J Polym Res (2016) 23:220 DOI 10.1007/s10965-016-1099-x
ORIGINAL PAPER
Cadmium acetate as a ring opening polymerization catalyst for the polymerization of rac-lactide, ε-caprolactone and as a precatalyst for the polymerization of ethylene Mrinmay Mandal 1,2 & Uwe Monkowius 2 & Debashis Chakraborty 3
Received: 2 March 2016 / Accepted: 23 August 2016 # Springer Science+Business Media Dordrecht 2016
Abstract In the present study, we have discussed the bulk ring opening polymerization (ROP) of rac-lactide (rac-LA) and ε-caprolactone (ε-CL) using Cd(OAc)2. Cd(OAc)2 appeared to be a good catalyst for the polymerization of rac-LA and εCL yielding high molecular weight (Mn) polymers with narrow molecular weight distributions (MWDs). The catalytic activity of the system can be increased markedly upon using catalytic amount of BnOH as external alcoholic initiator. There is a first order dependence of the rate constant with respect to monomer concentrations as understood from the kinetic studies. The rate was found to be faster in the presence of BnOH. The polymerization process was controlled. The polymerization proceeded via the coordination-insertion mechanism without BnOH as well as activated monomer mechanism in the presence of BnOH. In the absence of BnOH, the acetyl group initiated the polymerization as understood from the 1H NMR and MALDITOF analysis. The benzyloxy group initiated the polymerization in the presence of BnOH. Moderate activity towards the polymerization of ethylene was observed using MAO as alkyl aluminum activator. The polymerization parameters towards the polymerization of ethylene were widely investigated.
Electronic supplementary material The online version of this article (doi:10.1007/s10965-016-1099-x) contains supplementary material, which is available to authorized users. * Debashis Chakraborty
[email protected] 1
Department of Chemistry, Indian Institute of Technology Patna, Patna, Bihar 800 013, India
2
Institute of Inorganic Chemistry, Johannes Kepler University Linz, Altenberger str. 69, 4040 Linz, Austria
3
Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600 036, India
Keywords Cd(OAc)2 . ROP . Polyesters . Polyethylene . Coordination-insertion mechanism . Activated monomer mechanism
Introduction The use of biodegradable polymers as an alternative to the nondegradable plastics, are of growing interest as it prevents the accumulation of plastic material in the environment [1, 2]. Poly(lactic acid) (PLA) and poly(caprolactone) (PCL), the two important biodegradable polymers are the best fit to serve the cause [3, 4]. These polymers have path breaking applications in packaging, agriculture, medicine, pharmaceuticals, and tissue engineering [3, 5–9]. PLA has additional advantages as its monomer lactide is obtained from annually biorenewable resources such as corn, sugar-beet and dairy products thereby eliminating the possibility of the use of nonrenewable petroleum resources and fossil fuels [10, 11]. The degradation of PLA yields nontoxic lactic acid which degrades further to CO2 and water [12]. PLA can be produced from the conventional ROP of lactide employing metal catalysts comprising of a variety of metals [13–19]. Recently, the development of metal free catalysts for the ROP of lactide has become important since it prevents the residual metal contamination in the polymer [20–23]. In order to achieve high molecular weight (M n), controlled molecular weight distributions (MWDs) and stereoregularity in the polymer chain, the ROP of lactide is most preferable over the normal condensation pathway for the preparation of PLA [24–30]. Previously from our group, we demonstrated some excellent catalytic activity of copper and zinc acetate catalysts for the polymerization of lactide and lactones [31, 32]. Both the catalytic systems were very effective producing polymers with high Mn and controlled MWDs. The results of these catalytic systems induced
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and inspired us to study the catalytic activity for the ROP of rac-LA and ε-CL using cadmium acetate as a catalyst. Since the discovery of Ziegler-Natta polymerization and metallocene catalysts for the polymerization of ethylene, various successful attempts were reported with high activity for the polymerization of ethylene [33–35]. In spite of volumes of reports containing homogeneous single site catalysts [36, 37], the non-metallocene or post-metallocene catalysts with a variety of ancillary ligands have significant advantages such as wide range of structural modifications of the polymer and ability to govern the catalytic properties [38, 39]. In addition, the properties of the polyethylene produced using these compounds can be fine tuned efficiently [40]. Fujita and his research group reported some very good nonmetallocene catalytic systems for the polymerization of ethylene [41]. The catalytic activity was high for all the systems resulting in the formation of polyethylene with high Mn and narrow MWDs [42, 43]. To the best of our understanding, the polymerization of ethylene is not reported using simple metal acetate catalysts. This has influenced us for the extensive study of ethylene polymerization using Cd(OAc)2 as a catalyst. In this contribution, we have discussed the catalytic activity of Cd(OAc)2 for the polymerization of rac-LA and ε-CL. The results revealed a good degree of control in the polymerization process. In addition, using MAO as activator moderate activity for ethylene polymerization was noticed.
