Copolymerization of lactones and bioaromatics: Via concurrent ring-opening polymerization/polycondensation

Article abstract of DOI:10.1039/c6gc03238a

The general and efficient copolymerization of lactones with hydroxy-acid bioaromatics was accomplished via a concurrent ring-opening polymerization (ROP) and polycondensation methodology. Suitable lactones were l-lactide or ε-caprolactone and four hydroxy-acid comonomers were prepared as hydroxyethyl variants of the bioaromatics syringic acid, vanillic acid, ferulic acid, and p-coumaric acid. Copolymerization conditions were optimized on a paradigm system with a 20 : 80 feed ratio of caprolactone : hydroxyethylsyringic acid. Among six investigated catalysts, polymer yield was optimized with 1 mol% of Sb₂O₃, affording eight copolymer series in good yields (32-95% for lactide; 80-95% for caprolactone). Half of the polymers were soluble in the GPC solvent hexafluoroisopropanol and analyzed to high molecular weight, with M_n = 10 500-60 700 Da. Mass spectrometry and ¹H NMR analysis revealed an initial ring-opening formation of oligolactones, followed by polycondensation of these with the hydroxy-acid bioaromatic, followed by transesterification, yielding a random copolymer. By copolymerizing bioaromatics with l-lactide, the glass transition temperature (T_g) of polylactic acid (PLA, 50 °C) could be improved and tuned in the range of 62-107 °C; the thermal stability (T_95%) of PLA (207 °C) could be substantially increased up to 323 °C. Similarly, bioaromatic incorporation into polycaprolactone (PCL, T_g = -60 °C) accessed an improved T_g range from -48 to 105 °C, while exchanging petroleum-based content with biobased content. Thus, this ROP/polycondensation methodology yields substantially or fully biobased polymers with thermal properties competitive with incumbent packaging thermoplastics such as polyethylene terephthalate (T_g = 67 °C) or polystyrene (T_g = 95 °C).

Full text of DOI:10.1039/c6gc03238a

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Copolymerization of lactones and bioaromatics
via concurrent ring-opening polymerization/
polycondensation†
Cite this: DOI: 10.1039/c6gc03238a
Ha Thi Hoang Nguyen, Gabriel N. Short, Pengxu Qi and Stephen A. Miller*
The general and efficient copolymerization of lactones with hydroxy-acid bioaromatics was accomplished
via a concurrent ring-opening polymerization (ROP) and polycondensation methodology. Suitable
lactones were L-lactide or ε-caprolactone and four hydroxy-acid comonomers were prepared as
hydroxyethyl variants of the bioaromatics syringic acid, vanillic acid, ferulic acid, and p-coumaric acid.
Copolymerization conditions were optimized on a paradigm system with a 20 : 80 feed ratio of
caprolactone : hydroxyethylsyringic acid. Among six investigated catalysts, polymer yield was optimized
with 1 mol% of Sb2O3, affording eight copolymer series in good yields (32–95% for lactide; 80–95% for
caprolactone). Half of the polymers were soluble in the GPC solvent hexafluoroisopropanol and analyzed
to high molecular weight, with Mn = 10 500–60 700 Da. Mass spectrometry and 1H NMR analysis revealed
an initial ring-opening formation of oligolactones, followed by polycondensation of these with the
hydroxy-acid bioaromatic, followed by transesterification, yielding a random copolymer. By copolymeriz-
ing bioaromatics with L-lactide, the glass transition temperature (Tg) of polylactic acid (PLA, 50 °C) could
be improved and tuned in the range of 62–107 °C; the thermal stability (T95%) of PLA (207 °C) could be
substantially increased up to 323 °C. Similarly, bioaromatic incorporation into polycaprolactone (PCL, Tg
=
Received 23rd November 2016,
Accepted 5th January 2017
−60 °C) accessed an improved Tg range from −48 to 105 °C, while exchanging petroleum-based content
with biobased content. Thus, this ROP/polycondensation methodology yields substantially or fully bio-
based polymers with thermal properties competitive with incumbent packaging thermoplastics such as
polyethylene terephthalate (Tg = 67 °C) or polystyrene (Tg = 95 °C).
