH2CH-
CH2), 4.45-4.24 (m, OC(O)OCH2), 3.76-3.69 (m, CH2OH), 2.46 (t,
OH), 1.27 (s, CH3), 1.22 (s, C(CH3)CH2OH). 13C NMR (500 MHz,
CDCl3): δ 171.7 (CC(O)O), 154.4 (OC(O)O), 135.0 (ArC), 131.6
(CHvinyl), 128.6 (ArC), 118.5 (CH2-vinyl), 68.6 (OC(O)OCH2), 68.4
(CH2OH), 65.9 (OCH2CHCH2), 46.5 (CCH3), 17.5 (CH3). GPC
(THF, RI): Mn (PDI) = 3440 g mol-1 (1.15). GPC (DMF, RI): Mn
(PDI) = 3680 g mol-1 (1.10).
Figure 2. Plot of number-average molecular weight (Mn) against %
monomer conversion in the ring-opening polymerization of MAC.
Conditions: [MAC] = 2.0 M CDCl3 at 25 °C, 5 mol % 2, 10 mol % 3
[M]/[I] = 20 using benzyl alcohol as the initiator.
Synthesis of Block Copolymers. PEO-b-PMAC block copoly-
mer was synthesized using the above-mentioned method with commer-
cially available poly(ethylene oxide) monomethyl ether as initiator
([M]/[I] = 20).
For the synthesis of PLA-b-PMAC block copolymers, benzyl alcohol,
(-)-sparteine, and thiourea were weighed and dissolved in dry CDCl3.
rac-Lactide (or MAC) was dissolved separately in CDCl3 and added to
the initiator/catalyst solution. After 2 h (>90% conversion), a small
aliquot was taken from the mixture and used for GPC analysis, and the
reaction mixture was poured in a vial containing MAC (or rac-lactide).
The polymerization was carried out until >90% conversion was reached.
The mixture was then precipitated in hexanes, purified by column
chromatography on silica gel in hexanes/ethyl acetate (4:1). The
polymer containing fractions were concentrated and then dissolved in
a minimal amount of methylene chloride and precipitated into hexanes.
MeO-PEO114-PMAC20-OH. 1H NMR (300 MHz, CDCl3): δ 5.89
(CHvinyl), 5.35-5.19 (CH2-vinyl), 4.63 (OCH2CHCH2), 4.45-4.23
(OC(O)OCH2), 3.64 (OCH2CH2), 3.37 (OCH3), 2.49 (t, OH), 1.27
(s, CH3), 1.22 (s, C(CH3)CH2OH). GPC (THF, RI): Mn (PDI) =
11 900 g mol-1 (1.06).
BnO-PMAC16-PLA23-OH. 1H NMR (400 MHz, CDCl3) δ 7.37
(OBn-ArH), 5.89 (CHvinyl), 5.42-5.11 (CH2-vinyl, PLA-CH and
OBn-CH2), 4.63 (OCH2CHCH2), 4.41-4.25 (OC(O)OCH2 and
CHOH), 1.57 (PLA-CH3), 1.27 (s, PMAC-CH3). GPC (THF, RI):
Mn (PDI) = 6620 g mol-1 (1.28).
Figure 3. Plot of number-average molecular weight (Mn) against initial
monomer-to-initiator ratio, [M]0/[I]0, in the ring-opening polymeriza-
tion of MAC. Conditions: [MAC] = 2.0 M CDCl3 at 25 °C, 5 mol % 2,
10 mol % 3 using benzyl alcohol as the initiator.
BnO-PLA20-PMAC20-OH. 1H NMR (400 MHz, CDCl3) δ 7.31
(OBn-ArH), 5.83 (CHvinyl), 5.35-5.07 (CH2-vinyl, PLA-CH and O-
Bn-CH2), 4.59 (OCH2CHCH2), 4.41-4.23 (OC(O)OCH2), 3.68
(CH2OH), 1.53 (PLA-CH3), 1.23 (PMAC-CH3), 1.19 (C(CH3)
CH2OH). GPC (THF, RI): Mn (PDI) = 7130 g mol-1 (1.17).
Postpolymerization Functionalization of PMAC. Stock solu-
tions of polymer and AIBN (20%) in dioxane (50 mg polymer/mL) and
stock solutions of thiol in dioxane were prepared and degassed prior to the
reactions. In a typical experiment, an ampoule was charged with 0.78 mL of
polymer/AIBN stock solution (i.e., 38.8 mg polymer), after which the
dioxane was removed in vacuo. Under a nitrogen atmosphere, 0.15 mL of
the degassed benzyl mercaptan stock solution (3.68 M) was added, and the
ampule was placed in an oil bath at 90 °C and stirred for 24 h. The mixture
was then concentrated in vacuo; the residue was dissolved in a minimal
amount of methylene chloride and precipitated into petroleum ether.
