Montira Sriyaiabc,
Tawan Chaiwonac,
Robert Molloyd,
Puttinan Meepowpanabd and
Winita Punyodom*abd
aDepartment of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
bCenter of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
cGraduate School, Chiang Mai University, Chiang Mai 50200, Thailand
dCenter of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand. E-mail: winitacmu@gmail.com
First published on 8th December 2020
Novel soluble liquid tin(II) n-butoxide (Sn(OnC4H9)2), tin(II) n-hexoxide (Sn(OnC6H13)2), and tin(II) n-octoxide (Sn(OnC8H17)2) initiators were synthesized for use as coordination–insertion initiators in the bulk ring-opening polymerization (ROP) of L-lactide (LLA). In order to compare their efficiencies with the more commonly used tin(II) 2-ethylhexanoate (stannous octoate, Sn(Oct)2) and conventional tin(II) octoate/n-alcohol (SnOct2/nROH) initiating systems, kinetic parameters derived from monomer conversion data were obtained from non-isothermal differential scanning calorimetry (DSC). In this work, the three non-isothermal DSC kinetic approaches including dynamic (Kissinger, Flynn–Wall, and Ozawa); isoconversional (Friedman, Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW)); and Borchardt and Daniels (B/D) methods of data analysis were compared. The kinetic results showed that, under the same conditions, the rate of polymerization for the 7 initiators/initiating systems was in the order of liquid Sn(OnC4H9)2 > Sn(Oct)2/nC4H9OH > Sn(Oct)2 ≅ liquid Sn(OnC6H13)2 > Sn(Oct)2/nC6H13OH ≅ liquid Sn(OnC8H17)2 > Sn(Oct)2/nC8H17OH. The lowest activation energies (Ea = 52, 59, and 56 kJ mol−1 for the Kissinger, Flynn–Wall, and Ozawa dynamic methods; Ea = 53–60, 55–58, and 60–62 kJ mol−1 for the Friedman, KAS, and OFW isoconversional methods; and Ea = 76–84 kJ mol−1 for the B/D) were found in the polymerizations using the novel liquid Sn(OnC4H9)2 as the initiator, thereby showing it to be the most efficient initiator in the ROP of L-lactide.
Tin(II) 2-ethylhexanoate or stannous octoate (Sn(Oct)2) is the most common initiator used in the ROP of lactones and lactides due to its effectiveness and versatility. It is also easy to handle and is soluble in most common organic solvents and monomers.10–12 Moreover, it has been approved for use as a food additive by the US Food and Drug Administration (FDA). However, in order to improve its effectiveness, Sn(Oct)2 is usually used in combination with an alcohol co-initiator (ROH) as the initiating system.
Mechanistically, Kricheldorf and co-workers proposed the coordination of the Sn(Oct)2 and the alcohol with the cyclic ester monomer followed by ROP.13 In this mechanism, the Sn(Oct)2 serves as a catalyst while the alcohol is an initiator, as depicted in Fig. 1(a). However, Penczek et al. later suggested that the Sn(Oct)2 and the alcohol react together resulting in the formation of a tin(II) monoalkoxide ((Oct)Sn(OR)) and tin(II) dialkoxide (Sn(OR)2) which then become the ‘true’ initiators as shown in Fig. 1(b).14 This latter mechanism is now widely accepted as the true initiation pathway. Therefore, in order to produce high molecular weight polyesters via ROP, it is vitally important to know the exact concentration of the tin(II) alkoxide initiator which should be easily and completely soluble in the cyclic ester monomer.
Fig. 1 Comparison of the old (a) and the new (b) mechanistic proposals using Sn(Oct)2: (a) complexation of the monomer and alcohol prior to ROP and (b) formation of tin(II) alkoxide before ROP.13,14 |
As previously mentioned, it is now widely accepted that the tin(II) monoalkoxide, (Oct)Sn(OR), and/or the dialkoxide, Sn(OR)2, are the true initiators formed in situ via the reaction between Sn(Oct)2 and ROH. Since the coupled reactions in Fig. 1(b) are both reversible and interdependent, the exact initiator concentrations will be unknown. Moreover, stannous octoate is well known to be an effective transesterification catalyst, ROH can act as a chain transfer agent, and the octanoic acid (OctH) by-product can also catalyze other unwanted side-reactions. Thus, the kinetics of the ROP and the final molecular weight of the polymer cannot be accurately predicted. Therefore, in order to overcome this problem, it is logical to synthesize the tin(II) dialkoxide (Sn(OR)2) separately so that it can be used directly in an accurately known concentration.
