Photo-selective chain end transformation of polyacrylate-iodide using cysteamine and its application to facile single-step preparation of patterned polymer brushes

Article abstract of DOI:10.1039/c8cc08157c

Cysteamine, which is an inexpensive and non-toxic aminothiol, was successfully employed as a photo-selective chain end transformation agent of iodo-terminated polymer chains (polymer-I). Polymer-I was selectively transformed to hydrogen-terminated (polymer-H) and thiol-terminated (polymer-SH) polymers with and without UV irradiation, respectively. This method is applicable to acrylate polymers. This photo-selective reaction offered a single-step preparation of patterned polymer brushes with SH and H chain end functionalities as a unique application.

Full text of DOI:10.1039/c8cc08157c

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Photo-selective chain end transformation of
polyacrylate-iodide using cysteamine and its
application to facile single-step preparation of
patterned polymer brushes†
Cite this: Chem. Commun., 2018,
54, 13738
Received 11th October 2018,
Accepted 11th November 2018
Chen Chen, Chen-Gang Wang,
Longqiang Xiao and Atsushi Goto
*
DOI: 10.1039/c8cc08157c
rsc.li/chemcomm
Cysteamine, which is an inexpensive and non-toxic aminothiol, was has been achieved through either a thermal or photo-induced
successfully employed as a photo-selective chain end transformation reduction.20,21
agent of iodo-terminated polymer chains (polymer-I). Polymer-I was
Thiol- and hydrogen-terminated polymers (polymer-SH and
selectively transformed to hydrogen-terminated (polymer-H) and thiol- polymer-H, respectively) were synthesized independently. Recently,
terminated (polymer-SH) polymers with and without UV irradiation, an elegant approach was reported on the successive conversion of
respectively. This method is applicable to acrylate polymers. This polymer-trithiocarbonate to polymer-SH and subsequently to
photo-selective reaction offered a single-step preparation of patterned polymer-H using a photo-redox catalyst and amine and phosphine
polymer brushes with SH and H chain end functionalities as a unique additives (Scheme 1a).22 Herein, we propose a simple and unique
application.
approach for synthesizing polymer-SH and polymer-H selectively
under two different external stimuli (Scheme 1b).
Selective reactions, in which different products are selectively
Our group developed an organocatalyzed LRP using alkyl
generated from the same reactant in response to the applied iodides as dormant species (X = iodide) and organic molecules
environment, are useful tools in organic and polymer syntheses. as catalysts.23–26 The post-modification of the terminal iodide
Different products are targeted upon different external stimuli such was studied via treatment with primary amines (R–NH2), yielding
as temperature, light, redox, force, and solvent polarity, enabling chain end functionalized polymers (polymer-NHR) with various
unique molecular design by simply switching the external stimuli.1–6 functionalities at the R group.27
Living radical polymerization (LRP),7–9 also known as reversible-
In the present work, we use bifunctional cysteamine
deactivation radical polymerization, is a powerful approach for (NH2CH2CH2SH) containing thiol and primary amino groups
preparing polymers with narrow molecular weight distribution as the chain end modification agents (Scheme 1c). Cysteamine
and well-defined structures. The obtained polymer (polymer-X) is biosynthesized in mammals and biocompatible. Without
possesses a capping agent (X) at the chain end. The chain end photo irradiation, the chain end iodide reacts with the amino
functionalized polymers are obtainable via the conversion of X into group of cysteamine via a substitution reaction, yielding a thiol-
functional groups.10–13
terminated polymer-SH (Scheme 1c). With the UV irradiation, the
Thiol-terminated polymers can connect with other polymers carbon–iodide bond is photo-cleaved to generate a carbon-centred
and bio-molecules to create block copolymers, stars, and bio- radical (polymerꢀ), which undergoes a radical transfer reaction with
conjugates,14–18 and can also be anchored on gold surfaces to the thiol group of cysteamine and yields a hydrogen-terminated
generate polymer brushes on the surfaces.19
polymer-H (Scheme 1c). Therefore, by switching the UV irradiation
The reactive chain end (capping agent X) of the polymer
obtained via LRP often negatively influences the long-term
stability and the post polymer processing. To address this
negative aspect, the conversion of X into a simple hydrogen
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
637371, Singapore. E-mail: agoto@ntu.edu.sg
† Electronic supplementary information (ESI) available: Experimental section,
substitution with cysteamine, spectral (NMR and MALDI-TOF-MS) data, and
images of patterned brushes. See DOI: 10.1039/c8cc08157c
Scheme 1 (a) Successive and (b) selective conversion approaches for the
synthesis of polymer-SH and polymer-H. (c) This work.