DOI: 10.1039/c6gc03238a
www.rsc.org/greenchem
bility, PCL and PLA are both utilized in medical devices such
as sutures, implant scaffolds, tissue engineering media, and
Introduction
Polyesters are an indispensable component of the commercial drug delivery vehicles.6–13 Upon exploring the versatility of
polymer repertoire and continue to garner attention because these two polymers in the medical field, authors have reported
of their synthetic accessibility, real and potential bioavail- a variety of novel diblock, triblock, or graft copolymers
ability, biocompatibility, and typical degradability.1,2 For between lactide or ε-caprolactone and polyglycolic acid,14 poly-
example, polycaprolactone (PCL) and polylactic acid (PLA) are ethylene glycol,15–17 polyamides,18 polyhedral silsesquioxane,19
each synthesized via an efficient lactone ring-opening polymer- poly(vinyl acetate),20 polyurethanes,21 polypropylene adipate,22
ization (ROP) technique and have occupied leading positions polybutylene succinate,23 as well as chitosan,24 and other
as commercially available aliphatic polyesters with versatile natural fibers.25
applications.3,4 While PCL is a petroleum-based plastic pre-
Moreover, PCL and PLA can also be found in the disposable
pared from ε-caprolactone,4 PLA is one of the most heralded packaging and agricultural industry because of their scalability
100% biorenewable commodity plastics—made with L-lactide and potential biodegradability.6,26 Unfortunately, both PCL
from the fermentation of corn starch or other carbohydrates.3,5 and PLA have unsatisfactory thermal properties. For PCL, the
Owing to their biodegradability and their excellent biocompati- glass transition temperature (Tg) is −60 °C and the melting
temperature (Tm) is 60 °C,7 while for PLA, Tg = 50 °C and Tm
=
160 °C (Fig. 1).27 These relatively low values—especially the Tg
for the amorphous materials—have restricted their appli-
cations compared to environmentally malignant incumbents
such as polyethylene terephthalate (PET) with Tg = 67 °C 28 or
polystyrene (PS) with Tg = 95 °C.29
The George and Josephine Butler Laboratory for Polymer Research, Department of
Chemistry, University of Florida, Gainesville, Florida 32611-7200, USA.
E-mail: miller@chem.ufl.edu; Fax: +1-352-392-9741; Tel: +1-352-392-7773
†Electronic supplementary information (ESI) available: Synthetic details and
complete polymer characterization data. See DOI: 10.1039/c6gc03238a
This journal is © The Royal Society of Chemistry 2017
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readily found in lignin (syringic acid and vanillic acid) or
lignocellulose (ferulic acid and coumaric acid).43–45 Our pre-
vious work has demonstrated the utility of such bioaromatics
for synthesizing aromatic polyesters with thermal properties
and thermal stabilities comparable to those of incumbent
commodity plastics.46–49 Thus, this report describes the
improvement of Tg, Tm, and thermal stability of PCL and PLA
via novel copolymerizations, while maintaining or increasing
the biobased content of the polymers.
Fig. 1 The glass transition temperature (Tg) and melting temperature
(Tm) of petroleum-based polycaprolactone (PCL) and biobased poly-
lactic acid (PLA).
Results and discussion
Hydroxyethyl variants of the four aforementioned bioaromatics
are employed in the present study: hydroxyethylsyringic acid
(HESA), hydroxyethylvanillic acid (HEVA), hydroxyethylferulic
acid (HEFA), and hydroxyethylcoumaric acid (HECA). Our pre-
vious publications describe the polycondensation homopoly-
merization of these monomers and allude to the improved
polymerizability when an aromatic alcohol is replaced with an
aliphatic alcohol via hydroxyethylation.47,48
To address this issue, the thermal properties of PCL and
PLA were modulated by blending with other materials.
Initially, bulk blending was commonly investigated, especially
because of its apparent feasibility.30 However, this technique
proved to be problematic because of the immiscibility of struc-
turally diverse polymers, as well as the thermal instability of
PCL and PLA.31–34 For example, the successful blending of PCL
or PLA with high-Tg polyesters required precise control of
blending conditions (time, temperature, stirring rate, etc.); the
lack of desirable ester-interchange or decomposition of PCL or
PLA contributed to immiscible mixtures with poor mechanical
properties.35,36 Thus, more recent efforts have concentrated on
direct copolymerization of caprolactone or lactide with co-
monomers chosen to confer main-chain rigidity and increase
chain–chain interactions.
Optimization of polymerization conditions
The one-pot ROP/polycondensation conditions were optimized
with a model system utilizing ε-caprolactone and hydroxyethyl-
syringic acid (HESA) with a monomer feed ratio of 20 : 80,
respectively. Six different catalysts (Table 1, entries 1–6) were
screened with 1 mol% loading and Sb2O3 proved optimal,
yielding 97% polymer with a molecular weight (Mn) of 12 200
(Table 1, entry 6). Raising the final polymerization temperature
from 200 to 220 °C increased the polymer molecular weight
less than 10% (Table 1, entry 7). However, the inclusion of
Copolymerization with aromatics, for example, is a promis-
ing strategy because several aromatic polyesters are renowned
for their excellent thermal and mechanical performance.37
Through catalytic transesterification chemistry, a considerable
amount of work has been reported regarding the copolymeri-
Table 1 Optimization of ROP/polycondensation for the copolymeriza-
zation of aromatic oligoesters or aromatic polyesters with tion of ε-caprolactone : hydroxyethylsyringic acid (HESA) with
a
monomer ratio of 20 : 80a
ε-caprolactone, lactide, or lactic acid.38–42 For example,
Olewnik and coworkers reported poly(lactic acid-co-ethylene
terephthalate) synthesized from lactic acid and glycolysis pro-
ducts from PET.38 MacKnight et al. applied ring-opening co-
polymerization of cyclic poly(butylene terephthalate) oligomers
with ε-caprolactone.39 However, very few reports mention the
direct, random copolymerization of lactones or lactide with
bifunctional monomers (e.g., hydroxy-acids, diols, and di-
acids).
