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crack healing should be prohibited. Accordingly, polyurethane
networks containing monohydroxyl coumarin side groups as
crosslinkers were prepared in our proof-of-concept experiment [8].
Owing to reversible photodimerization and photocleavage charac-
teristics of coumarin [9e12] (Scheme 1), the crosslinked poly-
urethane has acquired repeated self-healing ability under
ultraviolet (UV) light. No detectable plastic deformation was
observed in the course of photo-remending.
structures. When products made from crosslinked version of the
polyurethane are scratched or cut, it is hoped that the cracked parts
could be re-bound through UV induced reversible photoreaction of
coumarin moieties (Scheme 3). The ether linkages offered by
DHEOMC that connect coumarin moieties with the macromole-
cules is believed to increase localized mobility of the former,
favoring photodimerization (i.e. healing reaction) of cleft coumarin
dimers in bulk polyurethane. Hereinafter, structure and properties
of IDHPEG with different molecular weights or contents of the PEG
soft segment are discussed to find out the optimal recipe.
As viewed from synthesis control of the photo-remedable poly-
urethane, however, the system with monohydroxyl coumarin deriv-
atives needs to be upgraded to prevent gelation and facilitate
structural adjustment. According to the structural requirements for
imparting photochemical reactivity to polyurethane, monohydroxyl
coumarin derivatives should play dual-role as side groups and
crosslinkers. It means that free isocyanate (NCO) groups have to be
present on the main chains of the polymer for the reaction with
monohydroxyl coumarin derivatives. As a result, tri-functional
homopolymer of hexamethylene diisocyanate (tri-HDI) that
contains three NCO groups per molecule is introduced. Ideally, mon-
ohydroxyl coumarin firstly reacts with one NCO group of tri-HDI, and
then polyethylene glycol (PEG) is incorporated for chain extension,
producingthedesiredpolyurethane.Owingtothefactthatreactivities
of the three NCO groups of tri-HDI are almost identical, synthesis of
the aforesaid intermediate from the reaction between monohydroxyl
coumarin and tri-HDI is hard to be controlled. In many cases, all the
three NCO groups of one tri-HDI molecule have reacted with mono-
hydroxyl coumarin, so that excessive unreacted tri-HDI is left. When
bifunctional PEG is added for the subsequent reaction, detrimental
gelation of the system appears. On the other hand, monohydroxyl
coumarin derivatives are attached to the hard segments of poly-
urethane (i.e. tri-HDI). Since both soft and hard segments should have
the same equivalent as required by stoichiometry, the ratio of soft and
hard segments in the polyurethane carrying monohydroxyl coumarin
can be regulated only bychangingmolecularweightofPEG, butnotby
changing content of PEG with a given molecular weight.
2. Experimental
2.1. Materials and reagents
Phloroglucinol, ethyl acetoacetate, ethyl acetate, 2-bromoethanol
(BrEtOH),1,4-dioxane, sulfuric acid(95.0e98.0%), andPEG (Mw ¼ 400
or 800 g/mol) were supplied by Alfa Aesar GmbH, Germany. Dibu-
tyltin dilaurate (DBTDL, T-12) was purchased from Sigma Aldrich Co.,
whereas isophorone diisocyanate (IPDI) was obtained from Bayer
Materials Science. All the above chemicals were used as received.
N,N-dimethylformamide (DMF) was purified by vacuum distillation
after being dried with anhydrous magnesium sulfate.
2.2. Synthesis of DHMC
DHMC was synthesized according to the route reported some-
where else [13]. Phloroglucinol (12.6 g, 0.1 mol) and ethyl acetoa-
cetate (13.0 g, 0.1 mol) were completely dissolved in 1,4-dioxane
(60 ml), and then concentrated sulfuric acid (3 ml) was dropped
in the mixture. The system was warmed up to 65 ꢀC for 1 h, cooled
down to room temperature, and poured into icy water (300 ml) to
get a yellowish precipitate. The crude product was dried in vacuum
and recrystallized twice in ethyl acetate to obtain white crystal
DHMC with a yield of 83%.
To overcome the above shortcomings, dihydroxyl coumarin
derivatives are employed as substitute for the monohydroxyl
counterparts. Similar to the case of conventional polyether diols,
polycondensation of dihydroxyl coumarin derivatives with bifunc-
tional isocyanate only gives birth to linear polyurethane. Moreover,
the ratio of soft and hard segments in the polyurethane can be
regulated by changing the ratio of dihydroxyl coumarin derivatives
and PEG, as well as by changing molecular weight of PEG. Compar-
atively, the synthesis becomes more flexible, favoring optimization
of polyurethane structure and the integrated photoreversibility.
In this work, we synthesize a dihydroxyl coumarin derivative
5,7-bis(2-hydroxyethoxy)-4-methylcoumarin (DHEOMC) by the
reaction between 5,7-dihydroxy-4-methylcoumarin (DHMC) and
2-bromoethanol. Then, a novel optical stimuli-responsive poly-
urethane (IDHPEG, meaning a condensation polymer of IPDI,
DHEOMC and PEG) is yielded (Scheme 2), which consists of iso-
phorone diisocyanate (IPDI) and DHEOMC moieties as hard
segments, while polyethylene glycol (PEG, Mw ¼ 400 or 800 g/mol)
as the soft segments. Upon UV irradiation, the photo-reversible
DHEOMC moieties are crosslinked forming cyclobutane
FTIR (KBr):
1389, 1303, 1238, 1161, 1099, 832, 761 cmꢁ1 1H NMR (300 MHz,
DMSO-d6, ): 10.47 (s, 1H, eOH), 10.25 (s, 1H, eOH), 6.23 (d,
n
¼ 3444, 3155, 2932, 2842, 1670, 1622, 1588, 1554,
d
J ¼ 2.3 Hz, 1H, Ar-H), 6.14 (d, J ¼ 2.3 Hz, 1H, Ar-H), 5.83 (d, J ¼ 1.2 Hz,
1H, C]CeH), 2.48(d, J ¼ 1.2 Hz, 3H, eCH3). 13C NMR (75 MHz,
DMSO-d6, d): 161.62, 160.64, 158.50, 157.07, 155.53, 109.49, 102.76,
99.75, 95.20, 24.27. EI-MS: 192 (Mþ). Anal. Calcd. for C10H8O4: C,
62.04; H, 4.14. Found: C, 62.50; H, 4.17.
2.3. Synthesis of DHEOMC
DHMC (0.86 g, 4.47 mmol) was dissolved in DMF (10 ml) under
continuous stirring. Potassium carbonate (6.3 g, 0.0456 mol) was
added to the mixture, and BrEtOH (1.67 g, 13.4 mmol) diluted in
DMF (5 ml) was added dropwise within 15 min. The reaction pro-
ceeded at 85 ꢀC for 18 h under Ar atmosphere and then cooled
down to room temperature. Anhydrous ethanol (20 ml) was added
to dissolve the organic layer and the inorganic salt was filtrated. By
removing the solvents, brown oily liquid was obtained. The crude
product was further purified by chromatography on a silica gel
O
350nm
O
254nm
O
O
O
or
O
O
O
O
O
O
O
Scheme 1. Reversible photodimerization and photocleavage reactions of coumarin upon irradiation with 350 and 254 nm UV light.