КИНЕТИКА И КАТАЛИЗ, 2014, том 55, № 4, с. 438-443
RING-OPENING POLYMERIZATION BY N-HETEROCYCLIC CARBENES AS CATALYSTS: CHARACTERISTICS AND KINETICS
© 2014 Y. Wang, N. Li, L. Zhang*, J. Guo, Q. Liu
Institute of Material Chemistry, Shanxi Normal University, Linfen 041004, People's Republic of China E-mail: email@example.com Received 20.06.2013
In this work catalytic ring-opening polymerization of cyclic esters in THF in the presence of benzyl alcohol is described. The polymerization is catalyzed by 1,3-bis(4-methoxyphenyl)imidazolium carbene, N-hetero-cyclic carbene (NHC). The ability of two different monomers, s-caprolactone and L-lactide, to enter into the polymerization via ring-opening polymerization with NHC as catalysts was evaluated. The plot of ln([M]0/[M]t) vs. reaction time yielded a straight line indicating that the kinetics of polymerization of s-caprolactone and L-lactide was first-order in monomer concentration. Moreover, a direct relation between the rate of ring-opening polymerization of s-caprolactone and the catalyst concentration suggested a first-order dependence of the rate of polymerization on the catalyst concentration.
Development of renewable polymers has been the subject of considerable interest over the recent years largely due to the needs of environmental protection and efficient utilization of natural resources. Aliphatic polyesters such as poly(s-capro-lactone) (PCL) and poly(L-lactide) (PLLA) are important class of biologically relevant macro molecules . Because of their biodegradability, high permeability, and compatibility with biopolymers, they have been widely used in agricultural [2—5] and as medical sutures and drug-delivery vehicles in pharmacy [6, 7].
A common synthetic route to PCL, PLLA, and other aliphatic polyesters is the ring-opening polymerization (ROP) of cyclic esters using metal-alkoxide initiators and catalysts based on alkali, transition, and rare-earth/lanthanides metals [8—11]. These metal catalysts can be applied in the preparation of polyesters with controlled molecular weight, molecular weight distribution, and microstructure. However, the presence of metal ions in catalysts suggested for the reactions with insertion of the monomer into a metal-alkoxide bond [12—16], has harmful consequences and the catalysts does not suit the synthesis of these polymers. For several reasons metal-free nucleo-philic N-heterocyclic carbine [17—19] appears to be good candidates as catalysts for the synthesis of PCL, PLLA, and polycarbonates as well as the polymers with novel macromolecular architecture. These catalysts [20, 21] are inexpensive, highly active and produce polymers of well-defined molecular weight and narrow polydispersity.
In this report the polymerization of s-caprolactone (e-CL) and L-lactide (LLA) in the presence of!,3-te(4-
methoxyphenyl)imidazolium carbene (diBnimY) as a catalyst is described. The catalyst is harmless and suitable for the synthesis of these polymers. Moreover, to improve the reaction rate of polymerization the catalyst is used for the first time with benzyl alcohol as an initiator (Scheme). An attempt is also made to optimize three experimental conditions governing the polymerization catalyzed by N-heterocyclic carbene. Among these are temperature, the monomer/catalyst molar ratio, and monomer/initiator molar ratio. The influence of these factors on the mass conversion and number-average molecular weight was also discussed. The optimization procedure provides a useful tool to select the proper experimental conditions for producing the polymer at a step preceding the synthesis and impregnation of biodegradable scaffolds.
s-Caprolactone ("Alfa Aesar", 99%) was dried and distilled over fresh calcium hydride (CaH2) powder under reduced pressure and stored over activated 4 A molecular sieves at room temperature prior to use. L-lactide was synthesized from L-lactic acid and purified by recrys-tallization three times from ethyl acetate, followed by drying to constant weight at 40°C under vacuum. Tet-rahydrofuran (THF) was dried by refluxing over the blue benzophenone-sodium complex and distilled prior to use. Benzyl alcohol (BnOH) was refluxed over CaH2 for 48 h and distilled from calcium hydride. Methanol was used without further purification.
Scheme. Synthesis of PLLA using diBnimY and BnOH.
General procedure for in situ imidazolium-based catalyst formation
Catalyst preparation was carried out with a glass ampoule and vacuum-line technique under purfied nitrogen [22—25]. Anhydrous 1,3-bis(4-methoxyphe-nyl)imidazolium hexafluorophosphate and equiv. of sodium hydride were combined in dry THF. After a reaction time of 30 min the reaction mixture was filtered.
Polymerization was carried out in previously flamed and nitrogen-purged glass ampoules. Monomer, solvent, an initiator and catalyst were injected into the ampoules with syringes successively. Then these ampoules were placed in a thermostat for a definite time needed for polymerization. The reaction mixture was quenched with a drop of methanol. The polymer was precipitated from cold methanol, and then dried to constant weight under vacuum at 40°C.
