Self-Buffering Organocatalysis Tailoring Alternating Polyester

Abstract

A major success has been made on organocatalytic ring-opening alternating copolymerization (ROAP) of phthalic anhydride and ethylene oxide (EO) by use of a simple phosphazene base (t-BuP 1 ). Polyesters with perfectly alternating sequence distribution, controlled molar masses, and low dispersities (Đ M < 1.1) are obtained. The ROAP exhibited a distinct living nature so that block, nonlinear, end-functional structures and postpolymerization modification of the alternating polyesters are readily achieved. Solvent, excess EO, and catalyst can be effortlessly recovered and reused. The ideally suitable basicity of t-BuP 1 is considered crucial for the high selectivity and neatness of the ROAP, which engenders a self-buffering mechanism rendering the carboxy and hydroxy terminals appropriately active to allow chain growth in an strictly alternating manner and complete avoidance of side reactions (epoxide self-propagation, transesterification) even at full conversion of the anhydride.

Phosphazene/Lewis Acids as Highly Efficient Cooperative Catalyst for Synthesis of High‐Molecular‐Weight Polyesters by Ring‐Opening Alternating Copolymerization of Epoxide and Anhydride

“…This is unexpected because a complete conversion of PA is reached in this experiment so that all the chain ends are supposed to be capped with PO units. The possible reason could be acidolysis of ester groups during the termination, precipitation, or MALDI TOF measurement, which is also observed in t ‐BuP 1 catalyzed ROAC of PA and phenylated epoxide 11a…”

“…Later, Merna and coworkers extended this research to phthalic anhydride (PA) and CHO monomers using the same catalyst . On the other hand, phosphazene base t ‐BuP 1 with a relatively low basicity was used as catalyst for a living ROAC of PA with various epoxides . Recently, this catalytic system was successfully employed to prepare sequence‐controlled diblock or multiblock copolymers from mixed feedstocks of anhydride, epoxide, and lactide .…”

An Investigation of the Organoborane/Lewis Base Pairs on the Copolymerization of Propylene Oxide with Succinic Anhydride

“…The LBs were chosen to span a range of basicity (the pKa values of their conjugate acid LB−H + listed are reported in MeCN, THF, DMSO, or H 2 O, see Table S1 in the Supporting Information) [38,39]: TEEA (pKa cal = 9.03) < TEA (pKa cal = 10.62, pKa MeCN = 18.7) < DBU (pKa MeCN = 24.3) < MTBD (pKa MeCN = 25.4) < t-BuP 1 (pKa MeCN = 26.9) < t-BuP 2 (pKa MeCN = 33.5). TEA, DBU, t-BuP 1 have been previously reported to copolymerize epoxides and anhydrides, but showed low activities [26,28,40]. The equimolar combination of these LBs (TEA, TEEA, t-BuP 2 , DBU) with TEB exhibited improved activity towards the PO/SA copolymerization, and the ester contents of the resultant poly(propylene succinate)s were ≥93%.…”

“…We therefore performed the PO/SA copolymerization with the TEB/t-BuP1 molar ratio of 1/1 at 30 °C with prolonged time (84 h, entry 2, Table 2), the conversion of SA was up to 98%, and Mn of the resultant poly(propylene succinate) was high (20.4 kg/mol) ( Figure S2), which is close to the calculated value (31.0 kg/mol). The catalytic mechanism of the LA/LB pair for the copolymerization of epoxide and anhydride has been investigated [26,27,41,42]. For the PO/SA copolymerization, the LB/OB pair firstly reacts with SA, which was determined by MALDI-TOF-MS spectrum (Figure 2).…”

“…Recently, there has been rapidly growing interest in implementing epoxide-based alternating copolymerization using metal-free catalysts [22][23][24][25][26][27][28][29][30][31][32]. The competence and potential of organocatalysts have been fully verified by the achievements of strictly alternating sequence distribution and high activity.…”