Diels Alder-crosslinked elastomers

The Diels Alder reaction is an easy and efficient way to synthesize architectural complex macromolecules, such as block, graft, star, crosslinked and cyclic polymers. In contrast to another prominent “click reaction”, the copper catalyzed Huisgen type [3+2] cycloaddition of azides with alkynes, the Diels Alder [4+2] cycloaddition is catalyst free and additionally offers the possibility of thermoreversibility.⁷⁷ This would allow the synthesis of polar-substituted, polyethylene-based elastomers which can be recycled to the non-crosslinked starting material.

For this reason, furfuryl acrylate was examined towards insertion copolymerization with ethylene at 5 bar ethylene pressure at 95 °C in toluene solution. HT-¹³C NMR spectroscopy (130 °C, C₂D₂Cl₄) unambiguously confirmed the copolymer nature of the materials. The distinct ¹³C NMR resonances of the methylene carbons adjacent to the carbonyl group -C^bH2C^aH2CH(COO(R)C^aH2C^bH2- (R: furfuryl) resonate at δ 32.80 (αδ⁺) and 27.74 (βδ⁺) (Table 3.6, entry 1). In terms of activity, polymerizations with furfuryl acrylate possess significantly lower polymer yields than polymerizations with e.g. 2-ethyl hexyl acrylate (Table 3.6, entry 2). This limitation of activity may be due to interactions of the furan moiety with the palladium center of 1-CH3-dmso.

In order to examine this possible interaction, a NMR tube was charged with 1-CH3-dmso (33 mM, CD₂Cl₂), which possesses a characteristic low-field shift of the dmso resonance due to the coordination to the palladium (coordinated dmso resonates at δ 2.94 ppm, free dmso at δ 2.54 ppm in CD2Cl2 at 25 °C). Gradual addition of furan resulted in fast exchange with dmso, indicated by high-field shifting of the dmso resonance towards free dmso. The estimated equilibrium constant Keq for the exchange of dmso by furan, however, is relatively small with a value of ~10^-2, which is e.g. in the same order of magnitude found for ethyl acetate.⁴⁷

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

and furanyl methyl propionate) indeed prove that even this weak ability to coordinate to the active metal center is sufficient to significantly hinder polymerization if furans are present in large excess to the catalyst during polymerization (Table 3.7). Activity e.g. decreases by a factor of 2 when polymerizing in a 0.3 M 2-ethyl furan solution in toluene. Retardation of the catalyst is even more pronounced by addition of furanyl methyl propionate and likely reflects the effect of furfuryl acrylate on activity more appropriate since coordination may occur via κ-O binding of the furan or the ester during polymerization. Polymerization in a 0.3 M solution of furanyl methyl propionate lowers activity by a factor of >10, thus the low activity of an ethylene copolymerization with furfuryl acrylate may be referred to coordination of the functional groups of the free monomer to the metal center.

Terpolymerizations of ethylene with furfuryl acrylate and 2-ethyl hexyl acrylate were performed (Table 3.6, entry 3 and 4). Figure 3.18 shows a representative ¹³C{¹H} NMR spectrum (CDCl₃, 100 MHz, 25 °C) which exhibits the characteristic methylene resonances adjacent to the esters and the resonances for the furan moiety as well as the 2-ethyl hexyl group.

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

Table 3.6Co- and terpolymerizations of ethylene with furfuryl acrylate and/or ethyl hexyl acrylate

entry ratio of aliphatic and end group resonances.

Table 3.7 Ethylene homopolymerization in the presence of 2-ethyl furan and furanyl methyl propionate^a

entry additive

a polymerization conditions: total volume toluene + monomers: 50 mL, 95 °C reaction temperature, 30 minutes reaction time, 5 bar ethylene pressure, 5 µmol Pd(II) from methylene chloride stock solution.

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

Figure 3.18 ¹³C NMR spectrum (100 MHz, CDCl₃, 25 °C) of poly(ethylene-co-2-ethyl hexyl acrylate-co-furfuryl acrylate) (Table 3.6, entry 3)

Scheme 3.6 Diels Alder model compounds DA1 and DA2.

