UV-induced photolysis of polyurethanes

Article abstract of DOI:10.1039/d1cc00124h

Waste production associated with the use of non-degradable materials in packaging is a growing cause of environmental concern, with the polyurethane (PU) class being notorious for their lack of degradability. Herein, we incorporate photosensitiveortho-Nitrobenzyl units into PUs to achieve controllable photodegradability. We performed their photolysis in solution and thin films which can inform the design of degradable adhesives.

Full text of DOI:10.1039/d1cc00124h

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UV-induced photolysis of polyurethanes†
ab
Charlotte Petit, ‡a Julian Bachmann,
‡
Lukas Michalek,a Yohann Catel,c
Cite this: Chem. Commun., 2021,
57, 2911
def
Eva Blasco,
James P. Blinco, *a Andreas-N. Unterreiner *b and
ag
Christopher Barner-Kowollik
*
Received 8th January 2021,
Accepted 8th February 2021
DOI: 10.1039/d1cc00124h
rsc.li/chemcomm
Waste production associated with the use of non-degradable materials properties, PUs’ main challenge remains achieving degradability.
in packaging is a growing cause of environmental concern, with the With such extensive applicability, this gives rise to high levels of
polyurethane (PU) class being notorious for their lack of degradability. waste unable to be recycled. High temperature treatment is the
Herein, we incorporate photosensitive ortho-Nitrobenzyl units into most used technology for the waste management of PUs,11
PUs to achieve controllable photodegradability. We performed their however combustion is not only energy intensive, but also
photolysis in solution and thin films which can inform the design of generates harmful gas emissions.12 On the other hand, light
degradable adhesives.
can trigger degradation and is a powerful tool as it allows for
spatial and temporal control.
Polyurethanes (PUs) are employed in a wide variety of applica-
ortho-Nitrobenzyl (oNB) derivatives are commonly used in
tions, ranging from industrial products such as shoe soles, photochemistry and -biology owing to their capability to cleave
wheels, construction, packaging, and adhesives1,2 to drug upon light irradiation.13–17 Indeed, under ultraviolet (UV) light,
delivery as well as for in vitro and in vivo biocompatibility oNB moieties undergo a Norrish II type reaction, leading to the
studies for enzymatic, tissue and cellular responses to the formation of a nitroso compound and the release of a carboxylic
material.3–8 The Bayer chemical company greatly contributed acid.18–22 Herein, we incorporate photodegradable moieties into
to expand the types of PUs in industry, particularly with the PU polymer chains (Scheme 1). A step-growth polymerization
invention of the diisocyanate polyaddition technique, thus process with oNB-containing dialcohol and diisocyanates readily
leading to the establishment of the PU industry in 1937.9 The allows for the insertion of these photodegradable moieties into
properties of PU resulting from the reaction of polyols and every second monomer unit. After investigating the light-induced
isocyanates depend mainly on the nature of starting com- degradation of the photosensitive monomer, the polymer photo-
pounds they are made of.2 For example, long and flexible lysis was performed in diluted conditions as well as on thin
polyols usually generate soft elastic polymers, while highly films. An oNB-based dialcohol (Scheme 1, orange) was synthe-
crosslinked polymers lead to rigid and hard materials.10 tized via a 3-step procedure (Fig. 1, experimental procedures are
Despite their outstanding mechanical, physical, and chemical available in the ESI†). The first step consists of the formation of
tert-butyl 2-(4-acetyl-2-methoxyphenoxy)acetate from acetovanil-
lone, followed by a nitration step to afford the carboxylic acid 3
and a final reduction driven by BH3, allowing for the formation
a Centre for Materials Science, School of Chemistry and Physics, Queensland
University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia.
of the oNB dialcohol monomer 4 (Fig. 1). Due to the presence of
boric acid as a by-product, a purification through amberlite 743
was carried out to obtain the oNB dialcohol in high purity. The
1H and 13C NMR spectra, as well as mass spectra of the
precursors, are available in the ESI† (Fig. S4–S12). In order to
verify the oNB cleavage when exposed to near visible light (close
to 400 nm), its physicooptical properties must be assessed.
Stationary UV/Vis absorption studies of the oNB dialcohol precur-
sors were performed in acetonitrile at 25 1C (Fig. 1). The addition of
the tert-butyl moiety (Fig. 1, 1 - 2) leads to a significant increase of
absorbance in the region around 330 nm. However, both the
acetovanillone and its ester derivatives exhibit no absorbance at
E-mail: christopher.barnerkowollik@qut.edu.au, j.blinco@qut.edu.au
b Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT),
Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany. E-mail: andreas.unterreiner@kit.edu
c Ivoclar Vivadent AG, Bendererstrasse 2, 9494 Schaan, Liechtenstein
d Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT),
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
e Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany
f Center for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225,
69120 Heidelberg, Germany
g Centre for a Waste-Free World, Queensland University of Technology (QUT), 2 George
Street, Brisbane, QLD 4000, Australia. E-mail: christopher.barnerkowollik@qut.edu.au
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1cc00124h
‡ These authors contributed equally.
