Functional Polyethylenes with Precisely Placed Thioethers and Sulfoniums through Thiol–Ene Polymerization

Abstract

The precise functionalization of polyethylenes, often accomplished through acyclic diene metathesis polymerization (ADMET), is a significant area of research that has improved polyethylene properties and performance. Here, the synthesis of precisely functionalized polyethylenes was accomplished using the thiol–ene step-growth (TES) polymerization. The simplicity and versatility of this technique allowed for the synthesis of a variety of polymers and enabled the study of carbon spacer length between and repeat unit symmetry about the resulting backbone thioether moiety. In addition, the backbone thioethers of some samples were functionalized postpolymerization with methyl triflate to produce polyethylenes containing sulfonium cations. All polymers were then characterized for their thermal stability, crystallinity, and morphology using differential scanning calorimetry (DSC) and X-ray scattering. While the carbon spacer length and repeat unit symmetry had no effect on polymer thermal stability, the incorporation of cationic sulfonium groups reduced the degradation temperature. Most polymers were polymorphic with respect to crystal structure, and increasing the carbon spacer length led to an increase in polymer melting temperature and percent crystallinity. Furthermore, the average carbon spacer length had a larger effect on polymer percent crystallinity and crystal structure than repeat unit symmetry, but the symmetry had a significant impact on polymer crystal melting temperature, as symmetric polymers had higher melting temperatures. Overall, TES polymerization was utilized to fabricate precisely functionalized polyethylenes, where the repeat unit symmetry improved polymer crystal perfection.

Crystallization of Long-Spaced Precision Polyacetals I: Melting and Recrystallization of Rapidly Formed Crystallites

“…More important is the fact that crystal transformations on heating and the formation of different polymorphs by changing undercooling appears to be a general feature of precision polyethylenes, whether the moiety is a pendant group, ethyl branch or is inserted in the backbone. ,, From the DSC heating runs, it appears that a common feature predictive of polymorphic transformations for polyethylene-like precision systems is a sharp melting–recrystallization–melting event in the heating curves. Melting–recrystallization features observed in the thermograms of short-spaced polyacetals, in precision polyethylenes with arylene ether groups, and in many other precision systems − are highly suggestive of the formation of two crystal forms.…”

“…This feature is also prominent in the thermal behavior of folded long-chain n -alkanes. Once folded, long-chain n -alkane crystals melt and quickly recrystallize into the extended chain form crystals. , Melting–recrystallization–melting events are usual features in the melting thermograms reported for many other types of precision polyethylene-like systems. − This analogy raises the possibility that precision polyethylenes with similar differential scanning calorimetry (DSC) melting–recrystallization–melting features also develop different crystalline structures with different thermal stabilities. It is feasible that analogous to the systems with Cl and Br, different crystalline structures are enabled in other types of precision systems by a different conformational packing of the moiety within layered crystallites.…”

Reversible Actuation via Photoisomerization-Induced Melting of a Semicrystalline Poly(Azobenzene)

“…Though semicrystalline polymers with azobenzene side chains have been reported previously, − we incorporate azobenzene into the polymer backbone to strongly couple isomerization with macromolecular conformation using the thiol-Michael addition of an azobenzene diacrylate (A6A) with 1,6-hexane dithiol (Scheme ). Similar step-growth polymerizations have been previously employed for the synthesis of both main-chain azopolymers , and semicrystalline polymers, − but to our knowledge, not one material containing both features. A diacrylate and dithiol (Scheme ) are combined in a 1.025:1 molar ratio in chloroform with catalytic triethylamine and stirred for 24 h at room temperature.…”

Crystallization of Long-Spaced Precision Polyacetals II: Effect of Polymorphism on Isothermal Crystallization Kinetics

“…Melting-recrystallization-melting events are commonly found in the melting thermograms of precision polyethylene-like systems, often indicating the transformation, on heating, from one crystal form to another with higher thermal stability. − Moreover, upon analysis of isothermal crystallization rates over a wide range of temperatures covering polymorphic transitions in other semicrystalline polymers, minima or discontinuities in the negative coefficient of the crystallization rates are often observed at temperatures within the transition between polymorphs. − Rate discontinuities found in PLA and other polyesters are explained as a competition between primary nucleation and radial growth of the two crystal modifications. ,, Similarly, fast scanning calorimetric measurements of the overall crystallization rates of different polyamides display two maxima with increasing temperature overlapping at the intersection between two polymorphic forms. Coupled with morphological studies, the maxima were interpreted as a combined change in crystal structure and nucleation mechanisms. − Recently, a deep minimum in the variation of the growth rate with temperature was found in precision polyethylenes with halogens .…”