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on and off, polymer-H and polymer-SH are selectively obtained disulfide (as observed at 2.51 and 2.72 ppm in Fig. 1c). Both with
from the same reactant polymer-I (iodo-terminated polymer). and without UV irradiation, HI was generated, forming a salt
Because the substitution occurs effectively for secondary alkyl (precipitation) with cysteamine. HI changed the pH value, resulting
chains but not for tertiary alkyl chains, this method is useful for in a slight difference in the NMR chemical shift among the samples
acrylate polymers but not methacrylate polymers. This work focuses (Fig. 1).
on acrylate polymers. This approach is a selective conversion
Instead of EA-I, the bromide analogue, i.e., ethyl 2-bromo-
(Scheme 1b) that is different from the successive conversion propanoate (EA-Br), was also studied. However, EA-Br was
(Scheme 1a). The selective conversion enables a single-step pre- converted to only EA-SH but not EA-H. EA-SH was also slowly
paration of patterned polymer brushes with SH and H chain end generated (85% for 1 h). Therefore, the dual (selective) and
functionalities as a unique application described below. This rapid reactions are unique to the alkyl iodide.
approach is attractive for the use of only two reactants, i.e.,
Both of the NH2 and SH groups of cysteamine are nucleo-
polymer-I and cysteamine without extra catalysts and additives. philes and may undergo substitution with alkyl halides.28 However,
The non-toxic nature of cysteamine is further attractive for bio- in our studied conditions, we observed only the substitution with
logical applications.
NH2 (hence EA-SH as a product) (in a mechanistic study in the
To probe this reaction, we first studied a low-mass model ESI†), meaning that NH2 is a much more reactive nucleophile than
alkyl iodide, i.e., ethyl 2-iodopropanoate (EA-I (Fig. 1)), which is SH in our conditions.
a unimer model of poly(acrylate)-iodide. We studied a mixture
We then studied polymer systems. Poly(butyl acrylate)-
of EA-I (80 mM) and cysteamine (400 mM) with and without UV iodide (PBA-I) was prepared via the organocatalyzed LRP of
irradiation (365 (Æ10) nm) at room temperature. The solvent butyl acrylate (BA) with 2-iodo-2-methylpropionitrile (CP-I) as
was a mixed solvent (dielectric constant e = 6.0) of toluene-d8 an alkyl iodide initiator and tetrabutylammonium iodide (BNI)
(e = 2.4) and methanol-d4 (e = 32.7) (w/w = 88/12), in which as a catalyst (ESI†). The PBA-I was purified by reprecipitation
methanol-d4 was added to dissolve cysteamine.
and subsequently by preparative GPC to remove trace amounts
Under the dark condition (without UV) after 1 h, EA-I was virtually of impurities. The Mn and dispersity (Ð = Mw/Mn) of the purified
quantitatively (499%) converted to the substitution product EA-SH PBA-I were 2900 and 1.29, respectively, where Mn and Mw are
(Fig. 1a, b, and d). In contrast, under UV irradiation for 10 min, the number-average and weight-average molecular weights,
EA-I was mainly converted to the radical transfer product EA-H (91%) respectively.
with minor generation of the substitution product EA-SH (9%)
Subsequently, polymer-I (1 eq., 20 wt%) and cysteamine
(Fig. 1a, c, and e). (While this reaction was optimized, a higher UV (20 eq.) were dissolved in a mixed solvent (e = 12.3) of diglyme
intensity and a lower temperature may even suppress the generation (e = 7.2) and 1-butanol (e = 17.4) (w/w = 1/1). An excess of
of EA-SH.) These results clearly demonstrate high selectivity of the cysteamine (20 eq.) was used, because the reaction is slower for
reaction with and without UV irradiation.
a polymer (polymer-I) than a low-mass analogue (EA-I). A more
Mechanistically, with UV irradiation, EA-I is photo-dissociated to polar solvent was used for polymer-I (e = 12.3) than EA-I (e = 6.0)
generate EAꢀ, which subsequently abstracts a hydrogen from the to accelerate the substitution reaction. The reaction time was
SH group of cysteamine to generate EA-H and NHCH2CH2Sꢀ. Two also prolonged to 12 h under dark conditions and 2 h under UV
molecules of NHCH2CH2Sꢀ subsequently combine to form a irradiation.
Fig. 1 1H NMR (400 MHz) spectra in a mixture of toluene-d8 and methanol-d4 (w/w = 88/12). (a) Pure EA-I. (b and c) A reaction mixture of EA-I (80 mM)
and cysteamine (400 mM) (b) without UV after 1 h and (c) with UV (365 (Æ10) nm) after 10 min. (d) Pure EA-SH. (e) Pure EA-H.
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one of polymer 2 (Fig. 2c). We also observed only a single series
of repeated peaks (i.e., only one product) (Fig. 2a (middle)),
suggesting a nearly quantitative conversion of PBA-I to polymer 2.
