Mechanism of the ring-opening polymerization of ecaprolactone promoted by Bu2SnCl2 as evidenced by
multinuclear NMR spectroscopy
Gaëlle Deshayes1, Frédéric A. G. Mercier2, Philippe Degée1, Rudolph Willem2 and Philippe Dubois1
of Polymeric and Composite Materials, University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, [email protected]
2High resolution NMR Centre and Laboratory of General and Organic Chemistry, HNMR & AOSC, Free University of Brussels, room 8G511, Pleinlaan 2 B, B-1050 Brussels, BELGIUM.
1Laboratory
Introduction:
Aliphatic polyesters can be obtained by ring-opening polymerization (ROP) of cyclic esters such as e-caprolactone (CL) initiated and/or catalyzed by a wide range of ionic and non-ionic
organometallic compounds. However, only few of them are able to initiate the ROP of cyclic esters through a totally controlled and elucidated mechanism. For instance, Bu2SnCl2 added with npropanol is shown to efficiently mediate the ROP of CL in toluene at 100°C and to yield polyesters chains of predetermined molar mass from the initial monomer-to-alcohol molar ratio, at least up
to high monomer conversion, though no comprehensive mechanistic study has been conducted yet. In order to gain fundamental understanding on the polymerization mechanism, advanced1H, 13C
and 119Sn NMR investigations have been performed in situ in toluene-d8, as well as with model solutions containing Bu 2SnCl2 and binary mixtures of the reagents and/or products at various
concentrations and temperatures.
Ternary mixtures (CL+ nPrOH + Bu2SnCl2)
Polymerization of e-caprolactone (CL):
Table 2 shows the time dependence of the 119Sn chemical shift and monomer conversion as
the ROP of CL proceeds.
The ring-opening polymerization of CL ([CL]0 = 4.51 mol.L-1 ) has been initiated with npropanol ([nPrOH]0 = 4.51 x 10-2 mol.L-1) in the presence of dibutyltin dichoride
([Bu2SnCl2]0 = 2.25 x 10-3 mol.L-1 ) in toluene at 100°C. Table 1 shows the time
dependence of monomer conversion, number average molecular weight (Mn) and molecular
weight distribution (Mw/Mn).
Table 2: Time dependence of 119Sn chemical shift and monomer conversion as determined
by NMR spectroscopy at 303 K ([CL] = 4.5 mol.L-1 , [nPrOH] = 0.145 mol.L-1 , [Bu2SnCl2]
= 0.1125 mol.L-1 , polymerization temperature = 353 K)
Table 1 : Time dependence of monomer conversion, number average molecular weight (Mn)
and polydispersity index (Mw/Mn)
Entry
1
2
3
4
Time (h)
24
48
72
94
Mntheor (1)
2700
5400
11200
Conv (%)
~1
24
47
98
MnNMR(2)
2200
6300
12400
MnSEC (3)
2600
6100
9800
Entry
1
2
3
4
5
6
7
8
Mw/M n
1.04
1.05
1.77
(1)Mn
theor = ([CL]0/[nPrOH] 0 x conv x MWCL) + MWnPrOH
(2)Mn
1
NMR = [(1 + (I CH2-O-CO/I CH2-OH) x MWCL] + MWnPrOH, as determined by H NMR
spectroscopy
(3)As determined by SEC with reference to polystyrene standards using an universal
calibration (KPS=1.25x10-4 dl/g, aPS=0.707, KPCL=1.09x10-3dl/g et aPCL=0.600)
O
C O CH2
CH2 C O
(polymer + propyl)
C CH2 C
Conv (%)
0.0
29.3
43.0
73.3
81.6
98.6
99.4
99.8
COpropyloxy
4.0
3. 5
CH3 (O
3 .0
2. 5
2 .0
1.5
1 .0
0 .5
t = 258 h
COCL
t = 160 h
t = 90 h
Pr)
t=0h
177
p pm
Figure 1:
NMR spectrum of poly(e-caprolactone) as obtained by polymerization of
e-caprolactone initiated by n-propanol in the presence of Bu2SnCl2 in toluene at
100°C for 72h (entry 3 table 1)
1H
30
10
175
174
173
172
171
ppm
Figure 6 shows that the experimental poly(ecaprolactone) molar mass (MnNMR) increases
linearly with monomer conversion meaning that
the number of active species remains constant
all along the ROP.
4000
3500
3000
M n (NMR )
Response (mV)
50
176
Figure 5: Time dependence of 13C NMR spectra of poly(e-caprolactone) as obtained by
direct polymerization of e-caprolactone in a NMR tube.
As far as the molecular weight distribution is
concerned, it is remarkably narrow (Mw/Mn <
1.1), at least up to monomer conversion of
50% (entries 2, 3 in Table 1). When the
reaction time exceeds 72h, transfer reactions
actually happen, which lead to a broadening
of the molecular weight distribution (Figure
2).
