Mechanochemical synthesis of poly(trimethylene carbonate)s: An example of rate acceleration

Article abstract of DOI:10.3762/bjoc.15.93

Mechanochemical polymerization is a rapidly growing area and a number of polymeric materials can now be obtained through green mechanochemical synthesis. In addition to the general merits of mechanochemistry, such as being solvent-free and resulting in high conversions, we herein explore rate acceleration under ball-milling conditions while the conventional solution-state synthesis suffer from low reactivity. The solvent-free mechanochemical polymerization of trimethylene carbonate using the organocatalysts 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) are examined herein. The polymerizations under ball-milling conditions exhibited significant rate enhancements compared to polymerizations in solution. A number of milling parameters were evaluated for the ball-milling polymerization. Temperature increases due to ball collisions and exothermic energy output did not affect the polymerization rate significantly and the initial mixing speed was important for chain-length control. Liquid-assisted grinding was applied for the synthesis of high molecular weight polymers, but it failed to protect the polymer chain from mechanical degradation.

Full text of DOI:10.3762/bjoc.15.93

Mechanochemical synthesis of poly(trimethylene carbonate)s:
an example of rate acceleration
Sora Park and Jeung Gon Kim*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 963–970.
Department of Chemistry and Research Institute of Physics and
Chemistry, Chonbuk National University, Jeon-Ju, Jeollabuk-do,
54896, Republic of Korea
doi:10.3762/bjoc.15.93
Received: 28 January 2019
Accepted: 11 April 2019
Published: 23 April 2019
Email:
Jeung Gon Kim* - jeunggonkim@jbnu.ac.kr
This article is part of the thematic issue "Mechanochemistry II".
Guest Editor: J. G. Hernández
* Corresponding author
Keywords:
aliphatic polycarbonate; green polymerization; mechanochemistry;
organocatalyst; poly(trimethylene carbonate)
© 2019 Park and Kim; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Mechanochemical polymerization is a rapidly growing area and a number of polymeric materials can now be obtained through
green mechanochemical synthesis. In addition to the general merits of mechanochemistry, such as being solvent-free and resulting
in high conversions, we herein explore rate acceleration under ball-milling conditions while the conventional solution-state synthe-
sis suffer from low reactivity. The solvent-free mechanochemical polymerization of trimethylene carbonate using the organocata-
lysts 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) are examined herein. The polymer-
izations under ball-milling conditions exhibited significant rate enhancements compared to polymerizations in solution. A number
of milling parameters were evaluated for the ball-milling polymerization. Temperature increases due to ball collisions and exother-
mic energy output did not affect the polymerization rate significantly and the initial mixing speed was important for chain-length
control. Liquid-assisted grinding was applied for the synthesis of high molecular weight polymers, but it failed to protect the
polymer chain from mechanical degradation.
Introduction
Nowadays mechanochemical syntheses are widespread in In the area of polymer chemistry, the use of mechanical forces
many areas of chemistry [1-4]. The efficient mixing and energy has a long history. Strong mechanical forces can break covalent
input induced by mechanical motions have promoted many bonds, including strong C–C bonds, thus their utilization has
chemical reactions with superior efficiencies [5]. Sometimes, generally focused on destructive approaches [7-9]. Recently,
unexpected outcomes that cannot be achieved by solution along with rapid progress in mechanochemical small molecule
synthesis occur, which makes mechanochemistry a topic of syntheses, the constructive polymeric material synthesis also
rigorous research [6].
succeeded. In 2014, Swager and co-workers demonstrated that
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poly(phenylene vinylene) could be obtained without any sol- of cyclic carbonates, such as trimethylene carbonate (TMC) and
vent after brief ball milling of monomer and base [10]. The its derivatives have been used for the controlled synthesis of
remarkable reactivity exemplified that the general concepts of high-molecular weight polymers. Among many catalysts,
mechanochemical synthesis are applicable to polymerization organocatalysts have attracted considerable attention, since the
reactions. Other examples of polymer syntheses have followed. use of nontoxic catalysts warrants a safe use in biomedical ap-
The Borchardt research team reported the efficient mechano- plications [24].
chemical synthesis of poly(azomethine) and poly(phenylene)
[11,12]. Our group also contributed to this area by developing a The amidine base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is
ball-milling promoted high-molecular weight poly(lactic acid) one of the best studied and most popular organocatalysts for
synthesis [13,14] and a solvent-free post-polymerization modi- ring-opening polymerizations of cyclic carbonates and lactones
fication of functional polystyrenes [15]. The Friščić team also [25-27]. In contrast to the high activity of lactone polymeriza-
showed that poly(ethylene oxide) end group modification is tion, cyclic carbonate polymerization usually requires long reac-
facile under ball-mill conditions [16]. Network polymeric mate- tion times to achieve high conversions (Table 1) [27]. The
rial fabrications were also realized using ball milling [17-19].