The synthesis of solid tin(II) alkoxides has been known for over 50 years.15,16 However, they have been found to have their drawbacks as initiators, in particular their difficult solubility in cyclic ester monomers and common organic solvents and their instability in contact with air and moisture which is caused by their solid-state molecular self-aggregations as shown in Fig. 2(a) and (b). According to their low solubility in most organic solvents and cyclic ester monomers of these solid tin(II) alkoxides, the polymerizations of monomers such as L-lactide, D-lactide, DL-lactide, ε-caprolactone and other cyclic esters are relatively slow and ineffective.
Fig. 2 Molecular self-aggregations in solid tin(II) alkoxides: (a) angular aggregation and (b) linear aggregation. |
Consequently, in this work, some novel liquid soluble tin(II) alkoxides were prepared for use in the ROP of L-lactide in order to overcome the molecular self-aggregation difficulties found in previous works with solid tin(II) alkoxides. Meepowpan and co-workers have proposed a method for synthesizing soluble liquid Sn(OR)2 using stoichiometric amounts of anhydrous tin(II) chloride (SnCl2), diethylamine ((C2H5)2NH), and an alcohol (ROH).17 Because the liquid Sn(OR)2 is completely soluble, it leads to more reproducible results which, in turn, leads to a more detailed understanding of the kinetics and mechanism of the ROP reaction. Using the Sn(OR)2 initiator directly instead of generating it in situ should give more reproducible and predictable results in terms of both kinetic and molecular weight control compared with the Sn(Oct)2/ROH initiating system or Sn(Oct)2 alone.
In order to investigate the efficiency of the liquid tin(II) alkoxide initiators, the kinetics of the ROP of LA were investigated by measuring the decrease in the monomer concentration as a function of time. This can be done by a variety of methods such as dilatometry, gravimetry, proton-nuclear magnetic resonance (1H-NMR) spectroscopy, infrared (IR) spectroscopy, Raman spectroscopy, and differential scanning calorimetry (DSC).18–23
Among those aforementioned methods, DSC has been found to be a fast and convenient method for studying polymerization kinetics and for determining kinetic parameters such as monomer conversion (α), rate of polymerization (dα/dt), order of reaction (n), and activation energy (Ea) of the ROP of both liquid and solid cyclic ester monomers. Furthermore, with the advent of convenient software-based data analysis programs, the ability to obtain such kinetic information has become more practical compared to other techniques.24–27
DSC kinetic experiments can be performed under either isothermal or non-isothermal conditions. In isothermal DSC, the polymerization is conducted at a constant temperature while in non-isothermal DSC, polymerization occurs during a temperature scan at a constant heating rate. The conversion of monomer to polymer can be determined from the amount of heat released from the reaction at any time t (ΔHt) divided by the total heat of reaction (ΔHm). Then, from the monomer conversion, various kinetic parameters such as rate of polymerization (dα/dt), activation energy (Ea), and rate constant (k) can be determined.
However, due to the overlap of the endothermic lactide melting peak and the exothermic peak from its polymerization in isothermal DSC, as reported in previous work,24 only non-isothermal DSC studies of the ROP of LLA polymerization in bulk were carried out in this work.
(1) |
The synthesis process employed anhydrous tin(II) chloride (SnCl2, Sigma-Aldrich) dissolved in n-heptane (nC7H16, Sigma-Aldrich) mixed with dry diethylamine ((C2H5)2NH, Panreac). An alcohol, n-ROH, in which the R group was either n-C4H9, n-C6H13, or n-C8H17 (Labscan) was added to the reaction mixture and stirred for 12 hours. The reaction mixture of the diethylamine hydrochloride salt, the n-heptane solvent plus any residual alcohol, ROH, and diethylamine was then filtered under a nitrogen atmosphere before being evaporated to dryness. All of the three tin(II) alkoxides, namely: tin(II) n-butoxide (Sn(OnC4H9)2), tin(II) n-hexoxide (Sn(OnC6H13)2), and tin(II) n-octoxide (Sn(OnC8H17)2) were obtained as viscous, dark yellow liquids which were readily soluble in most common organic solvents such as chloroform, toluene, and n-heptane. Moreover, they could all be stored under an inert atmosphere for long periods i.e. up to 1 month without any significant change in their reactivity and, therefore, in their efficiency as initiators for the ROP of L-lactide. Fig. 3 shows chemical structures of stannous octoate and the three liquid tin(II) n-alkoxides synthesized in this work.