b
b
c
Mn
Mw
Tg
PDIb (°C)
Yield
(%)
Entry Catalyst
(Da)
(Da)
1
Ti(OiPr)4
Sn(Oct)2
TBD
6600
5500
3200
14 900
9500
2.3
1.7
1.6
1.7
2.1
1.7
1.8
1.8
57
56
44
40
63
53
60
60
88.6
69.8
72.5
40.5
80.1
97.0
85.2
91.2
2
Aromatic polyesters are usually synthesized via step-growth
polycondensation techniques at very high temperatures while
PCL and PLA are typically obtained by chain-growth ring-
opening polymerization (ROP) at moderate temperatures.
Therefore, the successful combination of these two techniques
in a single reaction vessel could prove challenging. Herein, we
introduce a general and efficient one-pot ROP/polycondensa-
tion copolymerization of ε-caprolactone or L-lactide with
hydroxy-acid comonomers conducted in the melt using a cata-
lyst effective for both lactone ROP and Fischer esterification.
Importantly, this work employs bioaromatic hydroxy-acids
3
4
5
5200
4300
19 400
21 300
23 200
31 500
Hydrotalcite
Bu2SnO
Sb2O3
Sb2O3
Sb2O3
2500
9400
12 200
13 100
17 800
6
7d
8e
a 1 mol% catalyst except 0.5 mol% hydrotalcite; TBD = triazabicyclo-
decene; melted under argon for 5 hours with a temperature ramp from
150 to 200 °C; 8 hours dynamic vacuum at 200 °C. b GPC in hexafluoro-
isopropanol (HFIP) at 40 °C versus polymethyl methacrylate (PMMA)
standards. c Determined by DSC. d 4 hours dynamic vacuum at 200 °C
then 4 hours at 220 °C. e Bump trap charged with 1.5 equivalents of
P2O5.
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P2O5 as a drying agent (Table 1, entry 8) increased the Mn step-growth polymerization, resulting in lower polymer mole-
almost 50% to 17 800. Hence, subsequent polymerizations cular weight. It is also plausible that polymer rigidity
were conducted with 1 mol% Sb2O3 as catalyst and 1.5 equi- decreased GPC solvent solubility, affording a lower molecular
valents of P2O5 or 3 equivalents of MgSO4 to scavenge the weight analysis. All polymers of Table 2 showed polydispersi-
water by-product evolved from the polycondensation.
ties (PDI) fairly typical for step-growth polymerization, ranging
from 1.6 to 3.1 with the HESA copolymerization system
showing the best behaviour (1.7 to 2.0).
Copolymers of ε-caprolactone and bioaromatics
The optimized ROP/polycondensation conditions were applied
to the synthesis of four different polymer series, copolymeriz-
Copolymers of L-lactide and bioaromatics
ing ε-caprolactone with the bioaromatics HESA, HEVA, HEFA, After caprolactone, we next turned our attention to the con-
or HECA (Table 2). An oligomerization step was conducted current ROP/polycondensation of L-lactide with the same four
under argon for 5 to 22 hours, with a temperature gradient bioaromatic hydroxy-acids. As shown in Table 3, 1H NMR
from 150–175 to 200–240 °C. This step was designed to revealed the aromatic incorporation fraction to be consistently
consume monomer and prevent its volatilization during the higher than its feed fraction. This discrepancy is likely caused
next step, which applied dynamic vacuum for 6 to 8 hours at by the facile loss of L-lactide through sublimation51 before its
200 or 240 °C. Temperatures and times varied somewhat to polymerization and after back-biting depolymerization during
accommodate differences in the monomer melting tempera- prolonged heating of the polymer. Glass transition tempera-
ture and volatility. (See full details in the ESI.†)
tures (Tg) increased in response to the bioaromatic content,
As reported in Table 2, all polymers were obtained in high predictably improving upon the Tg of PLA (50 °C) to reach
yields of 80–96%. 1H NMR confirmed that the incorporation 113 °C (Table 3, entry 17). Note that Table 3 lacks any reported
fraction of bioaromatics into the copolymer chain closely polymer melting temperatures because these polymers failed
matched the feed fraction introduced to the reaction flask. The to exhibit any crystallinity by DSC. Copolymerization did sig-
most wayward entry is Table 2, entry 16, which used a 50% nificantly improve the thermal stability, as the T95% values
feed fraction of HEFA, but yielded a copolymer with 63% ranged from 209 to 343 °C while PLA itself loses 5% of its
HEFA incorporation. Over half of the polymers exhibited a mass at only 207 °C. Polymer yields ranged widely (32 to 95%),
melting temperature. Almost all of the HESA and HEVA copoly- but improved with greater bioaromatic feed fractions. The low
mers exhibited a melting transition, while none of the HEFA copolymer yields probably resulted from facile lactide sublima-
and only one of the HECA copolymers melted. When observed, tion.51 Overall, the copoly(caprolactone/bioaromatic) series
the Tm values (126–254 °C) were generally higher than that of (Table 2; Mn = 2700 to 60 700) exhibited higher molecular
the PCL homopolymer (60 °C). An exception is Table 2, entry weights than the copoly(lactide/bioaromatic) series (Table 3;
2, where an 11% presence of HESA in the PCL chain lowers the Mn = 1700 to 21 800). Both series were similar in that measured
Tm to 37 °C. As predicted by the Fox equation,50 all four co- molecular weight suffered in response to increased bio-
polymer series revealed a proportional increase in the glass aromatic content.
