A novel photodegradable hyperbranched polymeric photoresist

Article abstract of DOI:10.1039/c3cc47048b

We report the first synthesis of a photodegradable hyperbranched polyacetal, wherein every repeat unit carries a photo-labile 2-nitrobenzyloxy moiety. The pristine HBP serves as a positive photoresist to create micron-size patterns; furthermore, by changing the terminal groups to dipropargyl acetal, clickable photo-patterned substrates can be generated.

Full text of DOI:10.1039/c3cc47048b

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A novel photodegradable hyperbranched polymeric
photoresist†
Cite this: Chem. Commun., 2013,
49, 11041
Saptarshi Chatterjee and S. Ramakrishnan*
Received 14th September 2013,
Accepted 7th October 2013
DOI: 10.1039/c3cc47048b
www.rsc.org/chemcomm
We report the first synthesis of a photodegradable hyperbranched photo-deprotection of pendant groups to modify the solubility of the
polyacetal, wherein every repeat unit carries a photo-labile 2-nitro- polymer. However, very few reports describe polymers that contain
benzyloxy moiety. The pristine HBP serves as a positive photoresist this moiety in every repeat unit of the polymer backbone; a very recent
to create micron-size patterns; furthermore, by changing the terminal report by Nazemi et al. describes the use of the nitrobenzyloxy unit for
groups to dipropargyl acetal, clickable photo-patterned substrates can the preparation of photo-labile dendrimers, but only up to the third
be generated.
generation was prepared and no photopatterning was studied.13
In the present study, we have designed a straightforward
Photodegradable polymers have been exploited for a wide range of approach to prepare a photo-degradable hyperbranched polymer
applications, such as for photo-resist formulations in the electronics that breaks down into low molecular weight organic molecules upon
industry,1 for biomedical and drug delivery applications,2 etc. Several irradiation; this creates a dramatic difference in the solubility
photo-labile moieties3 have been explored over the years; among them between the exposed and unexposed regions, and, therefore, makes
the o-nitrobenzyloxy unit has recently gained special attention, even it a potentially useful candidate for photo-patterning. Hyperbranched
though the first observation of its photosensitive nature was reported polymers (HBP), just like their structurally well-defined dendrimeric
over 40 years ago.4 Much of the early efforts were mainly confined to analogues, are highly branched structures that adopt a very compact
exploiting its use as a protecting group.3 In the recent past, however, conformation and consequently exhibit very low solution and melt
this photo-cleavage reaction has been ingeniously utilized to photo- viscosities; in addition, they provide ample scope for modification of
modify the solubility of block copolymers and consequently modulate numerous peripheral terminal groups. Therefore, the development
their aggregation behaviour;5 this change in aggregation behaviour of a HBP that can be used for photo-patterning would be of immense
has, in turn, been exploited to trigger the delivery of drugs and other value; the low viscosity would enable the preparation of very thin
occluded molecules from micelles6 and vesicles,7 and it has also been uniform films and numerous peripheral functional groups would
used to alter their LCST.8 Suitably designed linkers based on the provide an excellent opportunity for easy variation of the surface
2-nitrobenzyloxy unit have also been used as the junction in diblock chemistry by post-patterning functionalization. Importantly, HBPs
copolymers; by fine-tuning the morphology followed by photo- can be prepared in a single step unlike the photo-labile dendrimers
cleavage and selective dissolution of one of the blocks, nanoporous previously reported.13
membranes have been generated.9 Besides these studies, there have
Recently, we showed that a simple AB2 monomer carrying a
been a few reports that have used this photo-cleavage to generate hydroxyl group and a dimethyl acetal unit (A) (Scheme 1) undergoes
photopatterns.10 For example, Ober and co-workers designed a block melt polymerization in the presence of an acid catalyst via a trans-
copolymer wherein one block carries a fluorinated hydrocarbon unit acetalization process to yield a hyperbranched polyacetal; this poly-
and the other block an o-nitrobenzyl protected methacrylate; selective mer degrades under mild acidic conditions, the rate of which could
dissolution of the unexposed regions using a fluorinated solvent was be tuned by varying the hydrophobicity of the peripheral alkyl
used to generate micropatterns.11 Similarly, block copolymers have groups.14 In an effort to develop a derivative that would be photo-
