Biobased chiral semi-crystalline or amorphous high-performance polyamides and their scalable stereoselective synthesis

Abstract

The use of renewable feedstock is one of the twelve key principles of sustainable chemistry. Unfortunately, bio-based compounds often suffer from high production cost and low performance. To fully tap the potential of natural compounds it is important to utilize their functionalities that could make them superior compared to fossil-based resources. Here we show the conversion of (+)-3-carene, a by-product of the cellulose industry into ε-lactams from which polyamides. The lactams are selectively prepared in two diastereomeric configurations, leading to semi-crystalline or amorphous, transparent polymers that can compete with the thermal properties of commercial high-performance polyamides. Copolyamides with caprolactam and laurolactam exhibit an increased glass transition and amorphicity compared to the homopolyamides, potentially broadening the scope of standard polyamides. A four-step one-vessel monomer synthesis, applying chemo-enzymatic catalysis for the initial oxidation step, is established. The great potential of the polyamides is outlined.

Fig. 1. Monomer synthesis overview.

Synthesis pathway with yields and diastereomeric excess (de) of intermediates for the production of the lactam isomers 5-3S and 5-3R (yields refer to the small-scale experiments from purified starting material). The labelling of the stereo-centre C3 at all intermediates follows the recommendation for terpene carbon skeleton numbering of M. W. Grafflin, which suggest that the initial carbon labels of (+)-3-carene are fixed also in case of functionalization.

Fig. 2. Up-scale of 5-3S.

Schematic one-vessel reaction cascade in a 4.0 L scale for the synthesis of lactam 5-3S. (1) Addition of 1 to the epoxidation reagent AcOOH, washing, addition of cyclohexane and subsequent azeotropic distillation; (2) Meinwald rearrangement of 2-3S to 3-3S with Fe(ClO₄)₂, washing, and solvent exchange to MeCN; (3) oximation to 4-3S using a buffered solution of HONH₂·HCl; and (4) Beckmann rearrangement to 5-3S in basic media with tosyl chloride (TsCl).

Fig. 3. Polymerization overview.

Anionic ring-opening polymerization of the (+)-3-carene based lactams 5-3S and 5-3R (a) and co-polymers of the general structures poly(5-3S_100%−x%·5-3R_x%) from 5-3S and 5-3R, poly(5-3S_100%−x%·CL_x%) from 5-3S and CL, and poly(5-3S_100%−x%·LL_x%) from 5-3S and LL (b).

Fig. 4. Polymerization of 5-3S.

Effect of the reaction temperature and the activator concentration on M_n and M_w (Supplementary Figs. 8 and 9, Supplementary Table 11). Conditions: 1.8 mmol 5-3S, 2.0–5.5 mol% NaH on paraffin, 180 or 220 °C, 1 h. Molecular weights refer to masses over 1.0 kDa (GPC).

Fig. 5. NMR and GPC investigations of the monomer conversion.

Conversion of the lactams and illustrative sigmoidal Boltzmann Fit depending on an increasing amount of activator Bz5-3S determined by NMR and GPC (Supplementary Figs. 11–16, Supplementary Table 13). Conditions: 3.0 mmol 5-3S or 5-3R, 3.0 mol% NaH on paraffin, 190 °C, 1 h.

Fig. 6. Effects of the amount of activator.

Influence of the activator concentration on the conversion and M_n of poly5-3R and poly5-3S (Supplementary Figs. 11–14, Supplementary Table 13). Conditions: 3.0 mmol 5-3S or 5-3R, 3.0 mol% NaH on paraffin, 190 °C, 1 h.

Fig. 7. Co-polymerization effects.

Influence of the inclusion of 5-3S on the T_g of co-polyamides with CL and LL (Supplementary Figs. 23–27, conditions: Supplementary Table 16).

Fig. 8. Transparent polyamide films.

Photographs taken under identical conditions of pure polymers and co-polymers in front of a sector of the Fraunhofer lines. a PA12; b copoly(5-3S_32%·LL_68%); c copoly(5-3S_33%·LL_67%); d copoly(5-3S_41%·LL_59%); e PA6; f copoly(5-3S_18%·CL_82%); g copoly(5-3S_48%·CL_52%); h poly5-3R.

Fig. 9. Crystal structures of the monomers.

Crystal structures and monomer molecule conformations of each 5-3R (a, c) and 5-3S (b, d) with bonding hydrogens only viewed in unit cell direction b.

Fig. 10. Crystal structure of poly5-3S.

Structure of poly5-3S, viewed in unit cell direction a (a) and the expanded repeating unit (b). In the unit cell view, only bonding hydrogens are shown.