The *H NMR was recorded in either CDCl3 or DMSO-d 6 on a Bruker AV-600 MHz spectrometer with the tetramethylsilane (TMS) as an internal standard. Differential scanning calorimetric (DSC) curves were taken on a DSC (204 F1) instrument in the temperature range from —60 to 150°C with a heating rate of10°C/min under nitrogen, the midpoint of the heat capacity change was taken as the glass transition temperature (Tg). The number-average molecular weight (Mn) and polydispersity (PDI) measurements were made with THF as an eluent (1 mL/min) at 40 °C by the gel permeation chromatography (PL-GPC220) with refractive index detector and a set of columns (PL gel 10 |im MIXED-B 300 mm x 7.5 mm, PL gel 10 |m Guard 50 mm x 7.5 mm) and calibrated using polysty-
rene standards. IR spectra were determined with Varian 660 IR spectrometer using KBr pellets.
RESULTS AND DISCUSSION
Characteristics of the polymerization
The data in Table 1 show that the diBnimY indicated in Scheme is a very effective catalyst for the controlled ROP of LLA. High molecular weight PLLA was readily synthesized within 60 min at 15°C. However, higher monomer concentration/initiator molar ratios ([M]/[I]) resulted in polymer polydispersities greater than 1.3. Since LLA and s-CL undergo similar ROP processes via a monomer-activated mechanism [26—30], it is useful to examine the efficiency of the carbene catalyst to catalyze the ROP of s-CL.
At a first stage, the catalytic activity of the diBnimY for polymerization of s-CL was evaluated. The diBnimY was found to be an active catalyst for polymerization of s-CL in the presence ofBnOH as an initiator, providing polymers with the expected molecular weights and low polydispersity indices (Table 2).
Table 1. Production of PLLA by the ring-opening polymerization of LLA in THF at 15°C*
Entry [M]/[I] Conversion, % Mn x 10-4**, g/mol PDI**
1 100 77.9 1.78 1.20
2 150 89.5 2.65 1.31
3 200 98.6 3.26 1.38
4 250 92.3 2.73 1.47
5 300 86.8 2.72 1.53
* Conditions: initiated by BnOH, [LLA] = 1.5 mol/L, 60 min. ** Measured by GPC.
Table 2. Production of PCL by the ring-opening polymerization of s-CL in THF at 15°C*
Entry [M], mol/L [M]/[I] Conversion, % Mn x 10-4**, g/mol PDI**
1 1.0 200 39.2 1.01 1.11
2 2.0 200 76.6 2.43 1.12
3 3.0 200 99.9 3.28 1.47
4 4.0 200 90.6 2.23 1.22
* Conditions: initiated by BnOH, 270 min. ** Measured by GPC.
At a second stage, the polymerization rates for the ROP of LLA and s-CL under identical reaction conditions were investigated. The polymerization processes were first-order in monomer concentrations as depicted in Fig. 1, with the rate constants summarized in Table 3. It can be seen that the diBnimY catalyzes the polymerization of LLA significantly faster than polymerization of s-CL with the kLLA/kCL ratio of 6.63 at 15°C. This difference in rates of polymerization for two monomers may reflect the difference in Lewis basicity of LLA vs. s-CL. Even if we assume that the Lewis basicities of two monomers are similar, an increased rate of polymerization with LLA can be accounted for by the presence two carboxyl groups in LLA which can be coordination at the carbene center, thereby resulting in a slower polymerization process of s-CL monomer.
The influence of the ([CL]/[C]) molar ratio on polymerization of s-CL is shown in Fig. 2, where CL is the monomer and C is the catalyst concentration. It can be readily seen that both monomer conversion and molecular weight of the polymer increase until the [CL]/[C] ratio reaches a value of 100, but decrease rapidly afterwards. According to these data, the monomer conversion and PCL molecular weight can be changed by varying the [CL]/[C] molar ratio in the test range. These phenomena suggest that as the catalyst concentration increases to the [CL]/[C] molar ratio <100 the number of active sites tends to rise. Accordingly, the number of monomer units per one active site decreases resulting in lower conversions and molecular weights. Meanwhile, it is also found from molecular weight distribution of PCL that the broader molecular weight distribution may be mainly attributed to the transesterification reaction occurring during the polymerization process. The same behavior occurs in the case of less active species because at lower catalyst concentrations ([CL]/[C] molar ratio >100) in the reaction medium reaction rate of polymerization decreases resulting in lower conversions and molecular weights.
The effect of the polymerization temperature and reaction time on the polymerization of s-CL was investigated in detail with the results summarized in Table 4. It can be seen that the polymer with 99.9% con
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