DBU-catalyzed polymerization of trimethylene carbonate in
chloroform, tetrahydrofuran, toluene, and methylene chloride
As mentioned, many mechanochemical reactions realized converted less than 5% monomer into poly(trimethylene
exceptional efficiencies that solution synthesis cannot afford carbonate) (PTMC) within 1 h (Table 1, entries 1–8). It general-
[5,20]. Chemical transformations at maximum concentrations ly took 24 hours to produce PTMCs with over 2,000 g/mol
benefit from no dilution, which results in fast conversions, as number average molecular weights (Mn). Among the tested sol-
long as efficient mixing is provided. We envisioned that poly- vents, the reaction in CH2Cl2 was the fastest with 43% conver-
merization systems with low propagation efficiencies under sion after 24 h (Table 1, entry 8).
solution conditions could be accelerated through mechanochem-
ical ball milling. The organocatalytic polymerization of trimeth- The same reaction was conducted using ball milling without
ylene carbonate to form aliphatic polycarbonates was found to any solvent added. All reagents were placed in a 10 mL stain-
be more efficient when using a mechanical ball-milling reac- less-steel milling jar with three 7 mm diameter stainless-steel
tion than a solution polymerization (Scheme 1). The detailed balls. The solid-state reaction mixture was placed into a high-
findings are disclosed in this article.
speed vibration ball mill. After 30 min of high-speed vibration
(30 Hz), 43% conversion was recorded, and PTMC with an Mn
of 3930 g/mol was obtained (Table 1, entry 9), which is compa-
Results and Discussion
Aliphatic polycarbonates are found in many biomedical applica- rable to that of PTMC obtained from a 24 h reaction with
tions since they have many desirable properties such as high CH2Cl2 (Table 1, entry 8). Longer milling times pushed the
biocompatibility, easy degradation, good mechanical properties, reaction to higher degrees of polymerization. A one-hour vibra-
and low toxicity [21-23]. Many synthetic methods have been tion resulted in 75% conversion with an Mn of 7380 g/mol, and
developed, and the chain-growth ring-opening polymerization the polymerization reached over 90% conversion after 2 h
Scheme 1: Fast trimethylene carbonate polymerization using a solvent-free ball-milling approach.
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Beilstein J. Org. Chem. 2019, 15, 963–970.
Table 1: DBU-catalyzed polymerization of trimethylene carbonate: solution vs ball milling.a,b.
entry
solvent
time (h)
conv (%)c
Mn
Mw
Mw/Mn
(g/mol)d
(g/mol)d
1
2
1
2
–
–
–
chloroform
THF
24
24
2120
2270
1.07
3
4
1
<1
5
–
–
–
–
–
–
24
5
6
1
<1
23
–
–
–
toluene
CH2Cl2
24
2660
2950
1.10
7
8
1
3
–
–
–
24
43
3990
4200
1.05
9
0.5
1
43
75
93
3930
7380
9230
4350
8350
1.11
1.14
1.15
ball mill
no solvent
10
11
2
10600
aPolymerization conditions: TMC (100 mg, 100 equiv), BnOH (1.02 µL, 1 equiv), and DBU (1.46 µL, 1 equiv) in 1 mL of the selected solvent at rt for
the solution reactions, or in a 10 mL stainless-steel jar with three 7 mm diameter stainless-steel balls for the ball-milling reactions. bThe average of
two runs is reported for the ball-milling reactions. cDetermined by 1H NMR spectroscopy. dDetermined by GPC calibrated with polystyrene standards
in tetrahydrofuran (THF) at 40 °C.
