Synthesis of Long-Spaced Polyamides

For the preparation of polyamides by ADMET copolymerization, N-(undec-10-en-1-yl)undec-10-enamide 38 as an amide functionalized ,ω-diene monomer and undeca-1,10-diene 28 as a non-functionalized ,ω-diene monomer were applied. Comparable to the route described by Cramail and coworkers,¹⁰³ monomer 38 was synthesized in a multistep procedure based on 11-bromoundec-1-ene 16 and 10-undecenoic acid 27 (Figure 7.4). In a substitution reaction of the bromide 16 with NaN3 the azide 36 was prepared. Afterwards, the

amine 37 was generated via reduction of the azide with LiAlH4 and finally used to prepare the amide monomer 38 by condensation with the acid chloride based on 10-undecenoic acid 27.

Figure 7.5. Preparation of long-spaced polyamides by ADMET copolymerization and post-polymerization hydrogenation. The designation ‘PA-X’ indicates the number (X) of amide groups per 1000 methylene units

determined by ¹H NMR spectroscopy, the additional ‘H’ designates saturated polyamides ‘PA-XH’.

Figure 7.6. ¹H NMR spectra of the unsaturated polyamide PA-38.8 (top, CDCl3, 400 MHz, 25 °C) prepared by ADMET copolymerization with G1 as a precursor for the olefin metathesis catalyst and the corresponding saturated PA-38.8H (bottom, C2D2Cl4, 400 MHz, 130 °C) from hydrogenation of the carbon-carbon double

bonds. Reprinted with permission from reference 157. Copyright 2015 American Chemical Society.

Since with the application of G1 as the olefin metathesis catalyst precursor satisfying conversions for other carbonyl functionalized dienes and no significant side reactions were

obtained during ADMET copolymerizations, this olefin metathesis catalyst precursor was also applied for copolymerization of different ratios of 38 and 28 to yield unsaturated polyamides (Figure 7.5 and Table 7.1). To keep the reaction mixtures liquefied, the copolymerizations were performed at reaction temperatures between 60 and 105 °C, depending on the melting points of the resulting copolymers (being significantly influenced by the amide group densities, vide infra) and under vacuum conditions to remove the ethylene byproduct. The unsaturated copolymers obtained were hydrogenated with H2 using Wilkinson’s catalyst [RhCl(PPh3)3]¹⁵⁸ in a pressure reactor to yield saturated, aliphatic polyamides with amide contents between 1.0 and 50.3 amide groups per 1000 methylene units (the latter originating from homopolymerization of the amide functionalized monomer 38). Complete hydrogenation of the carbon-carbon double bonds was confirmed by the absence of signals for the unsaturated protons by high temperature ¹H NMR spectroscopy (Figure 7.6).

Table 7.1. Compositions and molecular weights of long-spaced polyamides prepared by ADMET copolymerization using G1 as a catalyst precursor.

a Calculated from the monomer weight ratio applied. ^b Determined by ¹H NMR spectroscopy of the resulting unsaturated copolymers. ^c Determined by GPC in THF at 40 °C vs. polystyrene standards (unsat. polyamides).

d Determined by end-group analysis from ¹H NMR spectroscopy of the saturated polymers. ^e GPC analysis hampered by a low solubility in THF.

The compositions of the resulting polyamides were found to be identical with the initial monomer ratios (Figure 7.6), as determined by the ratios of signal intensities for the methylene units adjacent to the amide functionalities to all proton resonances in the ¹H NMR spectra. The molecular weights were analyzed by GPC analysis (in THF at 40 °C vs.

polystyrene standards) and ¹H NMR spectroscopy via end-group signal integration. While for

copolymers with low amide contents molecular weights around 10000 g mol^-1 were observed, moderate to high amide incorporation rates lead to molecular weights between 1000 and 5000 g mol^-1. Molecular weight distributions Mw/Mn around 2 were found, as expected for well-behaved polycondensation reactions. However, in comparison to previous findings from the synthesis of long-spaced polycondensates, ADMET copolymerizations with G1 yielded polyamides with significantly lower molecular weights. Possibly, the more polar character of the amide group together with the essentially high reaction temperatures to prevent polymerization mixtures with high amide contents from solidifying impair the performance of G1 in the copolymerizations. This consideration is further supported by the slightly decreased molecular weights observed in the synthesis of long-spaced polyketones with higher ketone contents (compared to polyesters and polycarbonates, cf. Chapter 5.2), where polymerization temperatures also had to be raised due to the highly polar character of the ketone groups (Table 5.6 and Figure 5.13).

Table 7.2. Compositions and molecular weights of long-spaced polyamides prepared by ADMET copolymerization using HG1 and HG2 as catalyst precursors.

Compound Monomer

a Calculated from the monomer weight ratio applied. ^b Determined by ¹H NMR spectroscopy of the re resulting polymers. ^c Determined by GPC in THF at 40 °C vs. polystyrene standards. ^d Determined by end-group analysis

from ¹H NMR spectroscopy of the saturated polymers. ^e GPC analysis hampered by a low solubility in THF.

To this end, Hoveyda-Grubbs 2^nd generation catalyst (HG2) was studied, since higher thermal stability at elevated reaction temperatures together with comparable high reactivities toward various substrates in olefin metathesis reactions have been reported for this catalyst

precursor.^26,159 Studies of ADMET polymerizations by HG2 demonstrated constant catalytic activities over a broad temperature range, even yielding polymers with molecular weights of 10000 g mol^-1 at temperatures of 120 °C (as shown for the polymerization of ester functionalized ,ω-dienes).^160,161 Also polymerizations of amide functionalized dienes have been reported for this catalyst precursor.^162,163 However, a significantly higher degree of carbon-carbon double bond isomerization,¹⁶⁴ has been proven for second generation olefin metathesis catalysts, especially at high reaction temperatures (compare Chapter 1.2).

In copolymerization experiments under the same conditions as applied with G1, indeed higher molecular weights could be achieved with HG2 as the catalyst precursor, ranging from 3000 to 11000 g mol^-1 (Table 7.2). For polymers with higher incorporations of functional groups, however, the molecular weights still remained lower compared to ADMET derived copolymers with other functionalities. A drastic decrease in the degree of polymerization was obtained in copolymerizations of mixtures containing very low amounts of the amide functionalized monomer 38. Due to an insufficient solubility of HG2 in apolar reaction media, only short-chain oligomers or monomer/dimer mixtures could be obtained. To resolve this issue, Hoveyda-Grubbs 1^st generation catalyst (HG1) was studied. Significantly higher solubilities were observed for this alkylidene (likely due to the more lipophilic PCy3 ligand) in the largely hydrocarbon reaction media and the copolymerizations yielded polyamides with molecular weights on the same order as prepared with G1. Also in the homopolymerization of the amide diene monomer, HG1 provided satisfactory molecular weights comparable to those obtained with HG2.

An exceeding carbon-carbon double bond isomerization behavior, evocating higher numbers of amide functionalities in the resulting copolymers (when besides the expulsion of ethylene longer-chain olefins like propylene or butylenes are released during the metathesis process) was not observed. The analysis of the polyamide compositions by ¹H NMR spectroscopy showed similar contents of functional groups compared with the initial monomer ratios.