Experimental General information and instrumentation All polymerization reactions were performed under a dry argon atmosphere. CDCl3 used for NMR spectral measurements was dried over calcium hydride for 48 h and distilled freshly before use. 1H spectrum was recorded with a Bruker Avance 400 instrument. Chemical shift for 1H spectrum was referenced to residual solvent resonances and are reported as parts per million relative to SiMe4. MALDI-TOF measurement of poly(lactic acid) (PLA) oligomers were recorded using Bruker Daltonics instrument in dihydroxy benzoic acid (DHBA) matrix. Molecular weights (Mn) and the molecular weight distributions (MWDs) of the polymer samples produced by the ROP of rac-LA and ε-CL were determined by using a GPC instrument with Waters 510 pump and Waters 410 Differential Refractometer as the detector. Three columns, WATERS STRYGEL-HR5, STRYGEL-HR4 and STRYGEL-HR3 each of dimensions (7.8 х 300 mm) were serially connected one after another. Measurements were done in THF at 27 °C for all the cases. Measurement of number average molecular weights (Mn), weight average molecular weights (Mw) and molecular
weight distributions (Mw/Mn) (MWDs) of the polymers were performed relative to polystyrene standards. Molecular weights (Mn) were corrected according to Mark-Houwink corrections [44]. Molecular weights (Mn and Mw) and the polydispersity indices (Mw/Mn) (MWDs) of polyethylene samples were obtained by a GPC instrument with Waters 510 pump and Waters 2414 differential refractometer as the detector. The column named WATERS STRYGEL-HR4 of dimensions (4.6 × 300 mm) was fixed during the experiment. Measurements were performed in trichlorobenzene (TCB). Mn and MWDs of polymers were determined relative to polystyrene standards. Materials Cd(OAc)2 was purchased from Sigma-Aldrich and used without further purification. The rac-LA and ε-CL were purchased from Sigma-Aldrich. The rac-LA was purified by sublimation repeatedly and stored in a glove box. The ε-CL was purified by drying over calcium hydride overnight and distilled fresh prior to use. Methylaluminoxane (MAO) was purchased from Aldrich and used without further purification. High pure ethylene was purchased from Indogas, Bangalore, India. General procedure for the bulk polymerization of rac-LA and ε-CL The polymerizations were done in 200:1 ratio between the respective monomers and Cd(OAc)2 under solvent free conditions. The polymerizations were performed by charging 236.6 μmol of Cd(OAc)2 (54 mg) and 5.4 g (5 mL) (47.3 mmol) of ε-CL or 173.4 μmol of Cd(OAc) 2 (40 mg) and 5 g (34.7 mmol) of rac-LA under an argon atmosphere into a 100 mL glass-lined reactor with stirring. The reactor was heated to 80 °C for ε-CL and 140 °C in case of rac-LA. The reaction mixture was quenched by cooling the reactor to ambient temperature in about 90 mins and subsequently pouring the contents (dissolved in minimum dichloromethane) into cold heptane for ε-CL and cold methanol in case of rac-LA. The polymer was isolated by subsequent filtration and dried till a constant weight was attained. General procedure for the kinetic experiment of rac-LA To determine the kinetics for the polymerization of rac-LA, we carried out the polymerization reactions in small scale in glass reactor at a temperature of 140 °C under argon atmosphere. At 200: 1 ratio the polymerization was performed by charging 34.69 μmol of Cd(OAc)2 (8 mg) and 1 g (6.94 mmol) of rac-LA under an argon atmosphere. Next, the aliquots were taken out at regular time interval from the glass reactor and 1H
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O O