(Table 1, entries 10 and 11). While the reaction rate was higher 75% conversion was recorded (Table 2, entry 3). The effect of
than that of the solution reactions, polydispersity under ball- vibration frequency was found to be less pronounced as in the
milling conditions remained low (Mw/Mn = 1.15). To maintain case of lactide polymerizations [13]. The changes in ball
low polydispersity, fast initiation and slow propagation are re- numbers and size were investigated as well, which will increase
quired [28]. In the case of ball-milling polymerization, the time overall mass of a vibration system. The use of five balls instead
required for the physical mixing of monomer, catalyst, and initi- of three improved the conversion to 84% and the vibration with
ator would result in a delayed initiation of the polymerization. a 12 mm ball gave 88% conversion. The mass increase in the
However, the relatively slow propagation rate of DBU-medi- vibration system resulted in the improvement of reaction effi-
ated trimethyl carbonate polymerization allowed for well-con- ciency.
trolled chain lengths. The previous examples on mechanochem-
ical poly(lactic acid) synthesis resulted in broader molecular The high impact collision energy [28,29] and exothermic nature
weight distributions due to the fast propagation rate [13,14]. of the given ring-opening polymerization [30] could increase
the temperature of a ball-mill system, which would speed up the
Variations of the ball-milling parameters were then scrutinized polymerization rate [31,32]. To gain insight into thermal
(Table 2). Firstly, the vibration frequency was varied from effects, we monitored the temperature of the reactor and the
10 Hz to 30 Hz. Even with the low vibration experiment at reaction mixture (Figure 1). After two hours of high-speed ball
10 Hz for one hour, 60% of trimethylene carbonate were con- milling, the temperature of the reactor and mixture increased to
verted into the corresponding polymer (Table 2, entry 1), which 36 °C. To allow a direct comparison, the solution reactions were
is much faster than conversions observed in solution reactions also conducted at 40 °C. However, their efficiencies remained
collected in Table 1. An increase in the vibration frequencies far behind those of the ball-milling polymerizations (Table 3).
exhibited only a marginal effect. At 20 Hz, only 3% increase in In chloroform (Table 3, entries 1 and 2) and toluene (Table 3,
conversion was observed (63%, Table 2, entry 2) and at 30 Hz, entries 5 and 6) rate enhancements by thermal energy were ob-
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Table 2: Vibration frequency and ball size and number effects.a,b.
entry
frequency and ball diameter
conv (%)c
Mn
Mw
Mw/Mn
(g/mol)d
(g/mol)d
1
2
3
4
5
10 Hz, 7 mm × 3 ea
20 Hz, 7 mm × 3 ea
30 Hz, 7 mm × 3 ea
30 Hz, 7 mm × 5 ea
30 Hz, 12 mm × 1 ea
60
63
75
84
88
5690
6000
7380
6660
6880
6320
6510
8350
7500
7810
1.11
1.09
1.14
1.13
1.14
aPolymerization conditions: TMC (100 mg, 100 equiv), BnOH (1.02 µL, 1 equiv), and DBU (1.46 µL, 1 equiv) in a 10 mL stainless-steel jar. bThe aver-
age of the two runs is reported. cDetermined by 1H NMR spectroscopy. dDetermined by GPC calibrated with polystyrene standards in THF at 40 °C.
served, however, their efficiencies remained much lower than achieved within only 5 min (Table 4). Interestingly, TBD-based
that of the ball-milling PTMC synthesis (Table 3, entries 9 and ball-milling polymerization did not allow for controlling the
10). The observed high efficiency of the mechanochemical molecular weight distribution, resulting in a broad polydisper-
transformation could originate from a large increase in concen- sity (Mw/Mn) of 2.01 (Table 4, entry 5). As mentioned, fast ini-
tration as well as a temperature difference [5]. In the synthesis tiation over chain propagation is one of the requirements in a
of poly(trimethylene carbonate), the observations imply that controlled polymerization. While TBD could chemically en-
concentration is a more influential factor than temperature hance both the initiation and propagation steps, the mixing of
increase for the rate enhancement under ball-milling conditions. catalyst, monomer, and initiator by heterogeneous ball milling
may physically limit the initiation rate. Thus, a relatively slow
initiation process resulted in poor molecular weight control.
Most solution polymerizations had no issues with mixing and
maintained good molecular weight control. The use of slower
polymerization systems is advised for controlled polymeriza-
tions under ball-milling conditions.
Next, a high degree of polymerization was pursued. Polymer-
izations were conducted under the same conditions but with a
higher monomer to initiator ratio ([TMC]:[I]:[DBU] = 200:1:2)
(Table 5). The reaction reached over 90% conversion after 3 h.