7a: 1H NMR (500 MHz, CDCl3): δ 7.37 (m, OBn-ArH), 5.15 (s,
OBn-CH2), 4.44-4.25 (m, OC(O)OCH2), 4.23 (m, OCH2CH2-
CH2S), 3.70 (m, CH2OH), 2.55 (m, OCH2CH2CH2S), 2.50 (m,
SCH2(CH2)10CH3), 1.91 (m, OCH2CH2CH2S), 1.58 (m, SCH2CH2-
(CH2)9CH3), 1.37 (m, S(CH2)2CH2(CH2)8CH3), 1.26 (m, CCH3 and
(CH2)8CH3), 0.88 (s, (CH2)8CH3). 13C NMR (500 MHz, CDCl3): δ
conducted on a system composed of a Varian 390-LC-Multi detector
suite fitted with differential refractive index (DRI) and ultraviolet (UV)
detectors equipped with a guard column (Varian Polymer PLGel 5 μM,
50 ꢀ 7.5 mm) and two mixed D columns (Varian Polymer Laboratories
PLGel 5 μM, 300 ꢀ 7.5 mm). The mobile phase was DMF with 5% LiBr
eluent at a flow rate of 1.0 mL min-1, and samples were calibrated
against Varian Polymer Laboratories Easi-Vials linear poly(methyl
methacrylate) standards (690-1.9 ꢀ 106 g mol-1) using Cirrus v3.3.
Differential scanning calorimetry (DSC) analyses in a T zero aluminum
pan were performed under a flow of nitrogen using a Q2000 DSC from
TA Instruments in a custom mode (stabilization at -90 °C, heating
ramp of 10 °C/min from -90 to 80 °C, cooling ramp of 10 °C/min from
80 °C to -90 °C, stabilization at -90 °C, second heating ramp of
10 °C/min from -90 to 80 °C). Data were reported from the second
scan. Thermogravimetric analysis (TGA) was performed under a
nitrogen flow using a Q5000 TGA from TA Instruments with a
conventional ramp of 20 °C/min from ambient to 700 °C.
Organocatalytic ROP of 5-Methyl-5-allyloxycarbonyl-1,3-
dioxan-2-one (MAC). In a typical experiment, alcohol initiator, (-)-
sparteine (5 mol % to monomer), and 1-(3,5-bis(trifluoromethyl)
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Figure 4. 1H NMR in CDCl3 of PMAC11 initiated from benzyl alcohol using 5 mol % 2 and 10 mol % 3 (400 MHz, 293 K; / = residual CDCl3, // =
H2O).
Figure 5. MALDI-TOF MS analysis of PMAC (DP = 11) initiated from
benzyl alcohol using 5 mol % 2 and 10 mol % 3.
171.9 (CC(O)O), 154.4 (OC(O)O), 68.7 (OC(O)OCH2), 64.1 (OC-
Figure 6. GPC traces of MeO-PEO114-OH (Mn = 7520 g mol-1, PDI =
H2CHCH2S), 46.6 (CCH3), 32.2 (SCH2(CH2)10CH3), 31.9 (CH2-
1.03) and MeO-PEO114-PMAC20-OH block copolymer (Mn = 11 900,
CH2CH3), 29.7 (CH2SCH2CH2(CH2)9CH3), 29.3, 29.0, 28.7, 28.5,
PDI = 1.06).
and 28.3 (CH2)7CH2CH2CH3), 22.7 (CH2CH2CH3), 17.4 (CH3), 14.1
(S(CH2)11CH3). GPC (THF, RI): Mn (PDI) = 25 000 g mol-1 (1.22).
7b: 1H NMR (500 MHz, CDCl3): δ 7.40-7.22 (m, backbone ArH
NMR (500 MHz, (CD3)2SO): δ171.7 (CC(O)O), 154.0(OC(O)O), 71.5
(CHOH), 68.8 (OC(O)OCH2), 64.9 (CH2OH), 64.5 (OCH2CHCH2S),
46.2 (CCH3), 35.1 (SCH2CHOH), 28.2 (OCH2CHCH2S), 16.8 (CH3).