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Scheme 1 Overall scheme of the step-growth polymerisation of oNB dialcohol and hexamethylene diisocyanate (HDI) followed by the photolysis
performed in solution, solvent-free and on thin film. The step-growth polymerisation of an oNB-dialcohol (in orange) with hexamethyl diisocyanate (HDI,
in blue) is performed from a stoichiometric solution. The photolysis of the resulting polymers is then achieved in solution or in thin films.
SEC trace is a sum of Gaussian curves resulting from the
successive addition of monomer units (Fig. S23 and S24, ESI†).
In addition, the polymers obtained via step-growth polymeri-
zation after 2 to 20 min were analysed by SEC-ESI-MS. The
successive addition of oNB-dialcohol and hexamethyl diisocyanate
can be assessed by the increase of the m/z value of 257.09 and
168.09, corresponding to their respective exact masses divided by
the charge (Fig. S26, ESI†). Additionally, when the last monomer
added is hexamethyl diisocyanate, an amine end-group has been
observed. We assume that during the ESI† process, the reaction of
the isocyanate group with water forms an unstable carbamic acid
intermediate, which decomposes to an amine releasing carbon
Fig. 1 Ultraviolet-visible absorbance spectra of the oNB-dialcohol and its
intermediates, (1) acetovanillone, (2) tert-butyl 2-(4-acetyl-2-methoxy-
phenoxy)acetate, (3) 2-(4-acetyl-2-methoxy-5-nitrophenoxy)acetic acid,
(4) 1-(4-(2-hydroxyethoxy)-5-methoxy-2-nitrophenyl)ethan-1-ol.
350 nm or higher. Insertion of a nitro group induces an
absorbance band around 400 nm. This feature is of key impor-
tance, as the transmittance of typical plastic materials is usually
below 350 nm.23 The final reduction step (Fig. 1, 3 - 4) further
increases the absorbance. As a result, the oNB dialcohol absorbs
light mainly below 400 nm, an essential feature for degrading
coloured polymeric material. It is expected that the photolysis of
the oNB moiety occurs between 350 and 400 nm, according to
the newly introduced absorption band in this region.
To proceed with the step-growth polymerization of the
dialcohol oNB with a diisocyanate, we used dibutyl tin dilaureate
(DBTL) as a catalyst. The nucleophilic addition on the CQN
bond is eased by the polarization of the isocyanate or the
hydroxyl moieties. The addition of an equimolar amount of
hexamethylene diisocyanate (HDI) and a catalytic amount of
Fig. 2 Top: Reaction scheme of the step-growth polymerization of
DBTL led to the formation of oNB-based PU after 2 to 20 min
oNB-dialcohol with hexamethylene diisocyanate (HDI) and its photolysis.
at 70 1C via a step-growth polymerization process. The resulting
Bottom: Size Exclusion Chromatography (SEC) traces of PUs after 2 to
polymers were analysed via size exclusion chromatography
20 min reaction at 70 1C and the spectra obtained after irradiation at
(SEC), the traces obtained with UV detection at 360 nm are
displayed in Fig. 2. As expected from a step-growth process, each
365 nm in a solvated environment. Ultraviolet detection at 360 nm; the
curves were normalized to the same area.
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dioxide. As a result, the increase of m/z of 142.11 instead of 168.09
due to the presence of an amine instead of an isocyanate end-
group (Scheme S4, ESI†).
The understanding of the oNB cleavage mechanism is
essential from the perspective of a multi-component-material
used in an industrial environment. Indeed, to achieve the
effective photodegradation of an oNB-based adhesive material,
side-reactions must be avoided in favour of the photolysis
process. The photolysis of nitrobenzyl compounds studied by
electron paramagnetic resonance has been reported in the
literature.24–27 EPR spectra of many para-substituted com-
pounds have been studied and the mechanism of the splitting
patterns is well understood,24,25 but to date few examples
involving ortho-substituted compounds are described.26,27 To
the best of our knowledge, all the examples with oNB moiety
describe the photolysis in an aqueous environment.
