Perfectly Linear α,ω‐Hydroxy‐Terminated Polyethylene with Near‐Ideal Crystallinity

A novel 1,5-(bis-borinane)pentane (1,5-BBP) has been developed for polyhomologation (C1 polymerization), enabling the synthesis of well-defined α,ω-hydroxy polymethylene (PM) with controlled molar masses and narrow polydispersity at elevated temperatures. The structural integrity and complete difunctionality of the PM were confirmed by ¹H NMR, ¹ ¹B NMR, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and SEC analyses. Achieving perfectly linear polyethylene (PE), an analog to PM, without branching remains a longstanding synthetic challenge, as even trace defects disrupt lamellar packing and reduce crystallinity. We address this limitation through the introduction of a monomer purification strategy for dimethyl sulfoxonium methylide in C1 polymerization that, for the first time, reduces residual branching to <0.04%, as confirmed by quantitative ¹ ³C NMR. The resulting nascent PM exhibits exceptional properties, including enthalpies of fusion up to 290 J·g^-1, melting temperatures of 138 °C-143.15 °C, and crystallinity values of 93.8%-99.1% (DSC) and 92.0%-95.9% (WAXS). Solid-state 2D ¹H-¹ ³C cross-polarization magic angle spinningwide-line separation (CP-MAS WISE) NMR spectra further confirm predominantly crystalline domains with minimal amorphous content. This study demonstrates that precise control over initiator design, monomer purity, and reaction conditions enables α,ω-hydroxy-terminated PM with crystallinity approaching the theoretical limits.