GPC (DMF, RI): Mn (PDI) = 29 340 g mol-1 (1.34).
and OBn-ArH), 5.14 (s, OBn-CH2), 4.39-4.13 (m, OC(O)OCH2)
and OCH2CH2CH2S), 3.70 (s, CH2(C5H6), 2.44 (m, OCH2CH2-
CH2S), 1.85 (m, OCH2CH2CH2S), 1.19 (s, CCH3). 13C NMR (500
MHz, CDCl3): δ 172.1 (CC(O)O), 154.4 (OC(O)O), 138.4 (CH2CAr),
129.0 (o-ArC), 128.6 (m-ArC), 127.2 (p-ArC), 68.8 (OC(O)OCH2),
64.2 (OCH2CHCH2S), 46.7 (CCH3), 36.7 (SCH2(C6H5), 28.2
(OCH2CHCH2S), 27.6 (OCH2CHCH2S), 17.6 (CH3). GPC (THF,
RI): Mn (PDI) = 20 710 g mol-1 (1.19).
7d: 1H NMR (500 MHz, (CD3)2SO): δ 7.38 (m, OBn-ArH), 5.14 (s,
OBn-CH2), 4.73 (m, OH), 4.31-4.17 (m, OC(O)OCH2), 4.13 (m,
OCH2CH2CH2S), 3.52 (m, CH2OH), 2.54 (m, OCH2CH2CH2SCH2),
1.81 (m, OCH2CH2CH2S), 1.18 (m, CCH3). 13C NMR (500 MHz,
(CD3)2SO): δ 171.7 (CC(O)O), 153.8 (OC(O)O), 68.8 (OC(O)
OCH2), 63.4 (OCH2CHCH2S), 60.8 (CH2OH), 46.2 (CCH3), 33.7
(SCH2CHOH), 28.2 (OCH2CHCH2S), 27.6 (OCH2CHCH2S), 16.8
(CH3). GPC (DMF, RI): Mn (PDI) = 26 050 g mol-1 (1.27).
7e: 1H NMR (500 MHz, (CD3)2SO): δ 12.62 (br s, COOH), 7.38
(m, OBn-ArH), 5.13 (s, OBn-CH2), 4.31-4.17 (m, OC(O)OCH2),
7c: 1H NMR (500 MHz, (CD3)2SO) δ 7.38 (m, OBn-ArH), 5.14 (s,
OBn-CH2), 4.72 (m, CH(OH)CH2OH), 4.53 (m, CH(OH)CH2-
OH), 4.34-4.08 (m, OC(O)OCH2 and OCH2CH2CH2S), 3.57 (m,
CH(OH)CH2OH), 3.34 (m, CH(OH)CH2OH), 2.63-2.39 (m,
OCH2CH2CH2SCH2), 1.81 (m, OCH2CH2CH2S), 1.18 (m, CCH3). 13
C
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Table 2. Telechelics and Block Copolymers of MACa
polymer
DPb
Mn(NMR)c (g mol-1
)
Mn(GPC)d (g mol-1
)
PDId
HO-PMAC-O(CH2)3O-PMAC-OH
HO-PMAC-O(CH2)2S-S(CH2)2O-PMAC-OH
MeO-PEO114-b-PMAC-OHe
BnO-PLA-OH
9f
3670
4960
12680
1550
5550
3310
4970
5452
4050
11900
2020
7130
4260
6620
1.16
1.23
1.06
1.17
1.17
1.34
1.28
12f
20f
20g
20f
16f
23g
BnO-PLA-b-PMAC-OH
BnO-PMAC16-OH
BnO-PMAC-b-PLA-OH
a Targeted degree of polymerization (DP) = 20 (block copolymers) or 10 (telechelics). Reactions were performed in CDCl3 at 25 °C, [MAC] = 2.0 M,
[M]/[I] = 20 using 5 mol % 2 and 10 mol % 3. b Experimental degree of polymerization measured by H NMR spectroscopy per OH group.
1
c Determined by 1H NMR spectroscopy. d Determined by GPC analysis in THF. e Poly(ethylene oxide) macroinitiator (DP = 114, determined as Mn =
7520 g mol-1, PDI = 1.03 by GPC analysis in THF against poly(styrene) standards). f DP of PMAC block. g DP of PLA block.