Prior to crosslinking experiments of poly(ethylene-co-2-ethyl hexyl acrylate-co-furfuryl acrylate) with bismaleimides, the Diels Alder reaction was investigated in two model reactions towards suitable reaction conditions (Scheme 3.6). The formation of the DA model compound DA1 was followed by periodically acquired ¹H NMR spectra using furfuryl alcohol and N-phenyl maleimide in equimolar amounts at 25 °C and 50 °C (concentration ~

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

150 mM, CDCl3). The reaction was found to be faster at elevated temperature, e.g. after 5 hours, conversion at 50 °C was determined to 67 % while conversion at 25 °C merely reached 22 %. The experiment at 25 °C showed a final conversion of 82 %. The endo- and exo- products were formed in a 54:46 ratio as calculated from the resonance of the allylic methine protons (¹H NMR and ¹³C NMR spectra of compound DA1 in experimental section Figure 3.47). In case of a reaction temperature of 50 °C, conversion was 86 % and the endo:exo ratio shifted towards the thermodynamically preferred exo-product (21:79). Model compound DA1 was isolated using furfuryl alcohol in large excess in diethyl ether in which the product precipitated within a few minutes and was isolated by filtration. ¹H NMR spectroscopy shows the formation of the endo- and exo- products in a ratio of 72:28. Upon heating this compound for 2 hours to 70 °C and 90 °C respectively in C2D2Cl4 solution, two reactions proceeded. On the one hand, the endo-exo ratio shifted towards the thermodynamically more stable exo-product (56:44 at 70 °C, 16:84 at 90 °C). In addition, retro Diels Alder (rDA) takes place with 31 % at 70 °C and 72 % at 90 °C (see experimental part Figure 3.48).⁷⁸

The optimized reaction conditions for the DA reaction found for furfuryl alcohol with N-phenyl maleimide were applied for the DA reaction of the terpolymer poly(ethylene-co-furfuryl acrylate-co-2-ethyl hexyl acrylate) (5.5 mol-% of each acrylate, Table 3.6, entry 3) at 50 °C as studies with model compound DA1 seem to be more promising at this temperature due to faster conversion. The polymer was reacted with one equivalent of N-phenyl maleimide at 50 °C for 2 days. ¹H NMR (Figure 3.19, upper spectrum) reveals 89 % conversion to the DA-product DA2 as calculated from the δ,γ-resonances of the furan. The DA product itself reveals an endo:exo ratio of 21:79 which is in good agreement with the ratio found for model compound DA1 at 50 °C.

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

Figure 3.19 ¹H NMR (400 MHz, CDCl₃) of poly(ethylene-co-furfuryl acrylate-co-2-ethyl hexyl acrylate (Table 3.6, entry 3) before (bottom) and after Diels Alder reaction with N-Phenyl maleimide (top).

O

Scheme 3.7 Crosslinking of furfuryl substituted terpolymers mit bismaleimide

Crosslinking of poly(ethylene-co-furfuryl acrylate-co-2-ethyl hexyl acrylate) (Table 3.6, entry 3 and 4) was performed using 1,1’-bis(methylenedi-4,1-phenylene)bismaleimide (BM) as a bifunctional crosslinker. In a typical crosslinking procedure, the polymer and BM (0.5 equiv referred to furfuryl groups) were dissolved in CH2Cl2 in order to obtain a homogeneous reaction mixture. The solvent was slowly removed by heating to 50 °C, and the material was

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

then cured for additional 24 hours at this temperature. Within this time, the sticky polymer turned into an elastomeric material. ATR-IR spectroscopy of the crosslinked polymers exhibit characteristic -C-N-C- bands at 1379 cm^-1 (sym) and 1167 cm^-1 (asym) as well as >C=O bands at 1781 cm^-1 and 1712 cm^-1 which were also found in the polymeric model compound DA2 (Figure 3.20).

Figure 3.20 ATR IR spectra of poly(ethylene-co-furfuryl acrylate-co-2-ethyl hexyl acrylate) (5.3 mol-% of furfuryl acrylate and 8.2 mol-% of 2-ethyl hexyl acrylate) (black), crosslinked with bismaleimide (red) and the model compound DA2 (blue).

A small band at 920 cm^-1 for the asymmetric out-of-plane vibration of the furan, which is absent in the model compound, is, however, still present in the crosslinked material indicating that crosslinking in bulk in not complete. The two crosslinked polymers were both found to be entirely insoluble in CH2Cl2 (gel content > 99±5 %). This verifies that partial crosslinking (according to IR) is sufficient to form a complete network. Figure 3.21 illustrates the elastomeric behavior of the crosslinked polymer with 5.3 mol-% of furfuryl acrylate and 8.2 mol-% of 2-ethyl hexyl acrylate. The polymer panel with a diameter of ~ 1 cm can be stretched to at least twice of its size, and recovers after release of the applied strain to its original shape.

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization

Figure 3.21 Crosslinked polymer pellet (Table 3.6, entry 3) left: before stretching, center: stretched, right: after deformation

Saturated polar-substituted Polyethylene Elastomers from Insertion Polymerization