transition temperature (Tg) as the rigid bioaromatic monomer
Copolymer microstructure analysis
content increased, from −60 °C (100% PCL)7 up to 113 °C
(100% HEFA).48 The aromatic content also proportionally con- Fig. 2 shows the four possible enchainment dyads for copoly
ferred greater thermal stability. The copolymers showed a 5% (caprolactone/hydroxyethylferulic acid). The other three copoly-
mass loss (T95%) at temperatures ranging from 277 to 343 °C mers of Table 2 will contain analogous dyads, wherein A
under nitrogen, while PCL itself showed this degradation represents enchainment of the aromatic monomer and L
below this range at 255 °C.
represents enchainment of the lactone. Fig. 2 also depicts two
For each copolymer series of Table 2, the highest molecular partial 1H NMR spectra in the region between 4.0 and
weight was recorded for low (11 to 19%) bioaromatic incorpor- 8.0 ppm: copoly(caprolactone/hydroxyethylferulic acid) pre-
ation: 29 700 (entry 2); 38 600 (entry 8), 60 700 (entry 14); and pared with a 60 : 40 caprolactone : HEFA feed ratio (black line)
29 200 (entry 20). Thereafter, increased aromatic content pro- and a 70 : 30 feed ratio (orange line). This region shows two
portionally lowered the molecular weight, although the HESA- distinct sets of resonances for protons in the vicinity of a dyad
based copoly(caprolactone/hydroxyethylsyringic acid) series juncture. In this case, the green resonances (the downfield set)
maintained the highest molecular weight range (10 600 to correspond to chemical shifts influenced by a neighboring
29 700; Table 2, entries 2–7) and did not drop below 10 000 as HEFA repeat unit; the blue resonances (the upfield set) corres-
the other series did. This effect could be related to tractability pond to chemical shifts influenced by a neighboring capro-
and melt viscosity; note that this highest molecular weight lactone repeat unit.
series is also the only one fully soluble in the GPC solvent
A non-random, blocky copolymer is expected to show a pre-
(hexafluoroisopropanol). This observation parallels our pre- ponderance of AA and LL dyads, whereas a random copolymer
vious study of copolymers from hydroxyethylferulic acid and is expected to show a Bernoullian distribution52 of AA, AL, LA,
its hydrogenated analogue, hydroxyethyldihydroferulic acid.48 and LL dyads statistically predicted by the monomer feed frac-
In this case, polymer rigidity (with the double bond intact) tions. For the 70 : 30 feed ratio, PL = 0.70 and PA = 0.30; thus,
translated to greater melt viscosity and a more challenged the predicted dyad distribution is: PAPA = 9%; PAPL = 21%;
This journal is © The Royal Society of Chemistry 2017
Green Chem.

Table 2 Polymerization results and characterization of copolymers prepared from ε-caprolactone and bioaromatics HESA, HEVA, HEFA, and HECAa
Monomer feed %
Entryb
Inc. HESAe (mol%)
Yield (%)
Mn f (Da)
Mw f (Da)
PDI f
Tg g (°C)
Tm g (°C)
T95% h (°C)
1
2
3
4
5
6
7
100
90
70
50
30
10
0
0
10
30
50
70
90
100
0
11
35
57
78
95
100
90.0
93.3
80.0
84.2
89.7
95.6
93.0
15 000
29 700
27 600
25 700
21 100
14 900
10 600
28 200
60 400
55 600
49 200
37 800
27 900
18 500
1.9
2.0
2.0
1.9
1.8
1.9
1.7
−60
−48
−18
20
47
67
53
37
n.o.
n.o.
