Understanding Molecular Structure to Understand Polymer Properties

Significance 

Our ability to manipulate, exploit and utilize a given material depends very much on our level of understanding of its chemical nature. This task is not always easy, especially when considering novel thermosetting materials. The inability to conduct absolute characterization using conventional techniques of the type employed for soluble molecules with uniform chemical structures is a well-known challenge for the study of insoluble thermosets. Consequently, this often leads to assumptions about materials produced that could ultimately affect applications, further functionalization and modification.

These characterization challenges are perhaps best illustrated by the polymer science community’s difficulties in coming to grips with the underlying chemical structure for the cross-links found in polydicyclopentadiene (PDCPD) – a tough, extensively crosslinked thermoset polymer that is used to make body panels for trucks and tractors. Despite the fact that PDCPD has been in industrial use for over 2 decades, there is still considerable disagreement about the type of cross-link that is present within the material. While most workers in the field envision a so-called “metathesis cross-link” that would result from simultaneous ring-opening metathesis of both carbon–carbon double bonds within the dicyclopentadiene monomer, others have argued that this reaction is impossible with the catalysts that are most commonly used for the polymerization! These workers advocate instead for an “olefin addition cross-link” that would connect residual bonds in such a way that less-substituted carbon–carbon double bonds are converted to more-substituted carbon–carbon single bonds.

The differences between these two outcomes is substantial, and the difficulties in deciding which one is present in such an industrially important material serve to underscore the challenges associated with characterizing insoluble thermosets. Several techniques including Raman spectroscopy and solid state NMR have already been seen as viable ways of providing clues as to the introduction of (or loss) of functionality in non-soluble materials. Unfortunately, these techniques are informative but often lack detail and resolution for definitive proof of the cross-linked structure when used individually. Together, however, they can contribute to a more thorough analysis.

Canadian researchers led by Professor Jeremy Wulff at University of Victoria in collaboration with Dr. Alexander Speed at Dalhousie University determined the exact cross-linked structure of C-linked methyl ester functionalized polydicyclopentadiene (fPDCPD). They achieved this by determining the functional groups involved in cross-linking and the various reaction pathways that could result within cross-linked material. Their work is currently published in the research journal, Macromolecules.

The research technique they employed for their study commenced with the consideration of several possible cross-linking scenarios: olefin addition through the backbone alkene only; olefin addition through non-backbone alkenes; oxidative cross-linking by exposure to air and lastly, further metathesis reactions induced by residual catalyst. Next, they undertook a full analysis of fPDCPD cross-linking and clarified the proposed structure of the resultant material based on solid-state 13C{1H} NMR, FT-IR, Raman spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry.

The authors observed that the ester-functionalized polydicyclopentadiene introduced in earlier work cross-links principally through thermal, self-initiated radical coupling of the pendent methyl methacrylate groups. Moreover, the researchers found no evidence of any secondary metathesis reactions occurring through the substituted cyclopentene.

The results obtained by postdoctoral fellow and first author Tyler Cuthbert and his colleagues ruled out the involvement of metathesis-derived inter-chain couplings and showed that the majority of cross-links were derived from thermally initiated radical polymerization of the embedded methyl methacrylate motif. Lastly, their work also showed that the degree of cross-linking could be tuned by the applied thermal conditions; rapid high-temperature heating froze the system at a relatively low cross-link density, while longer heating at a lower temperature allowed cross-linking to proceed nearly to completion.

In commenting on the work, Dr. Wulff explained, “By understanding the nature of the cross-link at a molecular level, we take the first step toward being able to undo those cross-links through rational means. This could lead to the world’s first truly recyclable form of polydicyclopentadiene, which would be a hugely exciting achievement with far-reaching consequences for the automotive sector and elsewhere.

Understanding Molecular Structure to Understand Polymer Properties (Polydicyclopentadiene)- Advances in Engineering

Reference

Tyler J. Cuthbert, Tong Li, Alexander W. H. Speed, and Jeremy E. Wulff. Structure of the Thermally Induced Cross-Link in C-Linked Methyl Ester-Functionalized Polydicyclopentadiene (fPDCPD). Macromolecules 2018, volume 51, page 2038−2047

Go To Macromolecules

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