also been used to generate micro-patterns, which have subsequently degradable, we designed new monomer B (Scheme 1) that carries an
been used as templates to pattern protein and cells.12 Most of these additional nitro-group ortho to the hydroxymethyl substituent; this
earlier studies, based on o-nitrobenzyloxy units, have exploited the monomer upon polymerization under similar trans-acetalization
conditions yielded a HBP that carries a photo-labile ortho-nitro-
benzyloxy moiety in every repeat unit. The monomer B was readily
prepared starting from terephthalaldehyde (see ESI†). Melt polymer-
Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore 560012, India. E-mail: raman@ipc.iisc.ernet.in
ization of the monomer was carried out in two stages in the presence
of 2 mol% p-toluenesulphonic acid; first the melt was maintained at
† Electronic supplementary information (ESI) available: Experimental details and
additional schemes and figures. See DOI: 10.1039/c3cc47048b
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statistically random growth.15 In addition, the aromatic region also
exhibits several sets of peaks, which again is reflective of the different
types of repeat units that are present in the HBP. Furthermore,
inadvertent hydrolysis of a small fraction of the terminal acetal
groups also generates an aldehyde moiety (B12%), which was
confirmed by the peak at B10.1 ppm and associated weak peaks
in the aromatic region (marked with arrows). One other interesting
feature is that the methoxy protons (peak d) appear as two singlets;
one due to the T units and the other due to L units.
The photo-degradation studies were first carried out by irradiat-
ing the polymer solution with a Hg-vapor lamp (150 watt); aliquots of
the solution after different exposure times were taken and analyzed
by GPC. From the GPC curves (Fig. S10, ESI†), it was clear that the
molecular weight, as expected, continuously decreased with irradia-
tion time. Although the postulated major final product of photo-
degradation of Nitro-HBPA would be 2-nitroso terephthalaldehyde
based on the currently accepted mechanism,16 (Scheme S4, ESI†);
experiments reveal the formation of several low molecular weight
products (Scheme S5, ESI†). FT-IR spectra of the photo-degraded
product at different stages of the degradation were also recorded; it
was evident that the intensity of peaks due to the nitro-groups at
1350 and 1530 cmÀ1 decreased with irradiation time and a new band
Scheme 1 Synthesis of the parent HB polyacetal (HBPA) and its photodegrad-
able nitro-derivatives.
100 1C under dry N2 purge for 45 min, which was followed by corresponding to the aldehyde carbonyl at around 1700 cmÀ1
1
application of reduced pressure (10 Torr) at the same temperature increased with time (Fig. S11, ESI†). Similarly, the variation of H-
for additional 30 min (see ESI†). The polymer was obtained as a light NMR spectra of a polymer solution with irradiation-time revealed the
yellow solid after purification. The molecular weight (Mw) of the formation of several aldehydic products, while prolonged irradiation
polymer was estimated to be about 11 800 (PDI of 2.9) by GPC and its of a thin polymer film yielded a slightly different set of products; this
Tg was B22 1C (Fig. S9, ESI†).
is possibly due to inadvertent hydrolysis of a photodegraded inter-
The proton NMR spectra of monomer B and Nitro-HBPA, along mediate in the solution studies (Fig. S6–S8, ESI†).
with their peak assignments, are shown in Fig. 1. Several interesting
In an effort to examine the utility of the nitro-HB polyacetal, as a
changes were noticeable when the monomer was transformed into potential photoresist material, the polymer was spin-coated on the
the polymer, the most interesting is the transformation of the single piranha-cleaned glass slide or silicon wafer; the film thickness was
methine proton (peak a) into three well-resolved peaks with an typically of the order of B100 nm, as measured by optical profilo-
approximate intensity ratio of 1 : 2 : 1 in the polymer spectrum; this metry. The photo-patterning was achieved by exposure to a 365 nm
is due to the presence of Dendritic (D), Linear (L) and Terminal (T) light source (600 mJ cmÀ2) using an EVG double-sided mask aligner;
repeat units.14 From the relative intensities of these three peaks, the typical exposure time was B5 min. The substrate was then devel-
degree of branching (DB) of the HBP was estimated to be around oped in isopropanol for 30 s to reveal the pattern. Fig. 2 shows the
51% (see ESI†), which is in accordance with the value expected for a scanning electron micrographs of the micropatterns generated using
Nitro-HBPA; it is evident that excellent pattern reproduction with
micron-level resolution is readily obtainable (see Fig. S12, ESI† for
additional SEM images).
