Ten different quat salts, tertiary ammonium salts and phosphazenium salts, mostly acetates and other carboxylates (succinate, citrate, betaine) were screened and found to be effective initiators for the polymerization of β-propiolactone to poly-3-hydroxypropionate.
Tetramethylammonium acetate (TMA OAc) showed greater control over the molar mass compared to longer chains such as tetrabutylammonium. TMA OAc also showed full beta-lactone conversion in less than 6 hours in both THF and MTBE as solvents. This was achieved with molar ratios of beta-lactone to TMA OAc of 500:1 and even 2,000:1.
Beta-propiolactone is a highly reactive chemical and reactions are performed in solvent to mitigate potential intense exotherms from the enthalpy of ring opening of the 4-membered ring ester monomer.
It is thought that acetate mitigates side reactions and this is evidenced by huge differences in molar mass when using DBUH OAc versus DBU at high monomer to initiator levels.
This polymerization in THF is likely not phase-transfer catalysis since TMA OAc is likely soluble in the THF-monomer mixture. It is not clear if TMA OAc is soluble in the MTBE-monomer system. The poly-3-hydroxypropionate polymer precipitates from the reaction solution, so the system is heterogeneous.
About Marc Halpern
Dr. Halpern is founder and president of PTC Organics, Inc., the only company dedicated exclusively to developing low-cost high-performance green chemistry processes for the manufacture of organic chemicals using Phase Transfer Catalysis. Dr. Halpern has innovated PTC breakthroughs for pharmaceuticals, agrochemicals, petrochemicals, monomers, polymers, flavors & fragrances, dyes & pigments and solvents. Dr. Halpern has provided PTC services on-site at more than 260 industrial process R&D departments in 37 countries and has helped chemical companies save > $200 million. Dr. Halpern co-authored five books including the best-selling “Phase-Transfer Catalysis: Fundamentals, Applications and Industrial Perspectives” and has presented the 2-day course “Practical Phase-Transfer Catalysis” at 50 locations in the US, Europe and Asia.
Dr. Halpern founded the journal “Industrial Phase-Transfer Catalysis” and “The PTC Tip of the Month” enjoyed by 2,100 qualified subscribers, now beyond 130 issues. In 2014, Dr. Halpern is celebrating his 30th year in the chemical industry, including serving as a process chemist at Dow Chemical, a supervisor of process chemistry at ICI, Director of R&D at Sybron Chemicals and founder and president of PTC Organics Inc. (15 years) and PTC Communications Inc. (20 years). Dr. Halpern also co-founded PTC Interface Inc. in 1989 and PTC Value Recovery Inc. in 1999. His academic breakthroughs include the PTC pKa Guidelines, the q-value for quat accessibility and he has achieved industrial PTC breakthroughs for a dozen strong base reactions as well as esterifications, transesterifications, epoxidations and chloromethylations plus contributed to more than 100 other industrial PTC process development projects.
Dr. Halpern has dedicated his adult life to his family and to phase-transfer catalysis (in that order!).