Cd(OAc)2
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O O
140 oC
O
O H O
O
n
PLA
LA
O O
Cd(OAc)2
O O H
80 oC
n
PCL
CL
Scheme 1 Polymerization of rac-LA and ε-CL
NMR spectra were recorded to determine the % conversion of monomer into the corresponding polymer by comparing the methine proton of the unreacted monomer and polymer. Using this data we got linear plot by plotting ln ([M]0/[M]t) vs. time. Apparent rate constant (kapp) were obtained from the slopes of the best fit lines. Next, we ran the GPC to get the molecular weight (Mn) and MWDs after quenching the aliquots were removed after the same time intervals as above. From this we plotted Mn vs. % conversion vs. MWDs. Kinetics of ε-CL polymerization High Performance Liquid Chromatography (HPLC) was used for carrying out the kinetic investigation in bulk scale as it determines the concentration of various starting materials
Table 1
and product present as a function of time. To carry out the experiment we used reversed-phase C18 HPLC column and HPLC grade methanol as an eluting solvent. Before running the HPLC we sonicated the solvent for 30 mins. The polymerizations (200: 1) were performed by charging 45.12 μmol of Cd(OAc) 2 (10 mg) and 1.03 g (1 mL) (9.02 mmol) of ε-CL under an argon atmosphere. We ran the HPLC after collecting the aliquots at regular time interval to determine the % conversion of monomer into the corresponding polymer. Using this data we got linear plot by plotting ln ([M]0/[M]t) vs. time. Apparent rate constant (kapp) were obtained from the slopes of the best fit lines. Next, we ran the GPC to determine the molecular weight (Mn) and MWDs after quenching the aliquots removed after the same time intervals as above. From this we plotted Mn vs. % conversion vs. MWDs. General procedure for ethylene polymerization The polymerizations were performed in a 100 mL stainless steel autoclave reactor with mechanical stirring under an argon atmosphere. The container was charged with an argon atmosphere with 50 mg of catalyst, 45 mL of hexane along with the required amount of MAO. Consequently, the autoclave was heated up to 50 °C and gas was continuously bubbled through the proper channel. The gas feed was passed for 30 min at a pressure of 8 atm and subsequently the polymerization was
Polymerization data based on changing the ratios in case of rac-LA and ε-CL using Cd(OAc)2 at 140 °C and 80 °C respectively Mnobs/kgmol−1
Mntheo/kgmol−1
Entry
Monomer (M)
[M]/[C] ratio
Yield (%)
a
b
c
d
e
Mw/Mn
1 2 3 4 5 6
rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA
100 200 400 600 800 1000
99 98 98 97 98 97
99 99 98 99 98 98
25 34 45 61 82 108
3.96 5.76 8.71 9.54 9.56 8.98
14.02 27.88 56.16 84.92 113.1 141.0
14.46 28.87 57.70 86.53 115.4 144.2
1.09 1.10 1.10 1.13 1.14 1.16
7 8 9 10 11 12 13 14
rac-LA ε-CL ε-CL ε-CL ε-CL ε-CL ε-CL ε-CL
1200 100 200 400 600 800 1000 1200
96 99 99 99 97 98 98 96
97 99 99 99 98 97 98 98
140 21 28 39 54 73 96 124
8.23 4.71 7.07 10.2 10.8 10.7 10.2 9.29
168.4 10.58 21.62 44.13 66.34 88.67 112.1 134.2
173.0 11.46 22.87 45.70 68.53 91.36 114.2 137.0
1.18 1.12 1.14 1.16 1.17 1.18 1.20 1.22
Conversion (%)
Time/min
TOF/min
a
Determined by 1 H NMR spectroscopy
b
Time of polymerization measured by quenching the polymerization reaction when all monomer was found consumed
c
Turnover frequency (TOF) = Number of moles of monomer consumed / (mole of catalyst × time of polymerization)
d
Measured by GPC at 27 °C in THF relative to polystyrene standards with Mark-Houwink corrections for Mn
e
