Alcohol-initiated
ring-opening alternating copolymerization (ROAP)
of phthalic anhydride (PA) and a variety of mono-, di-, and trisubstituted
epoxides has been performed with a weak phosphazene base (t-BuP1) as the catalyst. Each product exhibits
a perfectly alternating sequence distribution, controlled molar mass
(M
n up to 124 kg mol–1), and low dispersity (Đ
M <
1.15, mostly). Full conversion of PA can be reached in 0.5–24
h depending on the substituent of the epoxide, the targeted degree
of polymerization, and the amount of t-BuP1 used (0.2–5 mol % of PA) when the reactions are conducted
under solvent-free conditions at 100 °C with a small excess of
the epoxide (0.5 equiv of PA). The glass transition temperature of
the polyester ranges from −14 to 135 °C. The living nature
of the ROAP allows one-pot construction of well-defined block-alternating
copolymers through sequential addition of two epoxides. Statistical-alternating
copolymers have also been synthesized by copolymerization of PA and
two mixed epoxides. Thus, the structural diversity of aromatic alternating
polyesters synthesized by this simple organocatalysis has been largely
enriched.
One-step synthesis of block copolymer
from mixed monomers is of
great interest and challenge. Using a simple non-nucleophilic organobase
as the catalyst, we have achieved sequence-selective terpolymerization
from a mixture of phthalic anhydride (PA), an epoxide, and rac-lactide (LA). Alcohol-initiated alternating copolymerization
of PA and epoxide occurs first and exclusively because PA is substantially
more active than LA for reacting with base-activated hydroxyl. When
PA is fully consumed, LA polymerizes from the termini of the first
block while excess epoxide stays intact because of the mild basicity
of the catalyst. The two polymerizations thus occur tandemly, both
in chemoselective manners, so that an aromatic–aliphatic block
copolyester is generated in this one-step synthesis. The effectiveness
and versatility of this approach is demonstrated by the use of ethylene
oxide and several monosubstituted epoxides as well as mono-, di-,
or tetrahydroxy initiators.
Recently, Lewis pairs composed of
organobases and organoboranes
have shown high catalytic activity for the synthesis of polyethers
by ring-opening polymerization of epoxides. Surprisingly, copolymerization
of cyclic anhydride and a large excess of epoxide we conducted at
elevated temperatures were free of polyether formation when catalyzed
by a phosphazene base and triethylborane (Et3B), even after
complete anhydride consumption. As a result, polyesters with (near)
perfect alternating sequence distribution, controlled and narrowly
distributed molar mass were readily obtained. Experimental and calculational
results attributed the unexpected chemoselectivity to the protonolysis
of Et3B and the newly formed oxygenated boron species.
Most importantly, the reversible interchange of acyloxyborane and
borinic ester at the propagating chain ends suppressed the reaction
between hydroxy species and epoxide (formation of ethers) while maintaining
the catalytic activity for the reaction between carboxy species and
epoxide (formation of esters). The polyesters containing catalyst
residues exhibited good cytocompatibility despite these changes. The
in situ structure and activity evolvement of boron species revealed
here will pave new pathways for rational design of metal-free catalysts.
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