Catalyst switch strategy enabled a single polymer with five different crystalline phases

Semi-crystalline polymers provide superior mechanical properties, thermal conductivity, and phase separation due to their crystalline phases. Common semi-crystalline polymers include polyethylene (PE), poly(ethylene oxide) (PEO), poly(ε-caprolactone) (PCL), and poly(L-lactide) (PLLA), often prepared by anionic ring-opening polymerization. Advances in polymer chemistry have produced double- and multicrystalline block copolymers, but synthesizing multiblocks with more than two crystalline domains is challenging due to incompatibilities in polymerization methods and block properties. PE-based multiblocks typically use ω-hydroxyl-terminated PE (PE-OH) precursors, where polyhomologation (C1 polymerization) provides perfectly linear PE-OH as superior macroinitiators compared with hydrogenated polybutadiene-based routes. As the number of crystalline blocks increases, nucleation, crystallization, and morphology become increasingly complex, making multicrystalline systems valuable but synthetically demanding. The authors target integrating a high melting point polyglycolide (PGA) block—ideal for distinguishing crystalline phases but difficult to synthesize controllably—into a complex multiblock to explore crystallization and phase interplay. The research question is whether a catalyst-switch strategy can compatibilize the distinct chemistries required to assemble a single pentablock polymer exhibiting five distinct crystalline phases and how these phases interact during thermal transitions.

Prior double-crystalline diblocks include PEO-b-PCL, PEO-b-PLLA, and PCL-b-PLLA; PE-based double-crystalline polymers include PE-b-PEO, PE-b-PCL, and PE-b-PLLA. The first organic catalyst switch enabling base-to-acid switching produced polyether-b-polyester block copolymers in one pot, with subsequent developments including base-to-base, organic-to-metal, and biased Lewis acid/base switching; switchable metal catalysts have also enabled polycarbonate-b-polyester syntheses. A tetracrystalline PE-b-PEO-b-PCL-b-PLLA was reported previously via an organic-to-metal catalyst switch. PGA is a high-Tm biodegradable polyester (Tm ~220 °C) but is typically obtained by melt ROP at 150–230 °C with non-living characteristics, limiting block copolymer formation; literature reports only a few PGA-based double-crystalline copolymers and no PGA-based tricrystalline triblocks. Recently, a fluoroalcohol-assisted catalyst switch enabled the first living/controlled polymerization of glycolide at room temperature, suggesting a route to incorporate PGA into complex architectures. This work builds on these strategies to realize a pentacrystalline pentablock with five crystallites.

Synthesis strategy: Combine polyhomologation (C1 polymerization) to obtain linear PE-OH macroinitiators with sequential ring-opening polymerizations (ROP) using catalyst-switch approaches, including an organic-to-metal switch and a fluoroalcohol-assisted switch for glycolide.

