There is Signal in Your Noise: A Case for Advanced Mass Analysis

Abstract

■ WHY WE NEED NEW ANALYTICAL TOOLS Synthetic chemists often take modern characterization techniques for granted and do not appreciate the fortuitous process of analytical development. Imagine yourself as a 1950s chemist without easy access to spectroscopic instrumentation: how would you unambiguously assign chemical structures to small molecules? This question fueled an explosive growth of technologies that provide molecule-specific fingerprints. The advent of classical characterization tools (e.g., NMR spectroscopy, mass spectrometry) accelerated the rate of discovery in organic chemistry as they provide sufficient information to deduce the identity and purity of a sample. In the context of polymer synthesis, however, these classical tools only provide insights related to bulk composition and often fail to fully capture terminal group speciation. As an unintended consequence, many graphical representations omit the chainends. Despite the lack of comprehensive knowledge, ambitious research programs have sparked a renaissance of renewed interest in developing new analytical methodologies. Postpolymerization reactions, for example, often exploit end-groups for practical applications (e.g., upcycling, dynamic network cross-linking). A need, therefore, exists for new tools that uncover currently elusive structural details. Advanced mass analysis has begun to reemerge as an effective solution to these problems. While the initial development of Kendrick analysis dates to the early 1960s, only recent work from Fouquet and co-workers developed the tools necessary for adaptation to polymer characterization. The power of this mass spectral method is readily apparent in Figure 1. Signals are elegantly pulled from the noise in a traditional mass spectrum by deconvoluting the data and rendering it across multiple dimensions. The resultant Kendrick plot extracts compositional information within a homologous series of polymers. Despite its obvious utility, Kendrick analysis has not yet received the attention it deserves nor is it a common-place technique. Indeed, synthetic polymer chemists are often unaware of its exis\tence despite routinely acquiring mass spectra (e.g., MALDI). Indeed, we only learned of Kendrick analysis from an enlightening tutorial by Fouquet entitled “The Kendrick analysis for polymer mass spectrometry”. Our serendipitous introduction to this technique occurred while we were characterizing complex mixtures of oligomers derived from dicyclopentadiene (DCPD). Monomer resins with the second generation Grubbs catalyst (G2, [(SIMes)Ru(�CHPh)(PCy3)Cl2]) produced short chain oligomers when in the presence of a chain-transfer agent (CTA; e.g., styrene). Traditional characterization techniques approximated the molecular weights of the resultant materials but did not report on the chain-end speciation or fate of the pendent cyclopentene groups. While the MALDI pattern revealed the existence of multiple species, only Kendrick analysis provided definitive information as to their identities. At least seven types of species existed that varied by the number and type of chainends. Importantly, this technique revealed that cyclopentene ring-opening occurs; this is often invoked as, but rarely demonstrated to be, the cause for cross-linking in p(DCPD) thermosets. This technique also detected trace impurities within the monomer or CTA, oxidation products, and unusual monomer chain-transfer events (Scheme 1). ■ SO, HOW DOES KENDRICK MASS ANALYSIS WORK?