Mn (theoretical) at 100 % conversion = [M]0/[C]0 × mol wt (monomer) + mol wt (−OAc) (for 200: 1 ratio, Mn = 200/1 × 144.14 + 59.044 ) [29]
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Table 2 Polymerization data based on changing ratios in case of rac-LA and ε-CL using Cd(OAc)2 in the presence of benzyl alcohol at 140 °C and 80 °C respectively Entry Monomer (M) [M]: [C]: [BnOH] ratio Yield(%)
Mnobs/kgmol−1
Mntheo/kgmol−1 Mw/Mn
a
b
c
d
e
Conversion (%)
Time/min
TOF/min
1 2 3 4
rac-LA rac-LA rac-LA rac-LA
100: 1: 2 200: 1: 2 400: 1: 2 600: 1: 2
98 99 99 97
99 99 98 98
18 25 36 49
2.7 4.0 5.5 5.9
7.002 13.88 27.73 41.65
7.315 14.52 28.94 43.35
1.08 1.09 1.11 1.12
5 6 7 8 9 10 11 12 13 14
rac-LA rac-LA rac-LA ε-CL ε-CL ε-CL ε-CL ε-CL ε-CL ε-CL
800: 1: 2 1000: 1: 2 1200: 1: 2 100: 1: 2 200: 1: 2 400: 1: 2 600: 1: 2 800: 1: 2 1000: 1: 2 1200: 1: 2
98 97 96 99 99 98 97 98 97 97
98 97 98 99 99 98 99 98 97 97
65 85 112 14 19 27 38 53 69 88
6.0 5.7 5.1 3.5 5.2 7.2 7.6 7.4 7.0 6.6
54.92 69.19 83.33 5.343 10.98 22.02 32.95 43.38 55.05 65.70
57.76 72.18 86.59 5.823 11.52 22.94 34.35 45.76 57.18 68.59
1.13 1.15 1.17 1.10 1.11 1.13 1.15 1.16 1.17 1.20
a
Determined by 1 H NMR spectroscopy
b
Time of polymerization measured by quenching the polymerization reaction when all monomer was found consumed
c
Turnover frequency (TOF) = Number of moles of monomer consumed / (mole of catalyst × time of polymerization)
d
Measured by GPC at 27 °C in THF relative to polystyrene standards with Mark-Houwink corrections for Mn
e
Mn (theoretical) at 100 % conversion = [M]0/[C]0 × mol wt (monomer) + mol wt (end group) (for 200: 1: 2 ratio, Mn = 200/2 × 144.14 + 107.13) [29]
The ROP of rac-LA and ε-CL using Cd(OAc)2 as catalyst was studied in details (Scheme 1). It was observed that
Cd(OAc)2 can effectively catalyze the ROP of rac-LA and εCL in the presence and absence of benzyl alcohol (BnOH) generating polymer with high molecular weight (Mn) and narrow molecular weight distributions (MWDs). The representative GPC traces are provided in the supporting information (Figs. S1 & S2). The observed molecular weights (Mnobs) are closely parallel to the theoretical molecular weights (Mntheo) (Tables 1 and 2). Quantitative conversion was observed for rac-LA and ε-CL within a reasonable time of 34 min (rac-LA) vs. 28 min (ε-CL) in 200: 1 ratio. The TOF values for the polymerization of rac-LA and ε-CL in all the
Fig. 1 Plot of Mn and Mw/Mn vs. [M]0/[C]0 for rac-LA and ε-CL polymerization at 140 °C and 80 °C respectively using Cd(OAc)2
Fig. 2 Plot of Mn and Mw/Mn vs. % conversion for rac-LA and ε-CL polymerization at 140 °C and 80 °C respectively using Cd(OAc)2
quenched with acidic methanol. The polymer produced was collected by filtration and dried until a constant weight was achieved.
Results and discussion Polymerization studies
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Fig. 3 rac-LA and ε-CL conversion vs. time plot using Cd(OAc)2: [M]0/[Cat]0 = 200 at 140 °C and 80 °C respectively (left). Semi-logarithmic plot of racLA and ε-CL conversion in time initiated by Cd(OAc)2: [M]0/[Cat]0 = 200 at 140 °C and 80 °C respectively (right)
ratios are worth noting (Table 1 and 2). In general, one observed less control over the polymerization process resulting in the formation of low M n and broader MWDs on increasing the amount of monomer due to the occurrence of polymerization side reactions (intra- and intermolecular transesterification reaction) [45–47]. In our case we have not observed the presence of any polymerization side reactions. The Mn increased continuously with almost static MWDs with higher [M]0/[C]0 ratios indicated by the plot of Mn vs. [M]0/[C]0 vs. MWDs (Mw/Mn) for both the monomers. This suggested that the polymerization process is well controlled (Fig. 1). The plot of Mn vs. % conversion vs. MWDs (M w /M n ) suggested a controlled system as we witnessed the sharp increment of Mn with % conversion but the MWDs remained almost invariant (Fig. 2). We were able