To allow for further in-depth analysis via SEC and SEC-ESI-MS,
the polymer formed from nitrobenzyl-based monomeric unit must
be soluble in organic solvents. Even though the splitting pattern of
ortho-Nitrobenzyl relies on the position of the main substituent
group in relation to the –NO2 group on the aromatic ring, the
observed EPR spectrum is also dependant on the experimental
conditions (e.g. solvent). The light-induced degradation of the oNB
moiety in organic solvents (here CDCl3) leads to the heterolytic
Fig. 3 A Simulated and experimental electron paramagnetic resonance
(EPR) spectra of the oNB-dialcohol in CDCl3 at ambient temperature and
the proposed radical formation. B Action plot of compound 4 in acetonitrile,
indicating the conversion from 4 to 40, X, depicted in red and the yield Y in
blue, while the extinction coefficient of compound 4 as a function of
wavelength is provided in black.
cleavage of the NQO bond, thus generating free radicals on both protons a and b via 1H-NMR spectroscopy (Fig. 3B and Fig. S22,
the N and one of the O atoms. The abstraction of a proton from ESI†). The action plot of compound 4 is displayed in (Fig. 3B)
the N by the O atom induces the transfer of a free radical to the along with the evolution of its extinction coefficient as a function
carbon holding the secondary alcohol function (Fig. 3A).
of wavelength. Interestingly, the degradation product shows a
Nevertheless, radical formation represents only the first step peak at 310 nm which is not related to the absorption maximum
of the degradation process. To achieve an in-depth under- (lmax close to 350 nm). While the oNB dialcohol shows no
standing of the overall degradation of oNB dialcohol to a significant absorption around 400 nm, degradation is still taking
ketone-nitroso compound, FT-IR and 1H-NMR spectroscopy place until 425 nm, where the absorption is below the detection
were used. On the one hand, the infrared spectrum provides limit. Further, a gap between the conversion and yield was found
insight into changes of functional groups before and after which remained constant until 425 nm. Indeed, given the fact
irradiation (Fig. S16, ESI†) with the observation of a carbonyl that the formation of 40 is consistently less efficient than the
1
band after irradiation. On the other hand, H-NMR indicates a photolysis of the oNB dialcohol, the selectivity of this reaction
change in the environment of the protons on the methyl group can be estimated. Additionally, a similar kinetic study using
in a-position to the carbonyl (Fig. S15, ESI†). Both techniques LEDs ranging from 340 to 415 nm was conducted, thus allowing
confirm the pathway proposed (Fig. 3) and thus 1H-NMR can be to access the degradation performance of the compound 4 with a
used as a reliable quantitative technique for photolysis study.
more accessible method (Fig. S17–S20, ESI†).
To gain insights on the degradation efficiency of the oNB-dialcohol
Pulsed-Laser-Degradation hyphenated Size Exclusion Chromato-
and in order to determine the relation between physicooptical graphy (PLD-SEC) enables the direct control of polymers photolysis
properties and degradation efficiency, blue visible or UV light is with a precise number of photons. Although the degradation
used. A nanosecond pulsed laser with a tuneable light source efficiency of the oNB dialcohol subsides continuously throughout
allows the space and time-controlled irradiation of samples in the blue visible region (Fig. 1), an irradiation wavelength of 365 nm
solution. Tunable laser experiments with a constant photon was chosen in order to balance degradation efficiency and glass
count at specific wavelengths were carried out. Detailed descrip- absorption. In a first approach, the photolysis experiments were
tions of the setup and relevant calculations are available in the performed in solution in THF, using the polymers previously
ESI† (Page S4).28–30 Our team previously reported discrepancies synthetized (Scheme 1 and Fig. 2). As shown in Fig. 2 (purple
between photochemical reactivity and photon absorbance of dotted lines), a significant shift towards higher elution times (lower
photochemical moieties. These differences are typically captured molecular weights) is observed. We assume that the heterolytic C–O
in action plots.28 The action plot of the oNB compound 4 (Fig. 1 bond fission involving the radicals observed via EPR (Fig. 3A,
and 3B) has been investigated for a better understanding of its Scheme S3 and Fig. S21, ESI† for further details) leads to the
photolysis and to inform future reaction design. Employing formation of fragments as depicted in Fig. S25 (ESI†). Further, the
nanosecond pulsed laser irradiation, the wavelength was tuned. photolysis of oNB-based PU was performed on thin films using
Conversion, X4, and yield, Y4 , were then followed by tracking the 10 mL of a 7 mg mLꢀ1 polymer solution in dioxane or DMSO. The
0
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technical assistance at the Central Analytical Research Facility
(CARF) operated by the Institute for Future Environments (IFE).
Conflicts of interest
There are no conflicts to declare.
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