Mechanistic Study of Bu₂SnCl₂‐Mediated Ring‐Opening Polymerization of ε‐Caprolactone by Multinuclear NMR Spectroscopy

Abstract

The ring-opening polymerization (ROP) of epsilon-caprolactone (CL) was carried out in toluene at 100 degrees C with n-propanol (nPrOH) in the presence of Bu(2)SnCl(2). It comes out that the molar mass of the polyester chains can be predicted from the initial monomer-to-alcohol molar ratio in accordance with a controlled ROP mechanism involving an O-acyl cleavage of the monomer to selectively form (alpha-propyloxy)(omega-hydroxy)poly(epsilon-caprolactone) chains. In order to gain fundamental understanding of the mechanistic factors governing the polyester chain growth, advanced (1)H, (13)C, and (119)Sn NMR investigations were performed in situ in [D(8)]toluene, as well as with model solutions that contained Bu(2)SnCl(2) and binary mixtures of the components at various concentrations and temperatures. This has enabled us to propose a mechanism in which Bu(2)SnCl(2) behaves as a catalyst, while nPrOH is the actual initiator. It involves non-aggregated, six-coordinate Bu(2)SnCl(2) complexes in which ligands exchange fast on the (119)Sn NMR observational timescale, and the simultaneous interactions of CL and alcohol function in such a way that it favors insertion/propagation reactions over transesterification ones, up to high monomer conversion.

Cationic Ring Opening polymerization of ε-caprolactam by a Montmorillonite Clay Catalyst

“…This result suggests that intramolecular transesterification, i.e. ''back-biting'', which causes degradation and formation of cyclic oligomers and consequently may cause a decrease of the molecular weight [31,32,33]. On the other hand, the monomer conversion increase with the time and it can be noted that initially the polymerization proceeds very slowly; this can be considered as an induction period (2 h), which consumed approximately 5 % of the monomer.…”

Polystyrene‐Supported Organotin Dichloride as a Recyclable Catalyst in Lactone Ring‐Opening Polymerization: Assessment and Catalysis Monitoring by High‐Resolution Magic‐Angle‐Spinning NMR Spectroscopy

“…In the unused organotin derivatives (5, 6), the tin atom does not, of course, undergo coordination from the CDCl 3 NMR solvent molecules; in both cases, a rather broad 119 Sn resonance results at the well-known chemical shift of about 124 ppm, which is characteristic for nonpolar media (Figure 11, top). In a former study by our group with the homogenous organotin Bu 2 SnCl 2 catalyst, it was demonstrated [14] that in the presence of CL or PCL, a significant low-frequency shift is observed for the 119 Sn chemical shift of Bu 2 SnCl 2 , which is larger with CL than with PCL. This was explained by the fact that the monomer coordinates the tin atom more strongly than the polymer.…”

“…This was explained by the fact that the monomer coordinates the tin atom more strongly than the polymer. [14] Furthermore, the single 119 Sn resonance observed in both cases indicates fast exchange between noncomplexed Bu 2 SnCl 2 and tin di- Table 3). The white bar represents 500 mm.…”

“…[12,13] Moreover, some of us have reported a mechanistic study of the dibutyltin dichloride (Bu 2 SnCl 2 )-mediated ROP of CL in the presence of 1-propanol (nPrOH) by using multinuclear spectroscopy, mainly based on the 119 Sn nucleus. [14] At 100 8C in toluene, CL coordinates the tin atom and brings, at least partially, the four-coordinate Bu 2 SnCl 2 into five-and/or sixcoordinate configurations that are more favorable for initiation of the coordination-insertion polymerization. Although nPrOH is the actual initiator, it is prone neither to coordinating irreversibly nor to reacting substantially with Bu 2 SnCl 2 to form tin alkoxide species such as Bu 2 Sn(Cl) (2Àn) -(OC 3 H 7 ) n .…”

Polymerization of ε-caprolactone using a montmorillonite clay as catalyst

“…The molecular weight increases with polymerization time and reaches a maximum after 6 h. The results show that between 3 and 8 h molecular weight increases gradually and after 10 h it decreases. This result suggests that intramolecular transesterification, i.e., "back-biting", which causes degradation and formation of cyclic oligomers and consequently may cause a decrease of the molecular weight [25][26][27], takes place. Maghnite-H + /ε-caprolactone weight ratio = 10%, reaction temperature 80 • C.…”