However, the molecular weight did not increase at all. The
Figure 1: IR thermometer images showing reactor temperatures at the
competitive degradation of poly(trimethyl carbonate) became
significant after 100 degrees of polymerization. To validate me-
chanical degradation of PTMC under ball-milling conditions,
high molecular weight PTMC (Mn = 22900 g/mol) was synthe-
end of the two individual ball-milling reactions (averaged results
collected in Table 1, entry 11; individual results, see Supporting Infor-
mation File 1, left: Table S1, entry 11-1, right: Table S1, entry 11-2).
As another highly reactive organic catalyst, 1,5,7-triazabi- sized and grinded under the same mechanical conditions of
cyclo[4.4.0]dec-5-ene (TBD) was investigated next. The Table 5, entry 1, which led to degradation to lower molecular
bicyclic guanidine base TBD has shown better efficiencies than weight (Mn = 8220 g/mol). In our previous study on poly(lactic
DBU in many chemical transformations including the polymeri- acid) synthesis, liquid-assisted grinding (LAG), the addition of
zation of lactides and cyclic carbonates [27,33]. As expected, a very small amount of a liquid, prevented chain-degradation
TBD effectively promoted the polymerization of trimethylene from high impact collisions, and afforded PLA with over
carbonate both, in solution and under solvent-free ball-milling 100,000 g/mol. Thus, LAG was also tested in the PTMC syn-
conditions. Nearly quantitative conversions into polymer were thesis [13,14] with toluene and THF as liquids. A catalytic
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Table 3: DBU-catalyzed polymerization of trimethylene carbonate at 40 °C.a
entry
solvent
time (h)
conv (%)b
Mn
Mw
Mw/Mn
(g/mol)c
(g/mol)c
1
2
1
3
–
–
–
chloroform
THF
24
41
3500
3780
1.07
3
4
1
<1
6
–
–
–
–
–
–
24
5
6
1
<1
70
–
–
–
toluene
CH2Cl2
24
7130
8380
1.13
7
8
1
4
–
–
–
41
41
3000
3210
1.07
9
ball milling
1
75
7380
8350
1.14
10
ball milling (36 °C)
2
93
9230
10600
1.15
aPolymerization conditions: TMC (100 mg, 100 equiv), BnOH (1.02 µL, 1 equiv), and DBU (1.46 µL, 1 equiv) in 1 mL of the selected solvent at 40 °C.
bDetermined by 1H NMR spectroscopy. cDetermined by GPC calibrated with polystyrene standards in tetrahydrofuran (THF) at 40 °C.
Table 4: TBD-catalyzed polymerization of trimethylene carbonate: solution vs ball milling.a
entry
solvent
time (min)
conv (%)b
Mn
Mw
Mw/Mn
(g/mol)c
(g/mol)c
1
2
toluene
CHCl3
CH2Cl2
THF
5
5
5
5
5
99
96
86
76
99
12300
8650
9330
7720
12200
20000
9810
1.71
1.13
1.12
1.09
2.01
3
10500
8750
4
5d
ball mill
24400
aPolymerization conditions: (solution) TMC (100 mg, 100 equiv), BnOH (1.02 µL, 1 equiv), and TBD (1.4 mg, 1 equiv) in 1 mL selected solvent at rt;
(ball milling) in a 10 mL stainless-steel jar with three stainless-steel balls with 7 mm diameter. bDetermined by 1H NMR spectroscopy. cDetermined by
GPC calibrated with polystyrene standards in tetrahydrofuran (THF) at 40 °C. dAverage of two runs is reported.
amount of liquid (10 or 20 µL to 100 mg TMC), however, tained, regardless of LAG. The LAG for mechanochemical po-
failed to protect the poly(trimethylene carbonate) chain from lymerization reactions has been studied only in limited cases so
mechanical degradation and similar molecular weights were ob- far and the exact working mechanism is currently obscure. To
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Table 5: PTMC synthesis with a high monomer to initiator ratio.a
entry
liquid additives
time
conv (%)b
Mn
Mw
Mw/Mn
(g/mol)c
(g/mol)c
1
2
3
4
none
3 h
3 h
3 h
3 h
97
86
91
96
10900
11900
11500
11400
13600
13100
13700
13900
1.24
1.10
1.19
1.22
toluene (10 µL)
toluene (20 µL)
THF (20 µL)
aPolymerization conditions: TMC (100 mg, 200 equiv), BnOH (0.49 µL, 1 equiv), and DBU (1.46 µL, 2 equiv) in a 10 mL stainless-steel jar with three
7 mm diameter stainless-steel balls. bDetermined by 1H NMR spectroscopy. cDetermined by GPC calibrated with polystyrene standards in tetrahydro-
furan (THF) at 40 °C.