Recently, we reported several single-step methodologies for the
preparation of peripherally clickable HB polyethers and polyesters,17
using suitably designed AB2 monomers containing either propargyl-
or allyl-bearing B groups; these polymers could be quantitatively
Fig. 1 1H-NMR spectra of the monomer, Nitro-HBPA and Nitro-HPBA-P; the
peak at B10.1 ppm (marked by an arrow) is due to inadvertent hydrolysis of the
terminal acetal groups. Expansion of the monomer spectrum reveals two closely
spaced peaks at B7.75 ppm.
Fig. 2 Scanning electron micrograph of the positive pattern obtained using
Nitro-HBPA. Exposure to 600 mJ cmÀ2 (365 nm) UV radiation and isopropanol
was used as the developer.
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monomer bearing a dipropargyl acetal, a HB photoresist carrying
numerous clickable peripheral groups was synthesized. This click-
able HB photoresist was used to create reactive micro-patterns that
could be subsequently modified interfacially by clicking an organic
azide under standard Cu-catalysed conditions, as was demon-
strated using a fluorescent azide. The concept of introducing
multiple functionality into a single hyperbranched polymer, such
as photo-lability and clickability, opens up several interesting
possibilities for the creation of micro-patterned substrates for
specific applications, such as patterning of quantum dots, metal-
nanoparticles, proteins, cells, etc.
SR would like to thank DST, New Delhi, for the award of the
J C Bose fellowship for the period 2011–2016. Part of this work
was carried out at CeNSE, IISc, Bangalore, funded by MCIT and
DST, New Delhi.
Fig. 3 Schematic representation of the process for generating a reactive photo-
patterned film using Nitro-HBPA-P and thereafter clicking with a fluorescent azide.
The fluorescence microscopic image confirms the occurrence of the click reaction.
Notes and references
derivatized using either an organic azide or an organic thiol. Based
on a similar concept, instead of the dimethylacetal in the AB2
monomer B, we designed another AB2 monomer containing a
dipropargyl acetal group as the B2 functionality (see Scheme 1 and
Scheme S2, ESI†). Polymerization of this monomer under similar
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aspects were in accordance with the structure. Since the primary
backbone of both the polymers is identical, subjecting the polymer
Nitro-HBPA-P to UV (365 nm) irradiation using a mask generated
similar micro-patterns as before (Fig. S14, ESI†); the patterned
substrate could then be subjected to surface click reaction.18 To
achieve this, the photo-patterned glass substrates were dipped
into an ethanol solution containing a catalytic amount of CuSO4,
Na-ascorbate and 4-N,N-dimethylamino-N-(2-azido-ethyl)-1,8-
naphthalimide, which is a fluorescent dye. After 12 h, the glass
slide was rinsed with ethanol and dried under a slow stream of
dry nitrogen. The fluorescence microscopic image (Fig. 3) clearly
reveals the expected photopattern confirming the occurrence of
the click reaction with the residual polymer. Additionally, the IR
spectrum of the polymer after clicking showed a significant
reduction of the alkyne C–H stretching peak at B3350 cmÀ1
(Fig. S13, ESI†), which confirms that interfacial click reaction
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In summary, we have developed a novel photodegradable
hyperbranched polyacetal by suitably designing the AB2 mono-
mer that carries an ortho-nitrobenzyloxy unit; importantly, the
polymer was readily prepared by a solvent-free melt trans-
acetalization process. The intrinsic photo-lability of this poly-
mer permitted its use directly as a photoresist in the absence of
any photo-catalyst that is often required for most photoresists.
One very interesting and useful feature of this hyperbranched
polymer-based photoresist is the presence of numerous readily
accessible terminal groups, which can be varied by suitably
modifying the B-group in the AB2 monomer; thus by using a
¨
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This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11041--11043 11043