Fig. 3 Chemical structures of: tin(II) octoate; tin(II) n-butoxide; tin(II) n-hexoxide; and tin(II) n-octoxide initiators. |
(2) |
The dependence of a rate constant (k) on temperature (T) is given by the Arrhenius eqn (3):31
(3) |
The activation energy (Ea) of the polymerization reaction can be determined by using peak methods which consider using the peak temperature at maximum rate (Tp) such as the Kissinger and Ozawa methods and using the temperature at 50% conversion (T50%) such as the Flynn–Wall method. The Kissinger, Flynn–Wall, and Ozawa methods are DSC dynamic methods which rely on approximating the so-called temperature integral and require data on temperature only.32,33
The Kissinger method uses the temperature at which the rate of polymerization is at the maximum (Tp) and the activation energy, Ea, is obtained from the maximum reaction rate where d(dα/dt)/dt is zero under a constant heating rate condition leading to eqn (4) as follows:34,35
(4) |
The Flynn–Wall method is an integral method for determining the activation energies without any reaction order.36 This method combining with Doyle's approximation leads to eqn (5):37
(5) |
Similar to the Flynn–Wall method, the Ozawa method uses the temperature at which the rate of polymerization is at the maximum which, when combined with Doyle's approximation, leads to eqn (6):38
(6) |
In this method, plots of logβ against 1/Tp are used to determine the energy of activation (Ea) from the slopes.
These isoconversional methods employ multiple temperature programs (e.g., different heating rates) in order to obtain data from various rates at a constant fraction of conversion (extent of reaction), α. In other words, isoconversional methods allow complex (i.e., multi-step) processes to be determined via a variation in activation energy (Ea) with conversion α.41 This means that, if a significant variation in Ea occurs with conversion α, the process is complex. On the other hand, if Ea is independent of conversion α, then the reaction is a single-step process. Isoconversional methods are based exclusively on dynamic DSC analysis.
Based on eqn (2) and (3), the kinetic approach proposed by Friedman expresses the logarithm of reaction rate, ln(dα/dt), as a function of the reciprocal temperature, as shown in eqn (7).42 This enables the activation energy Ea to be determined for each fraction of conversion, α:
(7) |
This equation implies that the reaction rate is only a function of temperature at a constant value of α. It is obvious that if the function f(α) is constant for a particular value of α, then ln(Af(α)) is constant as well. Therefore, by plotting ln(dα/dt) against 1/T, a value for −Ea/R, and hence Ea, can be obtained.
The Kissinger–Akahira–Sunose (KAS) method is based on eqn (8) as follows:
(8) |
The activation energy Ea can then be obtained from the slope of a semi-log plot of ln(β/T2) against 1/T at constant conversion.
The Ozawa–Flynn–Wall (OFW) isoconversional method uses Doyle's approximation leading to eqn (9):43,44
(9) |
(10) |
(11) |
(12) |
Eqn (12) can be solved with a multiple linear regression of the general form: z = a + bx + cy (where z ≅ ln[dα/dt]; ln(A) ≅ a; b ≅ n; x ≅ ln[1 − α]; c ≅ −Ea/R; and y ≅ 1/T). The values for dα/dt; α; and T are experimental parameters obtained from a single linear heating rate DSC experiment scanning through the temperature region of the reaction exotherm as shown in Fig. 4.
Fig. 4 Idealized DSC curve for kinetic parameters determination by using Borchardt and Daniels (B/D) method. |
In this B/D method, assume a value for n = 1, then the value for ln[k(T)] can be calculated using eqn (13):
ln[k(T)] = ln[dα/dt] − nln[1 − α] | (13) |
Then Ea can be obtained from the slope of a plot of ln[k(T)] against 1/T at constant conversion (α = 0.1–0.9).
