N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)

The metal-free polymerization of propylene oxide (PO) using a special class of alkene—N-heterocyclic olefins (NHOs)—as catalysts is described. Manipulation of the chemical structure of the NHO organocatalyst allows for the preparation of the poly(propylene oxide) in high yields with high turnover (TON>2000), which renders this the most active metal-free system for the polymerization of PO reported to date. The resulting polyether displays predictable end groups, molar mass, and a low dispersity (ĐM<1.09). NHOs with an unsaturated backbone are essential for polymerization to occur, while substitution at the exocyclic carbon atom has an impact on the reaction pathway and ensures the suppression of side reactions.

Scheme S1. Schematic preparation of the NHO catalysts used in this study.

General Preparation of NHOs 1-3
Potassium hydride (KH, 770 mg, 19 mmol) was suspended in dry diethyl ether (25 mL) in a Schlenk flask. Precursor salts 1', 2' or 3' were slowly added (0.5 equivalents, mild gas development). The reaction was then stirred for 48 h at ambient temperature, under exclusion of light. This was followed by evaporation of the solvent in vacuo at 0 °C. The reaction vessel was subsequently transferred into the glove box, where the residues were extracted with pentane and filtrated to yield clear solutions. Compounds 1-3 were received after removal of the pentane under vacuum (0 °C).

Polymerization of PO
For a typical polymerization experiment, PO was mixed with BnOH and added to NHO 4.
The colorless mixture was transferred to a sealed glass vessel (50 mL ampoule, dried overnight at 160 °C) and submerged in a pre-heated oil-bath (50 °C). During the course of the reaction the polymerization turned a pale yellow. The polymerization was quenched by evacuation to remove excess PO (1-2 h). As a consequence of the volatility of PO, the degree of polymerization and conversion were calculated by 1 H NMR spectroscopy (CDCl 3 ), using the ratio of the methylene unit of the initiator ((Ar-CH 2 -O-) at δ = 4.5 ppm) versus the polymer signals at δ = 3.4 ppm and δ = 1.1 ppm. The PPO was stored under ambient conditions prior to further analysis during which time the pale yellow discoloration disappeared.

Characterization and Analysis
A Bruker DPX 400 spectrometer was used for recording of proton ( 1 H) and carbon ( 13 C) NMR spectra. All chemical shifts are reported in parts per million (ppm), relative to reference peaks for proton and carbon NMR experiments (CDCl 3 : δ = 7.26/77.16 ppm, C 6 D 6 : δ = 7.16/128.06 ppm). The molecular weight of the PPO was determined via gel permeation chromatography (GPC) using a system consisting of an Agilent 390-MDS and PLgel Mixed D-type columns in combination with a refractive index detector. Samples were run in chloroform (0.5% NEt 3 , 40 °C, 1 mL min -1 ) and a calibration from polystyrene standards was applied. Sample concentration was 7 mg mL -1 . MALDI-ToF (matrix-assisted laser desorption ionization-time of flight) mass spectrometry measurements were performed on a Bruker Autoflex Speed TOF/TOF mass spectrometer using a nitrogen laser delivering 2 ns pulses at 337 nm with positive ion ToF detection performed using an accelerating voltage of 25 kV.
Trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propylidene]malonitrile (DCTB) was applied as a matrix (0.2 μL of a 10 g L -1 solution in tetrohydrofuran), with sodium trifluoroacetate used as a cationization agent (0.1 μL of a 10 g L -1 solution in tetrohydrofuran). Analyte (0.1 μL of a 1 g L -1 solution in tetrohydrofuran) was applied in between separate loadings of DCTB and sodium trifluoroactetate, with solvent being allowed to evaporate between applications, to form a thin matrix-analyte-matrix film. All samples were measured in reflectron mode and calibrated against monodisperse Polymer Factory SpheriCal® dendritic standards (calibration range = 500-7000 Da). Figure S2. Figure S3. GPC chromatogram of PO polymers (Table 1, entries 3 and 4) by 3 (red) and 4 (black). Note the higher molecular weight impurity when 3 is used as catalyst.    (Table 1, entry 3). (a) full range spectrum, showing the dominating major distribution and the minor high molecular weight impurity (arrow, compare Figure S3). (b) Separate measurement for the low molecular weight region with denoted mass peak (compare Figure 3) and (c) the high molecular weight range with (d) expansion. Figure S7. 1 H NMR of benzyl alcohol (C 6 D 6 , 400 MHz, 298K). Note the typical convoluted aromatic region, the shift of the methylene unit at δ = 4.42 ppm and the -OH signal at δ = 1.23 ppm.

Figure S 8. 1 H NMR spectra of
BnOH combined with NHOs 1-4 (1:1, C 6 D 6 , 400 MHz, 298 K). Note the gradual shift of the methylene unit of BnOH (red), which is virtually unchanged when in the presence of 1 (a), but appears at  = 5.19 ppm when 4 is applied (d). Likewise, the aromatic region gets more resolved, strongest for NHOs 3 and 4, both of which polymerize PO. Notably in both latter cases distinct broad signals appear (green), most likely representing the deshielded hydroxylic proton that is partially abstracted by the strongly basic NHO. The effect is much more prominent for 4 ( = 13.17 ppm, d)) than for 3 ( = 4.65 ppm, c)), reflecting supposedly the difference in basicity for both catalysts.