126
171
183
255
278
281
316
316
300
318
76
Monomer feed %
Entryc
Inc. HEVAe (mol%)
Yield (%)
Mn f (Da)
Mw f (Da)
PDIf
Tg g (°C)
Tm g (°C)
T95% h (°C)
8
9
10
11
12
13
90
70
50
30
10
0
10
30
50
70
90
12
35
55
69
87
83.7
86.0
80.5
90.7
93.8
94.9
38 600
23 600
16 100
4400
74 800
49 600
32 300
10 200
1.9
2.1
2.0
2.3
−42
4
33
64
77
82
36
131
215
232
247
254
314
295
277
313
318
311
9500
14 800
1.6
i
i
i
100
100
Monomer feed %
Entryd
Inc. HEFAe (mol%)
Yield (%)
Mn f (Da)
Mw f (Da)
PDI f
Tg g (°C)
Tm g (°C)
T95% h (°C)
14
15
16
17
18
19
90
70
50
30
10
0
10
30
50
70
90
11
32
63
77
94
92.1
93.4
80.6
90.8
89.1
89.6
60 700
28 200
11 700
5000
2400
1600
179 100
88 300
21 500
10 600
4800
3.0
3.1
1.8
2.1
1.9
1.6
−39
6
59
87
105
113
42
311
310
302
307
331
331
n.o.
n.o.
n.o.
n.o.
n.o.
100
100
2500

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PLPA = 21%; and PLPL = 49%. The ester protons –(CO)OCH2–
resonate at different chemical shifts for each dyad: AA, 4.60;
AL, 4.21; LA, 4.48; and LL, 4.08 ppm. Spectral integration of
these baseline resolved resonances (Fig. 2, orange line) gives
an experimental dyad distribution of 8.5%, 22.1%, 20.1% and
49.3%. This is a near perfect match to the Bernoullian dyad
distribution with an RMS error of only 0.8%. Moreover, the
monomer incorporation ratio of 70.4 : 29.6 calculated from
this dyad distribution essentially mirrors the feed ratio.
A similar analysis for the 60 : 40 feed ratio (Fig. 2, black line)
predicts a Bernoullian dyad distribution of 16%, 24%, 24%,
and 36%. Spectral integration gives 21.6%, 26.9%, 23.1%, and
28.4% with an acceptable RMS error of 4.9%. The monomer
incorporation ratio of 53.4 : 46.6 is reasonably matched to the
60 : 40 feed ratio.
Fig. 3 shows the four possible enchainment dyads for
copoly(lactide/hydroxyethylvanillic acid). The other three co-
polymers of Table 3 will contain analogous dyads, wherein A
represents enchainment of the aromatic monomer and L
represents enchainment of the lactone. Fig. 3 also depicts four
partial spectra in the region between 4.6 and 5.4 ppm, with
varying lactide : HEVA feed ratios. Generally, the dyad distri-
bution analysis for the lactide copolymers of Table 3 is compli-
cated by spectral overlap. Copoly(lactide/hydroxyethylvanillic
acid) is the clearest copolymer and provides baseline-resolved
1H NMR peaks attributable to the AA, LA, and LL dyads. For
each comonomer feed ratio, Fig. 3 provides the calculated
Bernoullian dyad distribution, along with the experimental
(1H NMR integration) dyad distribution. Note that key 1H NMR
resonances for the AL dyad overlap with other peaks near
4.5 ppm; hence the AL dyad abundance is arbitrarily set to that
of the LA dyad, which would be true for a Bernoullian co-
polymer. While this copolymerization system deviates from
pure Bernoullian statistics (with RMS error values of 8.9%,
4.1%, 10.9%, and 18.8%, respectively), it is clear that the
lactide : HEVA incorporation ratio responds proportionally to
the feed ratio and that AA and LL dyads are not overly repres-
ented, consistent with a random, non-blocky copolymer.
Generally, all eight copolymerization systems (Tables 2 and 3)
adhere to a Bernoullian dyad model or otherwise possess a
distribution of all four dyads—showing no inclination toward
blocks or an alternating microstructure. Thus, our application
of the ROP/polycondensation methodology yields fully or
mostly random copolymers of lactones and bioaromatic
hydroxy-acids, with incorporation ratios that essentially track
the feed ratios. See the ESI† for a full analysis, including 12
additional polymers made with intervening feed ratios not
reported in Table 2.
ROP/polycondensation mechanism
Caprolactone and hydroxyethylsyringic acid (HESA) were co-
polymerized in equimolar amounts over a period of 13 hours.
Fig. 4 tracks the dyad content of polymer aliquots as a function
of time. After 10 minutes of polymerization, 1H NMR indicated
that 84% of the caprolactone and 43% of HESA reacted and
AL, LA, and LL dyads were each observed. However, no AA
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Green Chem.

Table 3 Polymerization results and characterization of copolymers prepared from L-lactide and bioaromatics HESA, HEVA, HEFA, and HECAa
Monomer feed %
Entryb
Inc. HESAe (mol%)
Yield (%)
Mn f (Da)
Mw f (Da)
PDIf
Tg g (°C)
Tm g (°C)
T95% h (°C)
1
2
3
4
5
6
100
80
60
40
20
0
0
20
40
60
80
0
44
61
75
91
86.1
47.4
63.9
82.1
81.5
93.0
21 800
7800
6900
4900
3600
49 300
16 000
13 400
9000
7000
18 500
2.3
2.0
1.9
1.9
1.9
1.7
50
62
68
70
74
76
161
n.o.
n.o.
n.o.
n.o.