The 1H NMR spectrum (Fig. S3b, ESI†) also confirmed the successful
attachment of the CH2CH2SH moiety.
In contrast, under UV irradiation for 2 h, we observed only
PBA-H as a single species (Fig. 2b (middle)). The experimental
mass (2781.60) (Fig. 2b (right)) matched the theoretical mass of
PBA-H (2781.81 (Fig. 2d)). The difference in the theoretical
masses of polymer 2 (2782.75 (Fig. 2c)) and PBA-H (2781.81
(Fig. 2d)) is only 0.94. Therefore, we look into the peak areas.
The theoretical area ratio of the far left and second left peaks of
PBA-H (2781.81 and 2782.81 (Fig. 2d)) is 0.571. The experi-
mental area ratio (0.605) of those peaks (2781.60 and 2782.60
Scheme 2 Detailed mechanisms for the selective reactions of PBA-I and (Fig. 2b)) was close to the theoretical one. If polymer 2 is also
cysteamine with and without UV irradiation.
generated, the experimental peak at 2782.60 should increase
because the peak of polymer 2 is overlapped. The close matching
Scheme 2 shows the detailed reaction mechanisms. Under of the experimental (0.605) and theoretical (0.571) ratios means a
dark conditions, PBA-I undergoes substitution with NH2 of nearly quantitative conversion of PBA-I to PBA-H. These results
cysteamine to generate polymer 1 (process (a) in Scheme 2), show high selectivity of the reactions with and without UV
followed by an intramolecular amidation to generate polymer 2 irradiation.
with a 5-membered ring at the chain end (process (b)). The chain
To extend the scope of polymers, we also used this method
end of polymer 2 possesses a SH group. Under UV irradiation, for other polymers, i.e., poly(2-methoxyethyl acrylate)-iodide
PBA-I was photo-dissociated to generate a polymer radical PBAꢀ (PMEA-I) (Mn = 3000 and Ð = 1.16) and carboxyl acid terminated
(process (c)), followed by a radical transfer with SH of cysteamine PMEA-I (HOOC-PMEA-I) (Mn = 3200 and Ð = 1.79) (ESI†). We
to generate a hydrogen-terminated PBA-H (process (d)).
successfully obtained the thiol-terminated polymers (PMEA-SH
Fig. 2a shows the matrix-assisted laser desorption/ionization and HOOC-PMEA-SH) and hydrogen-terminated polymers
time-of-flight mass spectrometry (MALDI-TOF-MS) spectrum of (PMEA-H and HOOC-PMEA-H) in a selective manner (Fig. S4
the product under the dark condition for 12 h. We used trans- and S5, ESI†). These results indicate good compatibility with
2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile functional groups in this approach.
(DCTB) as a matrix and CF3COONa as an additive salt for the
A unique application was the chain end functionalization of
MALDI-TOF-MS analysis. Fig. 2a (right) shows the isotope dis- polymer brushes on surfaces. Polymer brushes provide surfaces
tribution (13C distribution) in the mass region of 2780–2790. with advanced mechanical, optical, electrical, and biological
Fig. 2c shows the theoretical isotope distribution of polymer 2. properties.29 Surface-initiated LRP is a powerful method to
The far left peak (experimental mass = 2782.34) corresponds synthesize concentrated polymer brushes with high surface
to the polymer without a 13C atom (only with 12C atoms). The densities.30 Because of the steric hindrance of neighbouring
experimental mass matches the theoretical mass (2782.75). chains, the polymer chains are forced to be extended and the
The second peak from the left (experimental mass = 2783.33) growing chain ends tend to be localized at the outermost
corresponds to the polymer possessing one 13C atom, and the surface of the brush layer. The post chain end modification can
other peaks (experimental mass = 2784.32 and so on) correspond afford functionalities at the outermost surface. In the present work,
to those possessing two or more 13C atoms. The experimental we used selective reactions (not multi-step modification or successive
mass distribution (Fig. 2a (right)) well matched the theoretical reactions) and prepared patterned polymer brushes with SH and
Fig. 2 MALDI-TOF-MS spectra of products obtained via a reaction of PBA-I and cysteamine (a) without UV after 12 h and (b) with UV after 2 h. The
theoretical mass distributions of (c) polymer 2 and (d) PBA-H with n = 21.
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surfaces with inert (hydrogen-terminated) and reactive/binding
(thiol-terminated) areas may find sensing applications, e.g., for
biomolecular and ionic recognitions.
This work was supported by AcRF Tier 2 from the Ministry of
Education in Singapore (MOE2017-T2-1-018).
Conflicts of interest
There are no conflicts to declare.
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