Scheme 2. Radical Addition of a Thiol (RSH) to the PMAC
Table 3. Postpolymerization Radical Thiol-Ene Functiona-
lization of Poly(carbonates)a
polymer
thiol
Mn (g mol-1
)
PDI
BnO-PMAC11-OH (6)
3440b/3680c
15100b/17160c 1.23b/1.18c
1.15b/1.10c
BnO-PMAC100-OH (7)
6a
7a
6b
7b
6c
7c
6d
7d
1-dodecanethiol
1-dodecanethiol
benzyl mercaptan 3530b
benzyl mercaptan 20710b
5320b
25000b
1.16b
1.22b
1.18b
1.19b
1.16c
1.34c
1.17c
1.27c
4.13 (m, OCH2CH2CH2S), 3.21 (s, SCH2CO2H), 2.61 (m,
OCH2CH2CH2S), 1.84 (m, OCH2CH2CH2S), 1.17 (m, CCH3). 13C
NMR (500 MHz, (CD3)2SO): δ 171.6 (CC(O)O), 171.5 (COOH),
153.8 (OC(O)O), 68.8 (OC(O)OCH2), 63.4 (OCH2CHCH2S),
46.3 (CCH3), 33.0 (SCH2COOH), 28.0 (OCH2CHCH2S), 27.5
(OCH2CHCH2S), 16.7 (CH3).
1-thioglycerol
1-thioglycerol
mercaptoethanol 6170c
mercaptoethanol 26050c
6750c
29340c
a [PMAC] = ca. 0.15 M in 1,4-dioxane, 2 equiv of thiol, 10 mol % AIBN,
90 °C, 24 h. b Measured by GPC analysis using THF as eluent.
c Measured by GPC analysis using DMF as eluent.
’ RESULTS AND DISCUSSION
Monomer Synthesis and Polymerization. Owing to the
electrophilic nature of allyl bromide, 5-methyl-5-allyloxycarbo-
nyl-1,3-dioxan-2-one (MAC) was able to be synthesized in a
simple two-step procedure as previously reported (Scheme 1a).29
The organocatalytic ROP of MAC was investigated using a
range of organic catalysts, previously studied for the ring-
opening polymerization of lactide and trimethylene carbonate
(Figure 1).41,44,45,47,48
Initial polymerization studies were carried out in CDCl3 or
CH2Cl2 (2.0 M MAC) at 25 °C with [M]/[I] = 20 (Table 1).
Application of the highly active 1,8-diazabicyclo[5.4.0]undec-7-
ene (1, DBU) as catalyst (5 mol %) revealed a linear increase of
monomer conversion against time until ca. 70% monomer
conversion, at which point the polymerizations became severely
retarded with isolated polymers displaying broadening molecular
weight distributions (from PDI = 1.15 to >1.80) ultimately
leading to bimodal GPC traces (see Supporting Information).
As such, our focus turned to the bifunctional catalyst systems. For
the ROP of MAC using (-)-sparteine, 2 (5 mol %), in
combination with either thiourea, 3 or 4, or R,R,R0,R0-tetrakis-
(trifluoromethyl)-1,3-benzenedimethanol (5) (10 mol %), good
control of the polymerization was observed. Of these systems,
the 2/3 system had the highest activity and was therefore chosen
as catalyst system for the ROP of MAC.
against monomer conversion (Figure 2) and initial monomer-to-
initiator ratio ([M]0/[I]0) while retaining low PDI values throughout
the polymerization (Figure 3). After precipitation from CH2Cl2 into
cold hexanes, residual catalyst could be removed by column chroma-
tography to yield pure polymers. Analysis of DP10 and DP100
polymer using 1H NMR spectroscopy confirmed degrees of polym-
erization (DP) of 11 and 100, respectively, by comparison of the
integration of the resonances at δ = 7.37 and 5.15 ppm originating
from the benzyl alcohol initiator group and unsaturated allyl func-
tionality on the polymer, respectively (Figure 4). Further analysis of
the DP10 polymer by MALDI-ToF MS revealed a single distribution
with a spacing of 200 m/z, which is equal to that of a monomer unit
(Figure 5). The main peak at m/z = 2533 corresponds to a sodium
charged polymer chain of DP12 with a benzyl alcohol end group.
Initiator Versatility and Block Copolymer Synthesis. In-
itiator versatility was investigated by the synthesis of telechelic
PMAC ([M]/[I] = 10 per alcohol group) initiated from 1,3-
1
propanediol and 2-hydroxyethyl disulfide. H NMR analysis of
these polymers revealed resonances expected for the initiating
alcohols with integration against the main chain polymer reso-
nances being consistent with DP20 polymers in both cases.
MALDI-ToF MS analysis of these polymers both showed
distributions with a spacing of 200 m/z. The main peaks at
m/z 3502 (DP17) and m/z 4380 (DP21) both correspond to the
values expected for telechelic PMAC initiated from 1,3-propa-
nediol and dithiodiethanol, respectively. In both cases, however, a
second smaller distribution could be observed by MALDI-TOF
Investigation of the living characteristics of the polymerization
catalyzed by 3 and (-)-sparteine (2) led to the observation of a
linear correlation between number-average molecular weight (Mn)
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Figure 7. 1H NMR in CDCl3 of PMAC11 after postpolymerization radical “thiol-ene” functionalization with 1-dodecanethiol (400 MHz, 293 K; / =
residual CDCl3, // = H2O).