to reproduce the same results when the polymerizations were performed in the presence of BnOH (Figs. S3 & S4). The polymer chain was atactic as indicated by the careful observation of the methine region of the PLA using homonuclear decoupled 1H NMR spectra of PLA samples (Fig. S5) [29]. Kinetics of polymerization The kinetic studies for the polymerization of rac-LA and ε-CL were performed using Cd(OAc)2 in ratio [M]0/ [Cd(OAc)2]0 = 200 at 140 °C and 80 °C respectively. The % conversion of rac-LA to PLA was evaluated by comparing the methine region integration of 1H NMR peaks for the polymer and the unreacted monomer. In case of ε-CL, the conversion was calculated using High Performance Liquid
Fig. 4 MALDI-TOF of the crude product obtained from a reaction between rac-LA and Cd(OAc)2 in 15: 1 ratio
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Fig. 5 1H NMR spectrum of the crude product obtained from a reaction between rac-LA and Cd(OAc)2 in 10: 1 ratio
Chromatography (HPLC). The plot of % conversion of racLA and ε-CL against time depicted that initially the conversion of monomer to polymer was at a very high rate but almost negligible towards the end of the polymerization (Fig. 3, left). The linear plot of ln ([M]0/[M]t) vs. time passing through the origin indicated that the polymerization had a first order dependency on monomer concentrations (Fig. 3, right). In addition, no induction period was observed. The apparent rate constants (kapp) for rac-LA and ε-CL polymerization were extracted from the slope of the best fit lines and were found to be 10.26 × 10−2 min−1 and 14.26 × 10−2 min−1 respectively. This is indicative of the fact that the rate of polymerization is higher for ε-CL in comparison to rac-LA. Next, we also
performed the kinetics of polymerization in the presence of BnOH in ratio [rac-LA/ ε-CL]0/[Cd(OAc)2]0/[BnOH] = 200/ 1/2 at 140 °C (rac-LA) and 80 °C (ε-CL) respectively. Again, the % conversion of monomer (rac-LA and ε-CL) against time presented a sigmoid curve (Fig. S6). The plot of ln ([M]0/[M]t) vs. time was found to be linear with the absence of any induction polymerization (Fig. S7). From the slope of plot, the apparent rate constants (kapp) for rac-LA and ε-CL polymerization were found to be 14.72 × 10−2 min−1 and 22.01 × 10−2 min−1 respectively. The rate of polymerization is higher in the presence of benzyl alcohol than in the absence of alcoholic initiator (14.72 × 10−2 min−1 vs. 10.26 × 10−2 min−1 for rac-LA).
Cd(OAc)2
Scheme 2 Polymerization proceeds through the coordination-insertion mechanism for rac-LA
O
O
AcO O
O
Cd(OAc)
O
AcO
O O
O O O
O
O AcO
OCd(OAc)
O
OCd(OAc) O
O O
O O
O
O AcO
rac-LA
O
O AcO
O O
O
H n
OCd(OAc)
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Scheme 3 Polymerization proceeds through the activated monomer mechanism for rac-LA in the presence of BnOH
H BnO
O
O O
BnOH
O O
Cd(OAc)2
Cd(OAc)2
BnO O OH
O
O O
O rac-LA
O
OCd(OAc)2
H O
O BnO
O
O
OH
O O
O O
BnO
O BnO
O
O
O
O
H n
O
hydroxyl group at the other (Figs. S8 & S9). It is reasonable to conclude that the polymerization proceeded through the activated monomer mechanism in presence of BnOH (Scheme 3) [31, 32, 51–53]. Analogous result was obtained for the polymerization of ε-CL in the presence of BnOH (Scheme S2).