183
207
225
239
263
301
318
100
100
10 600
Monomer feed %
Entryc
Inc. HEVAe (mol%)
Yield (%)
Mn f (Da)
Mw f (Da)
PDIf
Tg g (°C)
Tm g (°C)
T95% h (°C)
7
8
9
10
11
80
60
40
20
20
40
60
80
41
55
72
77
100
48.1
63.8
79.0
91.8
94.9
10 500
8100
5200
21 900
16 800
10 900
2.1
2.1
2.1
66
71
77
79
82
n.o.
n.o.
178
240
254
229
233
271
281
311
4800
8500
1.8
i
i
i
0
100
Monomer feed %
Entryd
Inc. HEFAe (mol%)
Yield (%)
Mn f (Da)
Mw f (Da)
PDI f
Tg g (°C)
T95% h(°C)
12
13
14
15
16
17
90
80
70
60
20
0
10
20
30
40
80
26
38
50
59
89
32.0
46.5
47.9
66.3
83.9
89.6
16 300
10 900
10 100
6900
1500
1600
65 200
27 600
38 500
19 000
2700
4.0
2.5
3.8
2.7
1.9
1.6
64
69
74
82
107
113
220
229
244
249
323
331
100
100
2500

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dyads (from HESA-HESA condensation) were present. After
20 minutes, all caprolactone was consumed along with 45% of
HESA. Only a slight trace of AA dyads was observed, but this
dyad grew steadily thereafter. After approximately 4 hours, all
HESA was consumed. Signature ester protons –(CO)OCH2–
resonated at different chemical shifts for each dyad: AA, 4.57;
AL, 4.29; LA, 4.33; and LL, 4.04 ppm. After 5 hours, integration
of the nearly baseline resolved resonances gave an experi-
mental dyad distribution of 26.7%, 24.6%, 24.6%, and 24.1%
(corresponding to 48.7% caprolactone and 51.3% HESA). A
perfect Bernoullian dyad distribution is 25%, 25%, 25%, and
25%; thus the dyad distribution is nearly perfect with an RMS
error of 1.0%. Note from Fig. 4 (gray line) that the RMS error
achieves a global minimum at this time. Prior to T = 5 hours,
the slow reactivity of HESA dictates a low AA dyad fraction.
After T = 5 hours, the time at which vacuum is applied, it
seems that the AA dyad fraction continues to increase at the
expense of the LL dyad. At T = 13 hours, the experimental dyad
distribution reaches 31.0%, 24.8%, 24.8% and 19.4% (corres-
ponding to 44.2% caprolactone and 55.8% HESA) with an
increased RMS error of 4.1%. The deviation from equilibrium
after T = 5 hours is best explained by transesterification/back-
biting reactions that generate caprolactone, which is volatilized
and lost to vacuum at 200 °C.
GPC analysis after 1 hour showed Mn = 280 and after
4 hours, the Mn reached 3100. After 5 hours under argon, the
Mn reached 6400, whereupon dynamic vacuum was applied
and the Mn steadily progressed to 28 800 by T = 13 hours. The
oligomers formed during the reaction could be analyzed by
ESI mass spectrometry. At T = 10 minutes, the major oligomers
had the general formula of LnA1 with n = 1–6, where the dimer,
trimer, tetramer, pentamer, hexamer, and heptamer each con-
tained one or more lactones (L), but exactly one HESA (A)
monomer. By T = 20 minutes, the caprolactone monomer is
essentially consumed and the distribution of predominant oli-
gomers can be measured by ESI mass spectrometry, as
reported in Table 4 (mass + Na+, or caprolactone + H+). The oli-
gomer distribution evolves logically over 4 hours. The domi-
nant species early in the reaction are the LnA oligomers
formed relatively fast via caprolactone ring-opening. As the
reaction progresses, the LnA oligomers diminish in relative
abundance and yield to the AA, ALA, LAA, and LALA oligomers
(denoted in gray in Table 4), requiring relatively slower conden-
sation esterification. From Table 4, it is clear that initially
formed oligomers of formula LnA1 yielded to oligomers of
formula LnA2 over time, thereby increasing the aromatic
content.
The time-dependent dyad analysis, the molecular weight
evolution, and the temporal distribution of specific oligomers
can be integrated to yield the proposed mechanism in Fig. 5
for the concurrent ROP/polycondensation of caprolactone (L)
with HESA (A). Catalyst antimony oxide activates caprolactone
toward attack by the alcoholic end of HESA, which serves as
the necessary initiator. This ring-opening event generates an
LA dimer, and is the primary dimerization process. Fischer
esterification of two HESA monomers could yield the AA
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Fig. 2 Possible dyads AA, AL, LA, and LL for copoly(caprolactone/hydroxyethylferulic acid). The partial 1H NMR spectrum is shown for the copoly-
mer with 53% caprolactone and 47% HEFA incorporation (black line) and with 70% caprolactone and 30% HEFA incorporation (orange line).