MS. This distribution could originate either by initiation from
residual water within the system or by monomer initiated ROP
according to an active chain-end mechanism as recently proposed
by Delcroixet al.49 Initiatorversatility was furtherdemonstrated by
the synthesis of a MeO-PEO114-b-PMAC20-OH block copolymer
by initiation of MAC polymerization from commercially available
poly(ethylene oxide) monomethyl ether-5K that when character-
ized by GPC analysis in THF against poly(styrene) standards
(comparable to the PMAC polymers) revealed a Mn = 7520 g
mol-1 (PDI = 1.03). Chain growth was confirmed by GPC
analysis with the block copolymer revealing a shift to higher
molecular weight (Mn = 11 900 g mol-1; PDI = 1.06; Figure 6).
To further demonstrate the living nature of the system, block
copolymers of PMAC with poly(lactide), PLA, were synthesized
both by chain growth of PMAC from a PLA synthesized using the
same catalyst system, initiated from benzyl alcohol in a one-pot
process, and by lactide ROP from a PMAC macroinitiator. BnO-
PLA20-b-PMAC20-OH and BnO-PMAC16-b-PLA23-OH block
copolymer synthesis was confirmed by 1H NMR showing
resonances corresponding to both PLA and PMAC, only differ-
Figure 8. GPC traces of PMAC100 before (Mn = 15 100 g mol-1, PDI =
1.23) and after postpolymerization radical “thiol-ene” functionalization
with 1-dodecanethiol (Mn = 25 000 g mol-1, PDI = 1.22).
ing in the resonances of the final monomer unit (hydroxyl chain
end, δ = 3.68 and 1.19 ppm for MAC monomer unit; δ = 4.27
ppm for PLA monomer unit). Furthermore, in both cases an
increase in molecular weight was observed by GPC, while
maintaining a low polydispersity (Table 2).
Postpolymerization Functionalization. Optimization of the
required conditions for the radical addition of thiols to the allyl esters
(Scheme 2) was undertaken using a model system consisting of allyl
acetate and thiophenol, with azobis(isobutyronitrile), AIBN, as the
radical initiator. These reactions were performed at 90 °C in a range
of solvents. While dimethylformamide and dimethyl sulfoxide led
to no conversion being observed, 1,4-dioxane resulted in almost a
doubling in conversion over 24 h compared to reactions in which
other solvents were applied (chloroform, ethyl acetate, etc.). As
an excellent solvent for the poly(carbonate)s, 1,4-dioxane was
hence used in further thiol-ene reactions. As expected, the
amount of radical initiator did not greatly influence the reaction
whereas increased concentration of the reagents led to decreased
reaction times. Under optimal conditions (0.15 M PMAC in
dioxane at 90 °C for 24 h) with 1 equiv of benzyl mercaptan,
initial reactions led to the observation of a high molecular weight
tail by GPC analysis, most likely a consequence of cross-linking
of some pendant allyl groups. Variation of the equivalents of
benzyl mercaptan between 0.5 and 3 equiv observed cross-linked
chloroform-insoluble product with 0.5 equiv, whereas increasing
the equivalents of thiol to 2 or more resulted in the observation of
monomodal GPC traces with the narrow molecular weight
distributions of the initial PMAC retained.
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Table 4. Thermal Analysis of Functionalized Poly(carbonates)
polymer
thiol
Tga (°C)
Tmb (°C)
-2.9
ΔHm (J g-1
)
T5%d (°C)
T50%d (°C)
BnO-PMAC100-OH
-26.4
-15.7c
-5.1
307
282
280
229
284
355
313
347
275
329
343
7a
7b
7c
7d
7e
1-dodecanethiol
benzyl mercaptan
1-thioglycerol
27.0
-28.5
-16.3
-33.2
mercaptoethanol
mercaptoacetic acid
261
a Glass transition temperature, measured by DSC analysis at the second scan. b Melting point, measured by DSC analysis. Loss of accuracy due to
c
proximity to the melting endotherm. d Temperatures at 5 and 50% weight degradation.
expected for the modified side-chain groups. GPC data were also
consistent with the absence of adverse side reactions; however,
addition of mercaptoacetic acid resulted in the isolation of a
polymer that was not soluble in any available GPC solvents.
