Determining the Origins of Impurities during Azide–Alkyne Click

Note pubs.acs.org/Macromolecules

Determining the Origins of Impurities during Azide−Alkyne Click Cyclization of Polystyrene Ravinder Elupula,† Joongsuk Oh,‡ Farihah M. Haque,† Taihyun Chang,‡ and Scott M. Grayson*,† †

Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

‡

S Supporting Information *

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enabled their quantification: 2.8% in the case of c-PS2800 and 0.4% in the case of c-PS3700 (Table 1). Contrary to the observed results, the Jacobson−Stockmayer equation16 predicts that intramolecular coupling that occurs during the cyclization process would decrease for higher Mn polymers because the larger end-to-end distance would inhibit intramolecular cyclization, therefore intermolecular coupling might be more competitive. This model was reinforced by Lonsdale and Monteiro, who demonstrated in kinetic simulations a decreasing yield of monocyclic polymers with increasing molecular weight of polystyrene.17 Because the opposite was observed in this study, alternative explanations for the oligomeric byproducts were investigated. Potential sources of the higher molecular weight impurities include CuAAC oligomerization rather than cyclization resulting from either a missing azide end group (due to incomplete azidization or loss of the functional end group during polymerization) or a missing alkyne end group (due to Glaser coupling during polymerization, storage, or cyclization). In addition, premature coupling of the azide and alkyne end groups might occur during the azidization reaction, storage, or cyclization. Structural determination of the impurity by MALDI-TOF MS will be critical to confirming which of these pathways is responsible for the observed impurities. The MALDI-TOF mass spectra of 2 and 3 before HPLC purification confirm qualitatively that the majority of these materials are the expected unimers (Figure 2a,d). For unfractionated l-PS (2a), the spectrum acquired in linear mode appears to be dominated by unimer, with a trace amount of dimer. MS analysis of fractionated 2a (Figure 2b) exhibits the expected l-PS sodium adduct (25-mer m/zobs = 2793.8, m/ ztheo = 2794.0). Likewise, characterization of the HPLC isolated impurity fraction (Figure 2c) enables confirmation of its assignment as linear dimer (ld-PS), 4a, with a single triazole linkage as well as alkyne and azide end groups (46-mer m/zobs = 5148.2, m/ztheo = 5148.3). Furthermore, when these samples are analyzed by reflector mode, both 2a and 4a show a strong metastable signal, confirming the presence of a single azide functionality (Figure S3).18 Upon cyclization, the product again exhibits a major unimer distribution with a trace amount of dimer (Figure 2d). The mass spectrum of the major HPLC fraction, 3a (Figure 2e), shows only the sodium adduct of the

yclic polymers have been the subject of numerous investigations owing to their unique topology-related physical properties.1 Such studies have intensified in recent years because of improved routes for synthesizing cyclic polymers.1,2 In particular, the versatile copper-catalyzed azide−alkyne click (CuAAC) cyclization technique3 has been applied to generate cyclic homopolymers prepared by ATRP,3,4 ROP,5 RAFT,6 and cationic polymerizations.7 The technique is also amenable to preparing cyclic block copolymers,8 figureeight-shaped polymers,9 cyclic dendronized polymers,10 and more complex topologies.11 However, the presence of linear impurities in a cyclic polymer sample, even in trace amounts (