Resonances marked green (downfield set of peaks) are attributable to protons proximal to HEFA (A) units, while resonances in blue (upfield set of
peaks) are attributable to protons proximal to caprolactone (L) units. The measured dyad distributions suggest a random copolymerization.
Fig. 3 Possible dyads AA, AL, LA, and LL for copoly(lactide/hydroxyethylvanillic acid). The partial 1H NMR spectra are shown for four different feed
ratios of lactide : HEVA. Calculated (Bernoullian statistics) and experimental (1H NMR) dyad distributions are shown, but peak overlap precluded direct
measurement of the AL dyad content, which was constrained to equal the LA dyad fraction for this analysis. The measured dyad distributions
suggest a mostly random copolymerization.
dimer, but this condensation reaction is relatively slow. Copolymer thermal analysis
Instead, the abundant LA dimer reacts predominantly via
chain-growth ROP with one or more caprolactones to generate Fig. 6 plots the glass transition temperature (Tg) as a function
LnA oligomers. Condensation esterification of LA with HESA of bioaromatic feed fraction for the four caprolactone/
can generate either the ALA or LAA trimers. Condensation bioaromatic copolymers (Table 2, dashed lines) and the four
coupling of two LA dimers yields the LALA tetramer. As capro- L-lactide/bioaromatic copolymers (Table 3, solid lines).
lactone is depleted, ROP yields to the slower esterification of Increasing the bioaromatic feed fraction (and thus the compo-
all oligomers, along with remaining HESA, leading to high sition fraction) led to a proportional increase in the Tg. The
molecular weight poly(caprolactone/hydroxyethylsyringic acid). thermal range covered by the L-lactide/bioaromatic copolymers
After 5 hours, there is no statistical excess of LL dyads, was ΔT = 63 °C (50–113 °C) and that spanned by the capro-
suggesting that transesterifcation processes have separated lactone/bioaromatic copolymers was much larger, ΔT = 173 °C
caprolactone units initially adjacent in the LnA oligomers— (−60–113 °C).
resulting in a polymer with a largely random monomer
Structure/thermal property relationships are evident from
sequence. The polyesterifcation and transesterfication reac- Fig. 6. The number of pendent methoxy groups on the bio-
tions are both likely catalysed by antimony oxide.
aromatic monomer had an effect on the Tg. For the cinnamic
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Table 4 Relative abundance of monomers and oligomers as a function
of time according to ESI mass spectrometry. Oligomers in white are
formed early via lactone ring-opening, while oligomers in gray are
formed late via polycondensation esterification
Fig. 4 The equimolar copolymerization of caprolactone (L) and
hydroxyethylsyringic acid (A) shows a dyad distribution (by 1H NMR) that
changes as a function of time. The deviation from equilibrium (25% each
of AA, AL, LA, and LL) is quantified by the % RMS error and reaches a
minimum (1.0%) at 5 hours with 26.7% AA, 24.6% AL, 24.6% LA, and
24.1% LL.
group of HEVA (hydroxyethylvanillic acid). And, this effect was
acid derivatives with
a main-chain alkene group, HEFA more pronounced for the caprolactone series than the lactide
(hydroxyethylferulic acid), with one methoxy group, afforded series—up to ΔT = 17 °C for the 30 : 70 copolymers (Table 2,
higher Tg values than HECA (hydroxyethylcoumaric acid), with entries 5 and 11). In summary, the Tg is usually maximized
no methoxy group. And, this effect was more pronounced for with one methoxy group on the bioaromatic. Adding or sub-
the caprolactone series than the lactide series—up to ΔT = tracting a methoxy group lowers the Tg. One explanation for
24 °C for the 50 : 50 copolymers (Table 2, entries 16 and 21). this observation is that a single methoxy group (of HEVA or
For the benzoic acid derivatives without main-chain alkene HEFA) increases main-chain conformational barriers and thus
groups, the two methoxy groups of HESA (hydroxyethylsyringic Tg (vs. HECA), but the two methoxy groups (of HESA) introduce
acid) generally lowered the Tg compared to the one methoxy a free volume effect that lowers the Tg.
Fig. 5 Proposed mechanism for the concurrent ROP/polycondensation of caprolactone (L) with HESA (A), which serves as the initiating alcohol and
forms the LA dimer by ring-opening of activated caprolactone. Longer oligomers LnA are formed by subsequent ring-opening polymerization (ROP)
and ALA, LAA, and LALA oligomers are formed via slower Fischer esterification (condensation). High molecular weight poly(caprolactone/hydroxy-
ethylsyringic acid) is produced as all oligomers, along with available HESA, undergo polycondensation, while transesterifcation ensures an essentially
random monomer distribution.
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Fig. 6 The glass transition temperature (Tg) is proportional to the bioaromatic feed fraction for copolymers of HESA (diamonds), HEVA (squares),
HEFA (triangles), and HECA (circles) with either ε-caprolactone (CL, dashed lines) or L-lactide (LA, solid lines). The lactide copolymers generally have
higher Tg values and the Tg of PET (67 °C) or PS (95 °C) can be excelled with a 20% or 60% feed fraction of HEFA, respectively, in its copolymer with
lactide.
