End-Groups of Poly(p-phenylene sulfide) Characterized by DNP NMR Spectroscopy

Characterization of the local chemical structure of high-performance engineering plastics is essential for rational polymer blending to achieve outstanding physical properties. The inherent high chemical stability, however, has hampered their detailed characterization using conventional analytical methods. Herein, we demonstrate that dynamic nuclear polarization enhanced solid-state NMR combined with a thermal treatment approach for sample preparation enables characterization of end-groups of poly(p-phenylene sulfide) (PPS). Mildly heating PPS mixed with TEKPol/1,1,2,2-tetrachloroethane (TCE) solution resulted in a high sensitivity enhancement from 13C CPMAS signals of PPS, and further optimization of the conditions using 1-chloronaphthalene (1-CN) as a solvent led to an enhancement factor (ε) as high as ∼43, while a conventional incipient wetness impregnation method with TEKPol/TCE afforded an enhancement factor of only ∼4. Mechanistic studies with electron paramagnetic resonance and scanning electron microscopy analysis demonstrated that thermal treatment at 60 °C promoted the distribution of the PA solution into a surface layer of PPS while suppressing the decomposition of TEKPol. The developed sample preparation approach enabled us to observe 13C and even 15N signals assignable to end-groups of PPS in a reasonable measurement time by using a natural abundance sample. According to the observed signals, which were identified by comparing with signals from low-molecular-weight model compounds, sodium carboxylate is present at the chain-end position, suggesting that the termination step of PPS production involves an SNAr reaction of p-chlorophenyl intermediate with the secondary amine of the ring-opened N-methylpyrrolidone used as a solvent for PPS production.