• PE-OH preparation (polyhomologation): Generate dimethylsulfoxonium methylide in THF, filter into toluene, initiate with triethylborane (TEB) at 80 °C, then oxidize/hydrolyze the tris-organoborane junction with TAO·2H2O to obtain perfectly linear PE-OH. Example characterization: 1H NMR in toluene-d8 at 80 °C shows –CH2–OH (3.42–3.32 ppm) and main-chain –CH2– (1.47–1.18 ppm); Mn,NMR ≈ 1.5 kg/mol (PE-OH1.5k). HT-SEC indicates narrow dispersity (Đ = 1.06 for PE-OH1.5k; Đ = 1.15 for PE-OH7k).
• Block growth sequence (PE-OH1.5k series, pentablock-1a):
  1. PE-b-PEO: Activate PE-OH with ‘BuP4 (0.5 equiv) in toluene at 80 °C for 30 min; add ethylene oxide (EO, 240 equiv) and polymerize at 80 °C for 15 h; >99% conversion; Mn,NMR(PEO) ≈ 16.6 kg/mol.
  2. Organic-to-metal catalyst switch for CL: Neutralize with diphenyl phosphate (DPP, 0.6 equiv) and add Sn(Oct)2 (0.3 equiv). Add ε-caprolactone (CL, 90 equiv) in toluene and polymerize at 80 °C for 45 h; >99% conversion; Mn,NMR(PCL) ≈ 15.7 kg/mol.
  3. Continue ROP of L-lactide (LA, 140 equiv) at 80 °C for 24 h; 98.5% conversion; Mn,NMR(PLLA) ≈ 27.5 kg/mol.
  4. Fluoroalcohol-assisted catalyst switch for glycolide (GA): Dissolve GA (60 equiv) in toluene/HFAB [1,3-bis(2-hydroxyhexafluoroisopropyl)benzene] and add to the living solution; polymerize at 80 °C for 20 h; GA conversion 86.2%; Mn,NMR(PGA) ≈ 5.8 kg/mol. HFAB acts as co-solvent/co-catalyst to maintain solubility of the growing PGA and enable controlled ROP.
• Control without HFAB (pentablock-1b): Same conditions but no HFAB; GA conversion only 58.3% after 36 h; rapid precipitation/gelation indicates insoluble PGA obstructs living ROP.
• Reproducibility with higher-Mn PE-OH (≈7 kg/mol, pentablock-2): Synthesis analogous; solution-based SEC/1H NMR not feasible due to insolubility, but solid-state methods and WAXS used.

Characterization:

• Solution NMR: 1D 1H and 2D (COSY, TOCSY, HSQC, HMBC) in toluene-d8 or toluene-d8/HFIP-d2 to assign junctions and block-specific resonances; 19F NMR on trifluoroacetylated end-groups confirms presence/absence of secondary OH and lack of transesterification.
• Solid-state NMR: Variable-temperature (RT, 60 °C, 120 °C) 1H–13C CP-MAS (rigid domains) and INEPT (mobile domains); 2D CP-MAS WISE to map mobility via 1H linewidths versus 13C chemical shifts.
• SEC: HT-SEC (TCB, 150 °C) for PE-OH; SEC (THF, 50 °C) for other samples; monomodal traces with low dispersity across growth steps.
• DOSY (60 °C): Single diffusion coefficient for all block resonances indicates covalent linkage of all five blocks.
• Thermal analysis: TGA to determine Td,5% (most >250 °C; PE-b-PEO ~200 °C). DSC at 10 °C/min for heating and cooling; additional 1 °C/min scans for peak resolution.
• WAXS: Identify crystalline reflections of PE, PEO, PCL, PLLA, and PGA; assess crystallinity and phase coexistence.

• Successful synthesis of a pentacrystalline pentablock quintopolymer PE-b-PEO-b-PCL-b-PLLA-b-PGA via sequential ROP with catalyst-switching, starting from linear PE-OH made by polyhomologation.
• Fluoroalcohol-assisted catalyst switch (HFAB) is critical for PGA incorporation: with HFAB, GA conversion 86.2% in 20 h and clear solution until high conversion; without HFAB, GA conversion 58.3% even after 36 h with precipitation/gelation.
• SEC shows monomodal, low-dispersity traces at each growth step; DOSY reveals a single diffusion coefficient (6.55 × 10−7 cm2 s−1) for all block resonances, confirming all five blocks belong to the same macromolecule.
• DSC heating (10 °C/min) for pentablock-1a shows multiple melting endotherms consistent with five crystalline phases: PE Tm ≈ 102.1 °C; PEO/PCL overlapping Tm ≈ 39.5 °C; PLLA Tm ≈ 161.3 °C; PGA Tm ≈ 206.6 °C. On cooling: PE Tc ≈ 67.1 °C; PEO/PCL merged Tc ≈ 19.1/18.2 °C; PLLA Tc ≈ 88.2 °C; PGA Tc ≈ 150.0 °C. Slow scans (1 °C/min) partially resolve PEO and PCL melting but PGA peak may be obscured by decomposition.
• Variable-temperature solid-state NMR corroborates sequential melting/mobility changes: at 60 °C, PEO and PCL signals lose CP intensity and appear in INEPT (mobile), while PE, PLLA, PGA remain rigid; at 120 °C, PE becomes mobile, while PLLA and PGA remain rigid; WISE shows sharp 1H lines for PCL above its Tm and broad lines for higher-Tm blocks.
• WAXS identifies crystalline reflections of all five blocks in the pentablock: PE (110, 200 at 2θ ≈ 21.7°, 23.8°), PEO (120 at 19.4°, 200 at 23.8°), PLLA (010 at 15.1°, 200/110 at 17.0°, 203 at 19.4°), PGA (110 at 22.5°, 020 at 29.2°). Strong PGA reflections indicate high crystallinity despite low Mn (≈5.8 kg/mol).
• Thermal stability: Td,5% generally >250 °C (except PE-b-PEO ~200 °C). PE Tm in multiblocks remains ~101–103 °C; PEO/PCL Tm decreases with added blocks (from ~60.9 to ~39.5 °C), consistent with miscibility/dilution effects. PGA crystals nucleate PLLA, increasing PLLA Tc in the pentablock compared to the tetrablock.