70
Sn chemical shift (ppm)
26.2
34.7
37.6
49.7
54.5
71.8
72.4
72.2
The evolution of 13C NMR signals (focusing on
the carbonyl region) with the polymerization
time is shown in Figure 5. The formation of
cyclic oligomers can be excluded, at least within
the time required to reach maximum monomer
conversion since only carbonyl signals
characteristic of linear polyester chains (at
173.1 ppm) and of monomer (at 175.2 ppm) are
detected.
COPCL
t = 42 h
CH2 OH
119
It shows that the single narrow 119Sn resonance drifts to higher chemical shifts as the
polymerization proceeds. It worth noting that the 119Sn chemical shift at the end of the
polymerization process remains far from 120 ppm, which attests that pure four-coordinate
tin configuration is not preserved. In other words, polyester chains are also coordinated
onto tin atom extending tin coordination number.
Figure 1 shows the 1H NMR spectrum of a apropyloxy, w- hydroxy poly(e-caprolactone)
(entry 3, Table 1). It demonstrates that the
ROP of CL is actually initiated by n-propanol
and proceeds through the O-acyl rupture of
the monomer in the absence of any transfer
and irreversible terminaison reactions.
O
Time (h)
0.0
68.0
91.0
140.0
166.5
236.0
260.0
284.0
2500
2000
1500
1000
500
-10
12
13
14
15
16
17
0
18
0
Retention Volume (ml)
20
40
60
80
Figure 2: SEC traces of poly(e-caprolactone) as obtained by polymerization of ecaprolactone initiated by n-propanol in the presence of Bu2SnCl2 in toluene at 100°C for
various reaction times: 48h (full line with circle), 72h (full line with triangle) and 94h
(full line)
Figure 6: Dependence of poly(e-caprolactone) molar mass on monomer conversion in
toluene-d8 at 353 K ([CL] = 4.5 mol.L-1 , [nPrOH] = 0.145 mol.L-1 , [Bu2SnCl2] =
0.1125mol.L-1 )
The 119Sn NMR spectra of purified Bu2SnCl2 in toluene-d8 are consistent with a fourcoordinate tin atom and the virtual absence of coordinating expansion (d ~ 120 ppm).
O
O
Binary mixtures (Bu 2SnCl2 +CL or +nPrOH)
Bu
O
Cl
H
O
Bu
O
Sn (ppm)
Sn (ppm)
300
T (K)
320
340
360
of CL
Bu
O
O
220
Cl
Sn
O
Cl
Bu
CN = 6*
20
280
in absence
Cl
Bu
Scheme 1 illustrates the evolution of
tin configuration that likely prevails as
the polymerization proceeds. It is
worth noting that a fast dynamic
equilibrium between coordinated tin
species exists at the polymerization
temperature, especially for tin species
coordinating hydroxy groups.
CN = 6*
PCL
40
260
Cl
Sn
O
O
-40
O
O
60
-20
Bu
O
PCL
Bu
PCL
80
•
•
0
Bu
RO P
100
119
119
20
Cl
CN = 6*
Cl
Cl
Cl
Sn
O
Bu
Sn
Bu
O
CN = 6 *
CN = 6*
40
O
n Pr
Cl
O
CN = 4
O
120
nPrO H
Sn
O
140
60
Cl
O
Cl
PCL
80
H
Bu
Bu
Sn
Figures 3 and 4 show the temperature dependence of the 119Sn chemical shift for binary
compositions containing Bu2SnCl2 and CL or Bu2SnCl2 and nPrOH.
240
120
Conclusion:
NMR investigations:
220
100
conversion (%)
240
260
280
300
320
340
360
T (K)
Figure 3: Binary composition in tolueneFigure 4: Binary composition in toluened8 of Bu2SnCl2 (0.1125 mol.L-1 ) and CL
d8 of Bu2SnCl2 (0.1125 mol.L-1 ) and
nPrOH (0.225 mol.L-1 )
(4.5 mol.L-1 )
Clearly, tin species coordinated or not by n-propanol are in dynamic equilibrium. At high
temperature, this equilibrium is shifted towards not coordinated species (without actually
excluding that n-propanol does interact with tin). In contrast, CL coordinates the tin atom
and brings, at least partially, the four-coordinate Bu2SnCl2 in more favorable five and/or
six-coordinate configurations to initiate the coordination-insertion polymerization.
Scheme 1: Tin configuration as the polymerization of CL proceeds (*CN = 4 and 5 also
present)
Activation of the ROP of cyclic esters and control of molecular parameters can be achieved
by using tin (IV) derivatives, such as Bu2SnCl2 which likely behaves as a catalyst activating
the carbonyl group of the monomer while n-propanol acts as the initiator. The
electrophilicity of the metal center plays an important role in the controlled character of the
ROP sequence through the higher stability of the tin lowest unoccupied molecular orbital
with the monomer highest occupied molecular orbital rather than the hydroxyl group one.
Advanced 119Sn NMR spectroscopy has been shown to be a powerful tool for monitoring the
coordination of organotin catalysts and elucidating the polymerization mechanism.