Thermal Analysis. Analysis of the DP100 functional polymers
by differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) was undertaken to investigate the effect of
chemical structure on the thermal properties and degradation of
the functional polymers. Notably, DSC analysis of the DP100
PMAC prefunctionalization revealed a lower glass transition
temperature (Tg) of -26.4 °C than those previously reported
(-12.0 °C; Mn = 19 000 g mol-1, PDI = 1.6 and Mn = 9200 g
mol-1, PDI = 1.8).29,31 Postpolymerization functionalization of
the PMAC homopolymers revealed subtle changes in the Tg
values of the polymers with respect to DP100 PMAC displaying a
glass transition temperature (Tg) of -26.4 °C (Table 4). While
the addition of linear alkyl groups (1-dodecanethiol and
mercaptoethanol) led to a slight increase in Tg, the addition of
benzyl mercaptan led to the highest increase to -5.0 °C,
attributed to the steric hindrance and aromatic interactions.
Notably, the 1-dodecanethiol-functionalized polymer also dis-
played a melting transition at Tm = -2.9 °C, likely a consequence
of crystallization of the linear alkyl side chains.
Figure 9. MALDI-ToF-MS of PMAC (DP = 11) after postpolymeriza-
tion radical “thiol-ene” functionalization with 1-dodecanethiol. The
second lower distribution represents a PMAC chain that is not fully
functionalized and has one pendant allyl group left.
The thermal degradation of PMAC100 and functionalized PM-
AC100 samples (12a-12e) was also studied using TGA (Table 4).
Again, the initial degradation values were shown to be largely
invariant with polymer structure ranging from 311 to 360 °C,
attesting for a good thermal stability whatever the nature of the
pendant functional groups. The most notable outlier was the 1-
thioglycerol-functionalized polymer which degraded at significantly
lower temperature (initial degradation temperature = 247.5 °C)
most likely attributed to the dehydratation of this polyol derivative.
Notably, all investigated (functionalized) poly(carbonate)s
display higher thermal stability than poly(trimethylene carbonate)
(PTMC), which has a reported degradation temperature of ca.
230 °C.11
In order to demonstrate the versatility of this system, PMAC
homopolymers (DP11 and DP100) were functionalized with a
range of thiol-containing molecules (Table 3). 1H NMR spectro-
scopic analysis revealed >99% conversion of the allyl groups had
occurred, evidenced by the disappearance of the vinyl resonances
at δ = 5.87 and 5.37-5.20 ppm (Figure 7) and the appearance of
new signals consistent with the corresponding thiol groups.
Furthermore, in all cases, analysis by GPC revealed a shift to
higher molecular weight while maintaining narrow, unimodal
distributions with polydispersity indices similar to that of the
unmodified PMAC (Figure 8). MALDI-ToF MS analysis of the
polymers (Figure 9) revealed a new distribution with repeat units
that were consistent with the new polymer structure; for
example, addition of 1-dodecanethiol across the double bonds
resulted in an increase in the repeating unit from m/z = 200 to m/
z = 402. In most cases, additional peaks corresponding to a
polymer chain with a single residual unfunctionalized allyl group
is also observed, although in notably lower intensity.
’ CONCLUSIONS
In conclusion, the controlled ring-opening polymerization of
5-methyl-5-allyloxycarbonyl-1,3-dioxan-2-one was achieved suc-
cessfully by using a range of organic catalysts under mild
conditions. Using the preferred system of thiourea and (-)-
sparteine, the living nature of the polymerizations was demon-
strated by the observation of a linear evolution of molecular
weight with both monomer conversion and monomer to initiator
ratio, low PDIs, and the synthesis of telechelic polymers and block
copolymers of PMAC with PEO and PLA. Postpolymerization
functionalization of the pendant allyl groups in the polymer
The radical “thiol-ene” process was repeated for a range of
thiol-containing molecules including some with functional
groups that are not compatible with ring-opening polymerization
such as alcohol and carboxylic acid groups (Table 3). In all cases
1H NMR analysis of the modified polymers (Supporting In-
formation) demonstrated that addition of the thiol to the allyl
groups was occurring to >99% with data consistent with those
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backbone by facile free radical “thiol-ene” addition led to the
isolation of aliphatic poly(carbonate)s with a range of pendant
functionalities, without notable polymer degradation leading to
precise control over the thermal properties of the polymers. Such
a facile and versatile methodology provides an attractive route to the
synthesis of functional aliphatic poly(carbonates).