have a better understanding of LAG on chain protection, exten- ber-averaged molecular weights (Mn), weight-averaged molecu-
sive studies are currently in progress.
lar weights (Mw), and polydispersities (Mw/Mn). The RI mea-
surements were carried out using an instrument set composed of
a Waters 1515 isocratic pump, a 2414 differential refractive
Conclusion
A mechanochemical method, ball milling, was applied to the index detector, and a column-heating module with Shodex
synthesis of poly(trimethylene carbonate). The representative KF-804, KF-803, and KF-802.5 columns in series. The columns
organocatalyst, DBU, exhibited excellent polymerization effi- were eluted with tetrahydrofuran (preservative-free HPLC
ciency and good chain-length control under solvent-free condi- grade, Fisher) at 40 °C at 1.0 mL/min and calibrated using
tions. When compared to the very low rate obtained under solu- 14 monodisperse polystyrene standards (Alfa Aesar). The tem-
tion conditions, this demonstrates that mechanochemical reac- perature was recorded using a Fluke VT04 Visual IR ther-
tions can improve reaction efficiency and greenness. The use of mometer.
TBD truly enhanced the efficiency, and all polymerizations
reached completion within 5 min, despite physical mixing limi- Synthesis of trimethylene carbonate. 1,3-Propanediol
tations. However, the mechanochemical polymerization was (4.72 mL, 0.657 mol) and ethyl chloroformate (12.5 mL,
accompanied by degradation processes, which limited the mo- 0.131 mol) were dissolved in anhydrous THF (0.13 L). The
lecular weight to 10,000 g/mol. Liquid-assisted grinding did not mixture was stirred in an ice bath for 1 h and a solution of tri-
show any protective effect, and the search for other parameters ethylamine (19.2 mL, 0.138 mol) in THF (9 mL) was slowly
to mitigate polymer-chain breaking is currently in progress.
added. Then, the solution was transferred to ambient tempera-
ture and stirred for 2 h. The reaction mixture was filtered and
the volume of the solution was reduced to 40–50 mL. The mix-
Experimental
General considerations. Chemical reagents obtained from ture was kept in a freezer for 12 h and the precipitate was recov-
commercial sources were used without further purification. 1,8- ered by filtration. The recovered solid was recrystallized in
Diazabicyclo[5.4.0]undec-7-ene (DBU) was distilled over ethyl acetate and sublimed (2.9 g, 43%). 1H NMR (400 MHz,
CaH2. All solvents (THF, CH2Cl2, CHCl3, and toluene) were CDCl3) δ 4.47–4.44 (t, 4H), 2.18–2.12 (quintet, 2H).
dried over a mixture of pre-activated neutral alumina and 3 Å
molecular sieves. A Retsch Mixer Mill MM 400 was used for Representative procedure for mechanochemical solvent-free
the ball-milling experiments with a 10 mL stainless-steel vessel poly(trimethylene carbonate) synthesis (Table 1, entry 11).
and 7 mm stainless balls. 1H NMR spectra were recorded with a Three 7 mm stainless-steel milling balls were placed in a 10 mL
400 MHz Bruker Avance III HD Fourier transform NMR spec- stainless-steel milling container and trimethylene carbonate
trometer and all signals were referenced to residual protonated (0.100 g), benzyl alcohol (1.02 µL), and DBU (1.46 µL) were
solvent. Gel permeation chromatography (GPC) analyses with added. The milling vessel was placed in a vibrational ball mill
refractive index (RI) detection were used to determine the num- and vibrated at 30 Hz. After 2 hours, the vessel was opened and
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Beilstein J. Org. Chem. 2019, 15, 963–970.
15.Ohn, N.; Kim, J. G. ACS Macro Lett. 2018, 7, 561–565.
benzoic acid (10 mg) was added followed by an additional
5 minutes of milling to quench the polymerization. To avoid
data inconsistency due to inhomogeneity, all material was dis-
solved in methylene chloride and an aliquot was subjected to
analysis by 1H NMR spectroscopy and GPC measurements to
determine the conversion and molecular weight.
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Supporting Information File 1
Raw data for tables, GPC and NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
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