Polymer code | [Sn(OnC4H9)2] (mol%) | Temperature (°C) | Polymerization time (h) |
---|---|---|---|
PLLA 1 | 0.01 | 100 | 24 |
PLLA 2 | 0.01 | 110 | 24 |
PLLA 3 | 0.01 | 120 | 24 |
PLLA 4 | 0.01 | 130 | 24 |
PLLA 5 | 0.01 | 140 | 24 |
PLLA 6 | 0.01 | 150 | 24 |
PLLA 7 | 0.05 | 100 | 24 |
PLLA 8 | 0.05 | 110 | 24 |
PLLA 9 | 0.05 | 120 | 24 |
PLLA 10 | 0.05 | 130 | 24 |
PLLA 11 | 0.05 | 140 | 24 |
PLLA 12 | 0.05 | 150 | 24 |
PLLA 13 | 0.10 | 100 | 24 |
PLLA 14 | 0.10 | 110 | 24 |
PLLA 15 | 0.10 | 120 | 24 |
PLLA 16 | 0.10 | 130 | 24 |
PLLA 17 | 0.10 | 140 | 24 |
PLLA 18 | 0.10 | 150 | 24 |
PLLA 19 | 0.50 | 100 | 24 |
PLLA 20 | 0.50 | 110 | 24 |
PLLA 21 | 0.50 | 120 | 24 |
PLLA 22 | 0.50 | 130 | 24 |
PLLA 23 | 0.50 | 140 | 24 |
PLLA 24 | 0.50 | 150 | 24 |
PLLA 25 | 1.00 | 100 | 24 |
PLLA 26 | 1.00 | 110 | 24 |
PLLA 27 | 1.00 | 120 | 24 |
PLLA 28 | 1.00 | 130 | 24 |
PLLA 29 | 1.00 | 140 | 24 |
PLLA 30 | 1.00 | 150 | 24 |
Fig. 6 Ea determinations based on dynamic methods of: (a) Kissinger; (b) Flynn–Wall; and (c) Ozawa for the ROP of L-lactide using 1.0 mol% liquid Sn(OnC4H9)2 as an initiator. |
Initiators/initiating systems | Heating rate, β (°C min−1) | Tp (°C) | T50% (°C) | Ea (kJ mol−1) | ||
---|---|---|---|---|---|---|
Kissingera | Flynn–Wallb | Ozawac | ||||
a Kissinger: d[ln(β/Tp2)]/d(1/Tp) = −Ea/R = slope.b Flynn–Wall: logg(α) = log(Af(cat)Ea/R) − logβ − 2.315 − 0.457 (Ea/RT50%), slope = −0.457 (Ea/R).c Ozawa: logβ = constant − 0.4567 (Ea/RTp), slope = −0.4567 (Ea/R). | ||||||
Sn(Oct)2 | 5 | 142.0 | 141.8 | 57 | 65 | 61 |
10 | 157.8 | 157.0 | ||||
15 | 167.3 | 166.0 | ||||
20 | 176.0 | 173.0 | ||||
Liquid Sn(OnC4H9)2 | 5 | 128.9 | 131.1 | 52 | 59 | 56 |
10 | 145.3 | 147.2 | ||||
15 | 150.8 | 152.5 | ||||
20 | 160.3 | 161.7 | ||||
Sn(Oct)2/nC4H9OH | 5 | 139.2 | 138.8 | 60 | 67 | 64 |
10 | 152.3 | 151.7 | ||||
15 | 161.5 | 160.3 | ||||
20 | 170.0 | 169.0 | ||||
Liquid Sn(OnC6H13)2 | 5 | 144.8 | 144.2 | 57 | 60 | 61 |
10 | 157.5 | 157.1 | ||||
15 | 170.0 | 167.3 | ||||
20 | 177.0 | 174.3 | ||||
Sn(Oct)2/nC6H13OH | 5 | 148.8 | 148.5 | 61 | 67 | 66 |
10 | 160.0 | 159.0 | ||||
15 | 174.3 | 173.3 | ||||
20 | 179.3 | 178.0 | ||||
Liquid Sn(OnC8H17)2 | 5 | 152.3 | 152.8 | 65 | 71 | 69 |
10 | 168.5 | 167.3 | ||||
15 | 175.5 | 175.8 | ||||
20 | 185.3 | 184.0 | ||||
Sn(Oct)2/nC8H17OH | 5 | 160.3 | 158.3 | 71 | 74 | 72 |
10 | 169.3 | 168.0 | ||||
15 | 177.5 | 176.5 | ||||
20 | 187.0 | 186.7 |
From the original DSC thermograms of heat flow (normalized, W g−1) against temperature (°C) at the four different heating rates of 5, 10, 15, and 20 °C min−1, it was found that the temperature at the maximum peak (Tp) and the temperature of 50% L-lactide monomer conversion (T50%) were found to increase with increasing heating rate.
However, when considering the initial temperatures (i.e. onset temperatures, Tonset), it was found that Tonset only slightly increased when increasing the heating rate. This observation was seen for almost every initiator/initiating system. Interestingly, the lowest Tonset polymerization temperature of ∼108 °C was found using the liquid Sn(OnC4H9)2 initiator (Fig. 5(b)) indicating its potential as an initiator to synthesize the PLA at low temperature and yielding the polymer with controlled molecular weight and narrow molecular weight distribution in a short period of time.