The study demonstrates that an integrated catalyst-switch approach can reconcile the distinct chemistries required to assemble a single macromolecule bearing five crystallizable blocks. Incorporation of a high-Tm PGA segment, enabled by HFAB co-solvent/co-catalyst, overcomes solubility and precipitation issues that otherwise halt living ROP and prevents transesterification with neighboring blocks. Comprehensive spectroscopic, chromatographic, thermal, and diffraction analyses converge to confirm the coexistence of five crystalline domains in one polymer. The thermal sequence inferred from DSC and variable-temperature solid-state NMR aligns with block melting points: PEO/PCL melt near 40 °C, PE near 100–120 °C, PLLA near 160–170 °C, and PGA near 200 °C, enabling distinct identification of phases. The observed increase in PLLA crystallization temperature in the presence of PGA indicates a nucleating effect, illustrating how the high-Tm phase can direct the crystallization pathway in complex multiphasic systems. These results address the central hypothesis by proving that five distinct crystalline phases can be integrated and identified unambiguously within a single pentablock polymer, offering a model platform to explore crystallization/phase separation interplay and to tailor thermal/mechanical behavior in advanced materials.

A fluoroalcohol-assisted catalyst-switch strategy was developed to synthesize a pentacrystalline PE-b-PEO-b-PCL-b-PLLA-b-PGA pentablock quintopolymer, integrating polyhomologation-derived PE-OH and sequential ROP under organic-to-metal and fluoroalcohol-assisted switching. HFAB enables high GA conversion and prevents premature PGA precipitation, allowing incorporation of the high-Tm block. Multi-technique characterization (solution/solid-state NMR, SEC, DOSY, FTIR, DSC, TGA, WAXS) verifies five crystalline phases in one macromolecule. This platform advances the design of complex multicrystalline polymers and provides a robust model to study sequential crystallization and phase behavior. Future directions include in situ WAXS/SAXS at synchrotron sources to map precise crystallization/melting sequences, fast chip calorimetry to resolve Tg in fully amorphous quenched states, composition/molecular-weight studies to tune miscibility and thermal transitions, and extending HFAB-assisted PGA incorporation to diverse macromolecular architectures.

• Overlap of PEO and PCL melting endotherms complicates resolving individual transitions by conventional DSC; slow scans partially separate peaks but high-temperature analysis may be limited by thermal degradation masking PGA transitions.
• The GA conversion in the HFAB-assisted step (≈86.2%) indicates incomplete consumption under the chosen conditions; optimization may further improve conversion and block uniformity.
• One higher-Mn sample (pentablock-2) exhibited poor solubility, precluding solution-based SEC and NMR analyses; characterization relied on solid-state and diffraction methods.
• Glass transition temperatures (Tg) were not determined due to the complexity of multiphasic semicrystalline systems; specialized fast calorimetry would be required.
• Phase behavior and miscibility effects among multiple blocks (e.g., PEO/PLLA miscibility) were not fully dissected and are deferred to future studies.