Selective deuteration along a polyethylene chain: Differentiating conformation segment by segment

Abstract

To improve the circularity and performance of polyolefin materials, recent innovations have enabled the synthesis of polyolefins with new structural features such as cleavable breakpoints, functional chain ends, and unique comonomers. As new polyolefin structures become synthetically accessible, fundamental understanding of the effects of structural features on polymer (re)processing and mechanical performance is increasingly important. While bulk material properties are readily measured through conventional thermal or mechanical techniques, selective measurement of local material properties near structural defects is a major characterization challenge. Here, we synthesized a series of polyethylenes with selectively deuterated segments using a polyhomologation approach and employed vibrational spectroscopy to evaluate crystallization and melting of chain segments near features of interest (e.g., end groups, chain centers, and mid-chain structural defects). Chain-end functionality and defects were observed to strongly influence crystallinity of adjacent deuterated chain segments. Additionally, chain-end crystallinity was observed to have different molar mass dependence than mid-chain crystallinity. The synthesis and spectroscopy techniques demonstrated here can be applied to range of previously inaccessible deuterated polyethylene structures to provide direct insight into local crystallization behavior.

Figure 1.

ATR-FTIR absorbance spectra of CD₂ and CH₂ bending modes in perdeuterated PE (Cambridge Isotope Laboratories, top) and mid-chain-3k (entry 13, bottom). Crystal field splitting was absent in CD₂ bending modes (purple) for PE with deuterated segments, indicating phase segregation of deuterated segments did not occur.

Figure 2.

Representative transmission FTIR absorbance spectra of d-PE segment melting (mid-chain-6k, entry 5). Stretching modes of trace –CDH– isotopic impurities appear in the asterisked region.

Figure 3.

Conformational order indicators for (a) Raman and (b) transmission-FTIR spectra. Low ratios of Raman scattering intensity at 2176 cm⁻¹ and 2194 cm⁻¹ (I₂₁₇₆/I₂₁₉₄) and high ratios of FTIR absorbance at 2088 cm⁻¹ and 2195 cm⁻¹ (A₂₀₈₈/A₂₁₉₅) correlate with higher conformational order of the deuterated chain segment.

Figure 4.

Ratio of transmission-FTIR absorbances at 2088 cm⁻¹ and 2195 cm⁻¹ (A₂₀₈₈/A₂₁₉₅) for (a) alkyl-end-labeled samples (entries 1 and 7–12) and (b) mid-chain-labeled samples (entries 5 and 13-16) at several molar masses. (c) Same data at 46 °C replotted vs M_n. Higher values of (A₂₀₈₈/A₂₁₉₅) at a given temperature correlate with higher conformational order of the deuterated chain segment. Error bars represent standard deviation among three experiments, each using separate portions of the indicated sample.

Figure 5.

Ratio of transmission-FTIR intensities at 2088 cm⁻¹ and 2195 cm⁻¹ (A₂₀₈₈/A₂₁₉₅) near different structural features (entries 1–6). Inset shows side-by-side results at 46 °C for clarity. Higher values of (A₂₀₈₈/A₂₁₉₅) at a given temperature correlate with higher conformational order of the deuterated chain segment. Error bars represent standard deviation among three experiments, each using separate portions of the indicated sample.

Scheme 1.

(a) General polymerization scheme showing polyhomologation via sequential addition of protiated and deuterated monomers to an organoborane initiator. (b) Structural features with different polarity and bulkiness with adjacent deuterated segments explored in this work.