Furthermore, the Ea values for the L-lactide polymerizations using the seven initiators/initiating systems can be obtained from the slopes of the plots of (a) ln(β/Tp2) as a function of 1/Tp, (b) logβ as a function of 1/T50% and (c) logβ as a function of 1/Tp based on the Kissinger, Flynn–Wall and Ozawa methods respectively. The Ea (kJ mol−1) values obtained from linear equation fitting for these three dynamic methods are compared in Table 2 as previously mentioned.
From the results obtained, the Ea values are seen to be in the order of liquid Sn(OnC4H9)2 < Sn(Oct)2/nC4H9OH < Sn(Oct)2 ≅ liquid Sn(OnC6H13)2 < Sn(Oct)2/nC6H13OH ≅ liquid Sn(OnC8H17)2 < Sn(Oct)2/nC8H17OH.
According to isoconversional methods, values of Ea of the ROP of L-lactide were determined from the non-isothermal DSC data by the three methods of Friedman, KAS, and OFW. The plots of ln(dα/dt), ln(β/dT2), and lnβ against 1/T (K−1) based on these methods for the ROP of L-lactide using 1.0 mol% liquid Sn(OnC4H9)2 as an initiator are shown in Fig. 8(a)–(c). In general, linear plots were obtained for each of the initiating systems although the Friedman plots showed more scattering of the points than those of the KAS and OFW. The values of Ea at a particular conversion, α, can be obtained from the slopes of these linear plots and the values are summarized in Table 3. Again, the Friedman plots showed more variation in Ea with α than the KAS and OFW plots, indicating that the isoconversional KAS and OFW methods are more suitable for Ea determination in L-lactide polymerization. From Fig. 9, similar smaller Ea variations with α were observed for the KAS and OFW methods with the former being consistently <5 kJ mol−1 lower than the latter. The consistency of the Ea values over almost the complete range of conversion is an indication that the coordination–insertion mechanism ring-opening polymerization under the conditions is the sole mechanism which is operational in L-lactide employed in this study.
Fig. 8 Determination of Ea based on isoconversional methods of: (a) Friedman; (b) KAS; and (c) OFW for the ROP of L-lactide using 1.0 mol% liquid Sn(OnC4H9)2 as an initiator. |
Initiator/initiating system | Ea range (kJ mol−1) | ||
---|---|---|---|
Friedmana | KASb | OFWc | |
a Friedman: ln(dα/dt) = ln(Af(α)) − (Ea/RT), slope = −Ea/R.b KAS: ln(β/T2) = ln[(AR)/Ea] − lng(α) − Ea/RT, slope = −Ea/R.c OFW: lnβ = ln[(AE)/R] − lng(α) − 5.331 − 1.052 (Ea/RT), slope = 1.052 (−Ea/R). | |||
Sn(Oct)2 | 58–63 | 59–74 | 63–77 |
Sn(OnC4H9)2 | 53–60 | 55–58 | 60–62 |
Sn(Oct)2/nC4H9OH | 56–63 | 56–64 | 60–67 |
Sn(OnC6H13)2 | 56–61 | 56–60 | 60–64 |
Sn(Oct)2/nC6H13OH | 60–63 | 63–71 | 67–74 |
Sn(OnC8H17)2 | 66–76 | 66–72 | 70–76 |
Sn(Oct)2/nC8H17OH | 68–81 | 65–74 | 69–78 |
From Table 3, Ea values were found to be the lowest for the Sn(OnC4H9)2 initiator (Ea = 53–60, 55–58, and 60–62 kJ mol−1 for Friedman; KAS; and OFW respectively) and the highest for Sn(Oct)2/nC8H17OH (Ea = 68–81, 65–74, and 69–78 kJ mol−1 for Friedman, KAS, and OFW respectively). When comparing the results at the same heating rate of 5 °C min−1 (see Fig. 10(a) and (b)), the conversion and rate plots showed different values of Tonset with different conversions or rates of polymerization in the order of: liquid Sn(OnC4H9)2 > Sn(Oct)2/nC4H9OH > Sn(Oct)2 ≅ liquid Sn(OnC6H13)2 > Sn(Oct)2/nC6H13OH ≅ liquid Sn(OC8H17)2 > Sn(Oct)2/nC8H17OH.