Fig. 6 also reveals a Tg dependence on the main-chain struc- developed and applied to the melt copolymerization of lac-
ture of the bioaromatic. Generally, the Tg is greater for the cin- tones (ε-caprolactone or L-lactide) with bioaromatic hydroxy-
namic acid derivatives (HEFA and HECA) than the benzoic acids. This strategy expands upon existing methods for creat-
acid derivatives (HESA and HEVA). The presence of an alkene ing renewable aromatic/aliphatic polyesters using only
group in the main-chain increases the Tg for a given feed frac- polycondensation.53–55 Antimony oxide (1 mol%) was the best
tion. This can be explained by the greater conformational bar- catalyst among six tested and polymerization efficiency was
riers imposed by the increased fraction of main-chain sp2 increased with drying agents such as P2O5 or MgSO4, which
carbons vs. sp3 carbons, especially considering the conjugation served to capture the condensation by-product, water. A
1
of the akene unit with the adjacent aryl and ester groups. The kinetic analysis by H NMR and mass spectrometry suggested
most prominent effect on the Tg is the nature of the lactone initial, rapid ring-opening oligomerization of lactone, followed
employed. Clearly, the conformational flexibility of incorpor- by slower condensation polymerization of the oligolactones
1
ated caprolactone allows for very low Tg values, with a floor in with the bioaromatics. H NMR microstructure analysis of the
the range of −40 to −50 °C for a 10% bioaromatic feed. In isolated copolymers confirmed a random copolymerization,
comparison, the conformational rigidity and polarity of incor- suggesting that late-stage transesterification events effect
porated lactide command a much higher floor, with Tg values monomer sequence randomization. Compared to ε-capro-
near 65 °C for a 10% bioaromatic feed. As the bioaromatic lactone, the copolymerizations with L-lactide afforded poly-
content increases, the Tg values of all eight copolymer series mers with lower yields and lower molecular weights. This
increase and converge. The benzoic acid derivatives approach points to the greater reactivity of caprolactone compared to
the range of 76–82 °C, well above the Tg of PET. The cinnamic lactide, apparently because of the latter’s α-substitution56 and
acid derivatives approach the range of 109–113 °C, well above lesser ring-strain.57
the Tg of PS. Importantly, the Tg of PET (67 °C) can be excelled
Both the copoly(caprolactone/bioaromatic) series and the
with 60% of HEFA copolymerized with caprolactone copoly(L-lactide/bioaromatic) series revealed logical structure/
(Table S2,† entry 27) or as little as 20% of HEFA copolymerized property relationships, including: higher glass transition temp-
with lactide (Table 3, entry 13). And, the Tg of PS (95 °C) can erature (Tg) values for the lactide copolymers versus the capro-
be excelled with 90% of HEFA copolymerized with capro- lactone copolymers; an increase in the Tg with an increase in
lactone (Table 2, entry 18) or as little as 60% of HEFA co- the bioaromatic content; higher Tg values when the bio-
polymerized with lactide (interpolated).
aromatic comonomers possess an alkene segment (derived
from ferulic acid or coumaric acid) compared to those lacking
an alkene segment (derived from syringic acid or vanillic acid);
and higher Tg values when the bioaromatic bears one methoxy
group compared to zero or two methoxy groups. The control
Conclusions
A general and efficient one-pot, concurrent ring-opening over Tg was profound and ranged from −60 to 113 °C for
polymerization (ROP)/polycondensation methodology was copoly(caprolactone/bioaromatic) and from 50 to 113 °C for
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copoly(L-lactide/bioaromatic). The copolymerization strategy 16 M. J. Hwang, J. M. Suh, Y. H. Bae, S. W. Kim and B. Jeong,
also significantly improved polymer thermal stability, in
proportion to the bioaromatic content.
Biomacromolecules, 2005, 6, 885–890.
17 D. E. Discher and F. Ahmed, Annu. Rev. Biomed. Eng., 2006,
8, 323–341.
The tunability of the copolymer structure via this ROP/poly-
condensation method allows access to novel, biorenewable 18 G. E. Luckachan and C. K. S. Pillai, J. Polym. Sci., Part A:
copolymers with thermal properties competitive with those of Polym. Chem., 2006, 44, 3250–3260.
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607–621.
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23 C. Ba, J. Yang, Q. Hao, X. Liu and A. Cao,
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Acknowledgements
This research was supported by the National Science
Foundation
Bioplastics, LLC (usbioplastics.com), and the Florida High 25 T. Mukherjee and N. Kao, J. Polym. Environ., 2011, 19, 714–
Tech Corridor Council. The National Science Foundation is 725.
acknowledged for the ESI TOF instrument via the CRIF-MU 26 V. Siracusa, P. Rocculi, S. Romani and M. Dalla Rosa,
(CHE-1305794
and
CHE-1607263),
U.S. 24 C. S. Wu, Polymer, 2005, 46, 147–155.
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