Selectively Deuterated Poly(ε-caprolactone)s: Synthesis and Isotope Effects on the Crystal Structures and Properties

Abstract

Selective deuteration is an important tool for many analytical techniques including neutron scattering and spectroscopies. However, the availability of deuterated materials is limited because of the challenges in their synthesis. Here, we report the synthesis of partially and fully deuterated ε-caprolactone monomers and their corresponding polymers, poly(ε-caprolactone)s (PCLs), and the investigation of isotope effects on their crystalline structures and physical properties. Deuteration of PCLs leads to smaller crystal lattices and volumes compared to protiated PCLs by the amount proportional to the deuteration levels. The linear trend suggests that the volume isotope effect in PCL is primarily governed by the vibrations of C−D and C−H bonds. The large intrachain contraction of deuterated PCLs compared to that of polyethylene reported in the literature can be ascribed to the presence of polar ester groups in PCLs. Deuterated PCLs also display lower melting temperatures than protiated PCLs proportional to their deuteration levels because of weaker intermolecular interactions in deuterated polymers. FTIR spectroscopy, with support from density functional theory calculations, shows large red shifts of the stretching and bending frequencies of C−D versus C−H bonds as dictated by their relative reduced masses 13/7 , and to a smaller degree for the CO stretching frequency. This work is among the very few studies comparing the effects of partial versus full deuteration on the structures and properties in semicrystalline polymers. These results not only advance our understanding of isotope effects in polymeric materials but also provide an important avenue to design polymers with desirable properties.

Isotope Effects on the Crystallization Kinetics of Selectively Deuterated Poly(ε‐Caprolactone)

“… is the only variable for fitting with the parameter setting: σ =0.09 Jm −2 , Δh 0 =1.63 × 10 8 Jm −3 , β =1 and δL c =5 nm. This equation faithfully captures the trend of the overall data, as the differences between fitted curves and data points were less than 0.3 K. Herein, we extracted for D4 and D6 and summarized the values in Table , while the of D0 and D10 samples were taken from our previous work . The values of PCLs show the same trend as the T m : (D0) > (D6) > (D4) > (D10).…”

“…Nevertheless, the higher T m value of D6 than D4 suggests that there might be another factor contributing to the melting temperature of PCLs, which is associated with the deuteration sites. Our previous study revealed a weak hydrogen (H) ‐bond like interaction between the O atom (in the carbonyl group, CO) and three nearby H atoms (from neighboring chains), due to the larger electronegativity of C than H . This weak H‐bond like interaction is illustrated in Figure for D0, D4, D6, and D10, which functions as another attractive force that can hold the PCL chains together.…”

“…Fully deuterated ε‐caprolactone‐ d 10 (CL‐d10) and partially deuterated ε‐caprolactone‐ d 4 (CL‐d4) were synthesized through the Baeyer‐Villager oxidation as reported in our recent study . To achieve a better transition of the degree of deuteration from 0%, to 40%, to 60%, and then to 100%, ε‐caprolactone‐d 6 (CL‐d6), which is shown in Supporting Information Figure S1, was synthesized in this work (described later).…”

“…The flask was cooled in an ice bath under nitrogen flow, and a suspension of meta ‐chloro‐peroxybenzoic acid ( m ‐CPBA, 72 g at 70–75% purity) in 150 mL dry CH 2 Cl 2 was added. The reaction was conducted in the same manner as in our previous study for ε‐caprolactone‐d 10 with the exception that saturated aqueous Na 2 SO 3 was employed in place of saturated aqueous Na 2 S 2 O 3 during the workup. The crude ε‐caprolactone‐ d 6 was purified by fractional vacuum distillation from powdered calcium hydride (CaH 2 ).…”

“…Deuterated polyethylene (PE) was proved to be the key in elucidating the shish‐kebab superstructures and chain alignment of PE under cold‐draw through small‐angle neutron scattering (SANS) . Concurrently, various methods for the synthesis of deuterated polymers have been reported, including polymerizing the deuterated monomers, saturating polydienes with deuterium gas (D 2 ), and introducing deuterium at elevated pressure and temperature over different heterogeneous catalysts in the presence of D 2 . However, D substitution techniques sometimes exhibit nonsubtle isotope effects, such as different crystallization kinetics, which implicates the usage of D labeling in rigorously controlled crystallization studies .…”