Initiators/initiating systems | Heating rate, β (°C min−1) | Kinetic parameters | ||
---|---|---|---|---|
Eaa (kJ mol−1) | Ab (min−1) | kapp, 150 °Cc (min−1) | ||
a B/D: multiple linear regression of eqn (12), −Ea/R = slope.b ln(A) = intercept.c kapp = apparent rate constant at 150 °C = dα/dt/(1 − α)n. | ||||
Sn(Oct)2 | 5 | 79 | 5.42 × 1010 | 0.3150 |
10 | 89 | 2.35 × 109 | 0.3162 | |
15 | 89 | 1.49 × 109 | 0.3747 | |
20 | 91 | 6.32 × 108 | 0.4163 | |
Liquid Sn(OnC4H9)2 | 5 | 76 | 3.40 × 1010 | 0.5057 |
10 | 78 | 2.65 × 1010 | 0.5742 | |
15 | 80 | 2.19 × 1010 | 0.6729 | |
20 | 84 | 2.87 × 1010 | 0.7696 | |
Sn(Oct)2/nC4H9OH | 5 | 78 | 7.82 × 109 | 0.4230 |
10 | 84 | 2.59 × 1010 | 0.4322 | |
15 | 87 | 3.05 × 1010 | 0.4346 | |
20 | 88 | 2.57 × 1010 | 0.4936 | |
Liquid Sn(OnC6H13)2 | 5 | 80 | 2.63 × 1011 | 0.2323 |
10 | 87 | 3.40 × 1011 | 0.2352 | |
15 | 87 | 2.19 × 1011 | 0.2458 | |
20 | 87 | 2.07 × 1011 | 0.2773 | |
Sn(Oct)2/nC6H13OH | 5 | 89 | 3.64 × 1011 | 0.1710 |
10 | 90 | 3.79 × 1011 | 0.1722 | |
15 | 92 | 4.63 × 1011 | 0.1975 | |
20 | 94 | 1.13 × 109 | 0.2172 | |
Liquid Sn(OnC8H17)2 | 5 | 90 | 3.52 × 1011 | 0.1446 |
10 | 91 | 4.36 × 109 | 0.1475 | |
15 | 96 | 9.56 × 109 | 0.1617 | |
20 | 97 | 9.48 × 109 | 0.1639 | |
Sn(Oct)2/nC8H17OH | 5 | 97 | 7.66 × 108 | 0.0372 |
10 | 98 | 3.44 × 109 | 0.0943 | |
15 | 98 | 7.84 × 1010 | 0.1077 | |
20 | 100 | 1.27 × 1010 | 0.1082 |
Similar to previous dynamic and isoconversional approaches, liquid tin(II) n-butoxide shown to be the most efficient initiator due to its lowest activation energy range (76–84 kJ mol−1) with highest apparent rate constant values.
What the Ea value for L-lactide ring-opening polymerization in bulk means essentially is the Ea value for the propagation step since the number of propagation steps far outnumber the initiation and termination steps. Therefore, the actual meaning of Ea can be visualized as shown in the energy diagram in Fig. 11.
Fig. 12 DSC thermograms for (a) 1st run and (b) 2nd run of the purified poly(L-lactide) (PLLA 1) using 0.01 mol% liquid Sn(OnC4H9)2 as an initiator at 100 °C for 24 h. |
Information on the DSC thermal transition temperatures (i.e. the glass transition temperature (Tg), the crystallisation temperature (Tc), and the melting temperature (Tm)) as well as the heat of crystallization (ΔHc) and the heat of melting (ΔHm) from all synthesized PLLA polymer products were summarized in Table 5.
