Independent Control of Elastomer Properties through Stereocontrolled Synthesis

Abstract

In most synthetic elastomers, changing the physical properties by monomer choice also results in a change to the crystallinity of the material, which manifests through alteration of its mechanical performance. Using organocatalyzed stereospecific additions of thiols to activated alkynes, high-molar-mass elastomers were isolated via step-growth polymerization. The resulting controllable double-bond stereochemistry defines the crystallinity and the concomitant mechanical properties as well as enabling the synthesis of materials that retain their excellent mechanical properties through changing monomer composition. Using this approach to elastomer synthesis, further end group modification and toughening through vulcanization strategies are also possible. The organocatalytic control of stereochemistry opens the realm to a new and easily scalable class of elastomers that will have unique chemical handles for functionalization and post synthetic processing.

Keywords: click chemistry; elastomers; organocatalysis; step-growth polymerization; stereochemistry.

Scheme 1

Synthesis of thiol–yne elastomer materials from dialkyne and dithiol precursors.

Figure 1

A) Wide‐angle X‐ray diffraction, B) small‐angle X‐ray scattering, and C) differential scanning calorimetry (solid=first scan; dashed=second scan) for C_3A–C_6S materials.

Figure 2

A) Dependence of the normalized network true stress σ _true(t)/$\dot{ϵ}$ as a function of time ϵ(t)/$\dot{ϵ}$ for networks with different fraction of the cis‐isomers obtained from the constant strain rate experiments (2.34×10⁻⁶ to 9.38×10⁻² s⁻¹); B) Dependence of the network Young's modulus on the fraction of cis isomers. Inset: the linear dependence of E ₀−E_a on the degree of crystallinity φ _c.

Figure 3

A)–C) Exemplary stress vs. strain curves for A) 80 % cis C_3A–C_6S; B) 53 % cis C_3A–C_6S; C) 32 % cis C_3A–C_6S in the non‐linear region. Data for three samples are shown to illustrate the reproducibility. Expansions inset for clarity.

Figure 4

¹⁹F NMR spectrum of C_3A–C_6S thiol–yne step‐growth polymer following end capping with 2,2,2‐trifluoroethanethiol (376 MHz, CDCl₃+0.01 % v/v CF₃COOH).

Figure 5

Exemplary stress vs. strain curves for 80 % cis C_3A‐C_6S before (red solid line) and after (blue dashed line) vulcanization with 1 %wt dicumyl peroxide.