(17) Riva, R.; Schmeits, P.; Stoffelbach, F.; Jerome, C.; Jerome, R.;
Lecomte, P. Chem. Commun. 2005, 5334–5336.
(18) Riva, R.; Schmeits, S.; Jerome, C.; Jerome, R.; Lecomte, P.
Macromolecules 2007, 40, 796–803.
(19) Xu, N.; Wang, R.; Du, F. S.; Li, Z. C. J. Polym. Sci., Part A: Polym.
Chem. 2009, 47, 3583–3594.
(20) Mecerreyes, D.; Humes, J.; Miller, R. D.; Hedrick, J. L.;
Detrembleur, C.; Lecomte, P.; Jerome, R.; San Roman, J. Macromol.
Rapid Commun. 2000, 21, 779–784.
’ ASSOCIATED CONTENT
(21) Rieger, J.; Van Butsele, K.; Lecomte, P.; Detrembleur, C.;
Jerome, R.; Jerome, C. Chem. Commun. 2005, 274–276.
(22) Vaida, C.; Mela, P.; Keul, H.; Moller, M. J. Polym. Sci., Part A:
Polym. Chem. 2008, 46, 6789–6800.
(23) Chen, W.; Yang, H. C.; Wang, R.; Cheng, R.; Meng, F. H.; Wei,
W. X.; Zhong, Z. Y. Macromolecules 2010, 43, 201–207.
(24) Parrish, B.; Quansah, J. K.; Emrick, T. J. Polym. Sci., Part A:
Polym. Chem. 2002, 40, 1983–1990.
S
Supporting Information. Additional spectral and chro-
b
matographic data of the functionalized polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: a.p.dove@warwick.ac.uk.
(25) Hoyle, C. E.; Bowman, C. N. Angew. Chem., Int. Ed. 2010,
49, 1540–1573.
(26) Stanford, M. J.; Dove, A. P. Macromolecules 2009, 42, 141–147.
(27) van der Ende, A. E.; Harrell, J.; Sathiyakumar, V.; Meschievitz,
M.; Katz, J.; Adcock, K.; Harth, E. Macromolecules 2010, 43, 5665–5671.
(28) van der Ende, A. E.; Kravitz, E. J.; Harth, E. J. Am. Chem. Soc.
2008, 130, 8706–8713.
(29) Hu, X.; Chen, X.; Liu, S.; Shi, Q.; Jing, X. J. Polym. Sci., Part A:
Polym. Chem. 2008, 46, 1852–1861.
(30) Hu, X.; Chen, X.; Wei, J.; Liu, S.; Jing, X. Macromol. Biosci. 2009,
9, 456–463.
(31) Mullen, B. D.; Tang, C. N.; Storey, R. F. J. Polym. Sci., Part A:
Polym. Chem. 2003, 41, 1978–1991.
(32) Hu, X.; Chen, X.; Xie, Z.; Liu, S.; Jing, X. J. Polym. Sci., Part A:
Polym. Chem. 2007, 45, 5518–5528.
(33) Chen, X.; McCarthy, S. P.; Gross, R. A. Macromolecules 1998,
31, 662–668.
’ ACKNOWLEDGMENT
The Research Councils UK (RCUK) are acknowledged for
funding a fellowship to A.P.D. We gratefully acknowledge
financial support from EPSRC (EP/F068808/1) for funding a
studentship (S.T.) and (EP/C007999/1) the purchase of the
Bruker Ultraflex MALDI-ToF MS instrument. The GPC equip-
ment used in this research was obtained through Birmingham
Science City: Innovative Uses for Advanced Materials in the
Modern World (West Midlands Centre for Advanced Materials
Project 2), with support from Advantage West Midlands (AWM)
and part funded by the European Regional Development Fund
(ERDF). CIRMAP is grateful to the “Belgian Federal Governe-
ment Office Policy of Science (SSTC)” for general support in the
frame of the PAI-6/27 and thanks the Belgian F.R.S.-FNRS in the
frame of the ERA-Chemistry/FRFC 2.4.616.09.F project.
(34) Endo, T.; Kakimoto, K.; Ochiai, B.; Nagai, D. Macromolecules
2005, 38, 8177–8182.
(35) He, F.; Wang, Y.-P.; Liu, G.; Jia, H.-L.; Feng, J.; Zhuo, R.-X.
Polymer 2008, 49, 1185–1190.
(36) Miyagawa, T.; Shimizu, M.; Sanda, F.; Endo, T. Macromolecules
2005, 38, 7944–7949.