Polymer code | Tg (°C) | Tc (°C) | Tm (°C) | ΔHc (J g−1) | ΔHm (J g−1) | |||||
---|---|---|---|---|---|---|---|---|---|---|
1st | 2nd | 1st | 2nd | 1st | 2nd | 1st | 2nd | 1st | 2nd | |
PLLA 1 | 68.5 | 56.6 | — | 94.2 | 181.0 | 178.3 | — | 41.0 | 64.1 | 76.0 |
PLLA 2 | 67.0 | 54.3 | — | 98.8 | 178.0 | 175.5 | — | 40.4 | 64.5 | 70.5 |
PLLA 3 | 69.9 | 55.6 | — | 103.0 | 179.8 | 179.5 | — | 38.6 | 46.4 | 56.1 |
PLLA 4 | 68.6 | 59.8 | — | 106.8 | 177.8 | 177.7 | — | 34.2 | 38.0 | 45.9 |
PLLA 5 | 63.7 | 58.7 | — | 108.8 | 176.0 | 175.7 | — | 40.5 | 43.3 | 47.1 |
PLLA 6 | — | 59.1 | — | 105.3 | 176.5 | 176.8 | — | 36.2 | 42.2 | 50.9 |
PLLA 7 | 54.2 | 51.6 | — | 95.2 | 178.5 | 174.8 | — | 38.6 | 72.1 | 68.1 |
PLLA 8 | 69.9 | 55.2 | — | 118.0 | 178.7 | 177.0 | — | 53.6 | 52.5 | 47.5 |
PLLA 9 | 68.6 | 55.8 | — | 106.7 | 178.2 | 178.7 | — | 39.3 | 52.3 | 46.4 |
PLLA 10 | 72.3 | 60.1 | — | 108.7 | 178.8 | 178.8 | — | 32.1 | 34.1 | 36.9 |
PLLA 11 | — | 57.6 | — | 106.7 | 174.3 | 174.5 | — | 35.9 | 41.2 | 43.2 |
PLLA 12 | 71.0 | 59.1 | — | 107.5 | 176.0 | 176.5 | — | 36.6 | 43.2 | 41.5 |
PLLA 13 | 68.3 | 43.4 | — | 90.2 | 176.2 | 168.2 | — | 40.0 | 60.5 | 55.7 |
PLLA 14 | 67.0 | 51.7 | — | 99.3 | 177.2 | 173.9 | — | 37.8 | 64.0 | 56.1 |
PLLA 15 | 69.4 | 54.8 | — | 106.2 | 176.0 | 176.5 | — | 43.2 | 44.8 | 46.6 |
PLLA 16 | 71.7 | 60.2 | — | 109.3 | 179.5 | 180.2 | — | 35.1 | 39.0 | 45.6 |
PLLA 17 | 70.3 | 57.1 | — | 112.2 | 171.7 | 171.0 | — | 42.4 | 43.5 | 38.5 |
PLLA 18 | 69.0 | 58.2 | — | 108.2 | 174.2 | 176.2 | — | 37.0 | 51.6 | 40.2 |
PLLA 19 | 62.6 | 42.1 | — | 80.2 | 166.5 | 160.8 | — | 37.6 | 67.7 | 56.8 |
PLLA 20 | 60.4 | 39.6 | — | 86.3 | 169.3 | 163.7 | — | 40.4 | 63.4 | 47.5 |
PLLA 21 | 68.1 | 40.2 | — | 85.2 | 175.0 | 167.3 | — | 37.2 | 56.0 | 53.8 |
PLLA 22 | 74.7 | 48.8 | — | 95.0 | 175.5 | 169.0 | — | 38.5 | 50.5 | 48.1 |
PLLA 23 | 70.7 | 45.4 | — | 91.7 | 173.0 | 166.5 | — | 34.6 | 49.8 | 43.0 |
PLLA 24 | — | 43.9 | — | 87.3 | 173.3 | 166.0 | — | 31.6 | 49.3 | 45.2 |
PLLA 25 | — | 36.5 | — | 73.0 | 164.3 | 157.7 | — | 33.7 | 63.6 | 47.2 |
PLLA 26 | 61.9 | 37.0 | — | 77.0 | 165.0 | 157.0 | — | 34.9 | 67.8 | 50.3 |
PLLA 27 | — | 39.0 | — | 78.8 | 171.5 | 164.2 | — | 33.6 | 60.1 | 53.9 |
PLLA 28 | 75.6 | 38.8 | — | 78.8 | 172.3 | 162.3 | — | 27.3 | 49.3 | 45.3 |
PLLA 29 | — | 43.6 | — | 83.5 | 171.8 | 163.0 | — | 31.0 | 51.0 | 45.2 |
PLLA 30 | 69.4 | 41.9 | — | 80.8 | 171.2 | 163.2 | — | 29.1 | 50.6 | 46.8 |
Concentration (g dL−1) | Flow-time (sec) | ηrel | ηsp | ηred (dL g−1) |
---|---|---|---|---|
0 | 132.4 | — | — | — |
0.2012 | 207.6 | 1.567 | 0.567 | 2.819 |
0.4120 | 301.8 | 2.278 | 1.278 | 3.103 |
0.6056 | 423.6 | 3.198 | 2.198 | 3.630 |
0.8012 | 562.0 | 4.243 | 3.243 | 4.048 |
Fig. 13 depicts plots of reduced viscosity (ηred) and inherent viscosity (ηinh) against concentration (g dL−1). Double extrapolating to zero concentration giving a value of intrinsic viscosity [η] of the PLLA sample of 2.26 dL g−1 was obtained.