Mechanism of polymerization Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was used to understand the mechanism for the concerned polymerization process. The low Mn oligomer of PLA was synthesized at a monomer: catalyst ratio of 15: 1 at 140 °C. The product was precipitated in cold methanol and filtered. According to MALDI-TOF data, the polymer is capped with –OAc and H end groups (Fig. 4). The result was also supported by 1H NMR studies (Fig. 5). Polymerization side reactions such as inter- and intramolecular transesterification products were absent since the base line of Fig. 4 is very sharp [45–47]. The appearance of peaks at an interval of 72 amu between the adjacent peaks in the MALDI-TOF indicated that the polymer was linear in nature (Fig. 4). These polymerizations are believed to undergo through the conventional coordination-insertion mechanism [15, 48–50]. The polymer chain started growing via the coordination of rac-LA with metal first followed by the cleavage of acyl oxygen bond of rac-LA (Scheme 2). In case of ε-CL the same conclusion were drawn (Scheme S1). In presence of BnOH, the oligomer sample was prepared by carrying out the polymerization in 15: 1: 2 (rac-LA: Cd(OAc)2: BnOH) at 140 °C. MALDI-TOF and 1H NMR analyses indicated the presence of the benzyloxy ester group at one end and the
Table 3 Polymerization data for ethylene using Cd(OAc)2 along with MAOa
Entry 1 2 3 4 5
Scanning electron microscopy (SEM) Next, we performed the SEM-EDX (energy dispersive X-ray) analysis in order to calculate the residual of cadmium content in the polymer. SEM-EDX analysis indicated that the polymer contains only 0.23 % (weight %) of cadmium in the resulting polymer (Fig. S10). Ethylene polymerization Next, Cd(OAc)2 was examined as a suitable pre-catalyst for the polymerization of ethylene using methylaluminoxane (MAO) as cocatalyst. The polymerizations were performed at 50 °C in hexane. The results with different MAO: catalyst ratios for ethylene polymerization are collated in Table 3. The representative GPC traces are provided in the supporting information (Figs. S11 & S12). It is evident that we achieved moderate activity to produce polyethylene with moderate
MAO: catalyst
Ab
Yieldc(g)
Mw (kg /mol)
Mn (kg/mol)
Mw/Mn
500: 1 1000: 1 2000: 1 3000: 1 4000: 1
2.3 3.0 1.8 1.3 0.9
2.49 3.25 1.95 1.41 0.985
88.05 89.54 87.93 84.56 80.55
42.13 45.22 40.15 37.58 35.02
2.09 1.98 2.19 2.25 2.30
a
All experiments were performed in hexane at ethylene pressure = 8 atm, 50 °C for 30 min, catalyst =50 mg, solvent =45 mL
b
A = Activity in (g PE per mol cat х h) х 104
c
g of PE obtained after 30 min
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Fig. 6 Catalytic activity of Cd(OAc)2 in different solvents for ethylene polymerization
molecular weight [Mn = 45.22 kg/mol for 1000: 1 ratio (MAO: Cd(OAc)2)]. The reaction parameters such as time of polymerization, temperature of the reaction and solvents played an important role in the polymerization. Under the optimized conditions the MAO: catalyst ratio of 1000: 1 was the best as it displayed the highest activity amongst other ratios investigated in this study (Table 3) (Fig. S13). Next, we studied the effect of solvent polarity by choosing a range of different solvents with varying polarity. We observed that least polar hexane was suitable for carrying out the polymerization reactions. With increasing solvent polarity, due to the stronger solvation effect on the monomer, the propagation of the polymer chain is restricted by preventing the attack of the monomer. Hence, the activity is maximum in least polar hexane and minimum in most polar CHCl3 (Fig. 6) [54, 55]. The effect of temperature was studied by carrying out the polymerization over a temperature range of 25–60 °C. The activity increased
with increasing temperature and peaked at 50 °C and decreased with further rise in temperature (Fig. S14). However, the Mn decreased significantly with increase in temperature due to the increase in viscosity of the polymerization mixture [56]. Later, the influence of polymerization time on the yield and activity was researched in details. There is a steep rise in activity initially but decreased drastically for prolonging the polymerization time over a time span of 10–150 min (Fig. 7). On the other hand, the yield increased continuously with time (Fig. 7). The increase in yield is negligible towards the end because of the fact that the viscosity of the polymerization mixture increased to a maximum extent while carrying out the polymerization for long periods [57]. The activity of this catalytic system is lower than the Fujita’s catalysts [41–43].
Conclusions
Fig. 7 Effect of time on the yield and activity for Cd(OAc)2 towards ethylene polymerization
In summary, Cd(OAc)2 was found to be effective catalyst for the ROP of rac-LA and ε-CL yielding polymers with high Mn and narrow MWDs. It showed potent catalytic activity in the presence and absence of BnOH as external alcoholic initiator. The kinetic studies revealed a first order dependency on monomer concentrations.1H NMR and MALDI-TOF studies depicted that acetyl group was responsible for initiating the polymerization in the absence of benzyl alcohol suggesting coordination-insertion mechanism. We observed the presence of benzyloxy group as one of the end terminal when we carried out the polymerization in the presence of BnOH suggesting activated monomer mechanism. Overall, the polymerization system was controlled as understood from the linearity of the plot of Mn and Mw/Mn vs. % conversion. Good activity was observed in ethylene polymerization when Cd(OAc)2 was activated by MAO. Good polymerization results in the
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Page 9 of 9 220 28. 29.
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