(37) Wang, C.-F.; Lin, Y.-X.; Jiang, T.; He, F.; Zhuo, R.-X. Biomater-
’ REFERENCES
(1) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004,
43, 6618–6639.
ials 2009, 30, 4824–4832.
(38) Dove, A. P. Chem. Commun. 2008, 6446–6470.
(39) Jerome, C.; Lecomte, P. Adv. Drug Delivery Rev. 2008,
60, 1056–1076.
(2) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388–2410.
(3) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.; Billodeaux,
D. R. Acc. Chem. Res. 2004, 37, 836–844.
(40) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.;
Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107, 5813–5840.
(41) Dove, A. P.; Pratt, R. C.; Lohmeijer, B. G. G.; Waymouth, R. M.;
Hedrick, J. L. J. Am. Chem. Soc. 2005, 127, 13798–13799.
(42) du Boullay, O. T.; Saffon, N.; Diehl, J.-P.; Martin-Vaca, B.;
Bourissou, D. Biomacromolecules 2010, 11, 1921–1929.
(43) Pounder, R. J.;Dove, A. P.Biomacromolecules 2010, 11, 1930–1939.
(44) Nederberg, F.; Lohmeijer, B. G. G.; Leibfarth, F.; Pratt, R. C.;
Choi, J.; Dove, A. P.; Waymouth, R. M.; Hedrick, J. L. Biomacromolecules
2006, 8, 153–160.
(45) Pratt, R. C.; Lohmeijer, B. G. G.; Long, D. A.; Lundberg,
P. N. P.; Dove, A. P.; Li, H. B.; Wade, C. G.; Waymouth, R. M.; Hedrick,
J. L. Macromolecules 2006, 39, 7863–7871.
(46) Becker, J. M.; Tempelaar, S.; Stanford, M. J.; Pounder, R. J.;
Covington, J. A.; Dove, A. P. Chem.—Eur. J. 2010, 16, 6099–6105.
(47) Coulembier, O.; Sanders, D. P.; Nelson, A.; Hollenbeck, A. N.;
Horn, H. W.; Rice, J. E.; Fujiwara, M.; Dubois, P.; Hedrick, J. L. Angew.
Chem., Int. Ed. 2009, 48, 5170–5173.
(4) Ochiai, B.; Endo, T. Prog. Polym. Sci. 2005, 30, 183–215.
(5) Rokicki, G. Prog. Polym. Sci. 2000, 25, 259–342.
(6) Keul, H. Polycarbonates; Wiley-VCH Verlag GmbH & Co.
KGaA: New York, 2009; pp 307-327.
(7) Pratt, R. C.; Nederberg, F.; Waymouth, R. M.; Hedrick, J. L.
Chem. Commun. 2008, 114–116.
(8) Sanders, D. P.; Fukushima, K.; Coady, D. J.; Nelson, A.; Fujiwara,
M.; Yasumoto, M.; Hedrick, J. L. J. Am. Chem. Soc. 2010, 132, 14724–14726.
(9) Suriano, F.; Pratt, R.; Tan, J. P. K.; Wiradharma, N.; Nelson, A.;
Yang, Y.-Y.; Dubois, P.; Hedrick, J. L. Biomaterials 2010, 31, 2637–2645.
(10) Mespouille, L.; Nederberg, F.; Hedrick, J. L.; Dubois, P.
Macromolecules 2009, 42, 6319–6321.
(11) Nederberg, F.; Trang, V.; Pratt, R. C.; Mason, A. F.; Frank, C. W.;
Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 3294–3297.
(12) Gauthier, M. A.; Gibson, M. I.; Klok, H. A. Angew. Chem., Int.
Ed. 2009, 48, 48–58.
(13) Hoyle, C. E.; Lowe, A. B.; Bowman, C. N. Chem. Soc. Rev. 2010,
39, 1355–1387.
(48) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.;
Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth,
R. M.; Hedrick, J. L. Macromolecules 2006, 39, 8574–8583.
(49) Delcroix, D.; Martin-Vaca, B.; Bourissou, D.; Navarro, C.
Macromolecules 2010, 43, 8828–8835.
(14) Iha, R. K.; Wooley, K. L.; Nystrom, A. M.; Burke, D. J.; Kade,
M. J.; Hawker, C. J. Chem. Rev. 2009, 109, 5620–5686.
(15) Pounder, R. J.; Dove, A. P. Polym. Chem. 2010, 1, 260–271.
(16) Jiang, X.; Vogel, E. B.; Smith, M. R.; Baker, G. L. Macromolecules
2008, 41, 1937–1944.
2091
dx.doi.org/10.1021/ma102882v |Macromolecules 2011, 44, 2084–2091