From the value of [η] = 2.26 dL g−1 (Fig. 13), the polymer's viscosity-average molecular weight, v, can be calculated from the Mark–Houwink–Sakurada49 eqn (14) for PLLA in chloroform at 25 ± 0.1 °C:
[η] = 5.45 × 10−4 v−0.73 dL g−1 | (14) |
[2.26] = 5.45 × 10−4 v−0.73 dL g−1 |
v = 9.03 × 104 |
Therefore, the number-average molecular weight (n), can be calculated from the gamma function50 eqn (15); assuming an approximately “most probable” molecular weight distribution, as:
(15) |
n = 4.80 × 104 g mol−1 |
The values of [η], v, and n obtained from dilute-solution viscometry for PLLA products were provided in Table 7.
Polymer code | Intrinsic viscosity, [η] (dL g−1) | Viscosity-average molecular weight, v (g mol−1) | Number-average molecular weight, n (g mol−1) |
---|---|---|---|
PLLA 1 | 0.66 | 2.22 × 104 | 1.18 × 104 |
PLLA 2 | 1.05 | 3.15 × 104 | 1.68 × 104 |
PLLA 3 | 2.15 | 8.42 × 104 | 4.48 × 104 |
PLLA 4 | 1.69 | 6.09 × 104 | 3.24 × 104 |
PLLA 5 | 1.92 | 7.20 × 104 | 3.83 × 104 |
PLLA 6 | 1.90 | 7.12 × 104 | 3.79 × 104 |
PLLA 7 | 0.98 | 2.89 × 104 | 1.54 × 104 |
PLLA 8 | 1.67 | 5.98 × 104 | 3.18 × 104 |
PLLA 9 | 2.26 | 9.03 × 104 | 4.80 × 104 |
PLLA 10 | 2.35 | 9.51 × 104 | 5.05 × 104 |
PLLA 11 | 1.84 | 6.83 × 104 | 3.63 × 104 |
PLLA 12 | 2.17 | 8.52 × 104 | 4.53 × 104 |
PLLA 13 | 0.85 | 2.36 × 104 | 1.25 × 104 |
PLLA 14 | 1.01 | 2.99 × 104 | 1.59 × 104 |
PLLA 15 | 2.38 | 9.67 × 104 | 5.14 × 104 |
PLLA 16 | 2.02 | 7.73 × 104 | 4.11 × 104 |
PLLA 17 | 1.58 | 5.51 × 104 | 2.93 × 104 |
PLLA 18 | 1.65 | 5.90 × 104 | 3.14 × 104 |
PLLA 19 | 0.31 | 6.06 × 103 | 3.22 × 103 |
PLLA 20 | 0.48 | 1.07 × 104 | 5.71 × 103 |
PLLA 21 | 0.84 | 2.31 × 104 | 1.23 × 104 |
PLLA 22 | 1.05 | 3.17 × 104 | 1.68 × 104 |
PLLA 23 | 0.89 | 2.53 × 104 | 1.35 × 104 |
PLLA 24 | 0.85 | 2.35 × 104 | 1.25 × 104 |
PLLA 25 | 0.23 | 4.06 × 103 | 2.16 × 103 |
PLLA 26 | 0.29 | 5.48 × 103 | 2.91 × 103 |
PLLA 27 | 0.63 | 1.56 × 104 | 8.32 × 103 |
PLLA 28 | 0.73 | 1.91 × 104 | 1.02 × 104 |
PLLA 29 | 0.64 | 1.61 × 104 | 8.55 × 103 |
PLLA 30 | 0.52 | 1.42 × 104 | 6.86 × 103 |
Therefore, liquid tin(II) n-butoxide (Sn(OnC4H9)2) is regarded as being the most efficient initiator for ring-opening polymerization of L-lactide via coordination–insertion mechanism as confirmed by the non-isothermal DSC kinetic studies which gave the lowest Ea values and the fastest rates of polymerization at the lowest temperature.
The results also confirm that the use of liquid tin(II) n-alkoxide (Sn(OR)2) initiator directly is more efficient than generating it in situ via the Sn(Oct)2/ROH reaction. It also has the important advantages of (a) knowing the [Sn(OR)2] concentration accurately for the purpose of being able to predict polymerization rates and polymer molecular weights and (b) to avoid any unwanted side-reactions due to the use of Sn(Oct)2 alone and/or Sn(Oct)2/ROH system.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra07635j |
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