Nucleobase-templated polymerization: Copying the chain length and polydispersity of living polymers into conjugated polymers

Full text of DOI:10.1021/ja809613n

Published on Web 03/10/2009
Nucleobase-Templated Polymerization: Copying the Chain Length and
Polydispersity of Living Polymers into Conjugated Polymers
Pik Kwan Lo and Hanadi F. Sleiman*
Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada
Received December 9, 2008; E-mail: hanadi.sleiman@mcgill.ca
Scheme 1. (a) Templated, (b) Nontemplated Polymerization and
(c) Polymerization Using Noncomplementary Template of 1aa
Nucleic acid templated polymerization is a fundamental biologi-
cal process that enables the precise copying of the sequence and
length of a DNA or RNA strand into a daughter polymer.1 Over
the past few years, nonenzymatic nucleobase-templated polymer-
ization has been examined, notably to construct DNA analogues
and peptide nucleic acids that reproduce the information of their
templates.2-9 A particularly attractive goal would be to use
nucleobase-templated polymerization to address problems in syn-
thetic polymer chemistry. For instance, polymers synthesized by
step polymerization mechanisms, and in particular conjugated
polymers, suffer from poor molecular weight control and broad
molecular weight distributions.10 By using a polymer generated by
living methods as the template strand, nucleobase-templated po-
lymerization can possibly result in copying the controlled molecular
weight and narrow polydispersity of this parent strand into a
daughter conjugated polymer. However, to our knowledge, this
approach, which would significantly enhance the properties of
conjugated polymers, has not been previously reported.
We here report the construction of a thymine-containing block
copolymer 2 using living ring-opening metathesis polymerization
(ROMP),17 resulting in a polymer with a narrow molecular weight
distribution. Aligning complementary adenine-containing monomers
1a on this parent template using hydrogen-bonding interactions,
and subsequently carrying out a Sonogashira polymerization, leads
to the templated synthesis of a conjugated polymer. Remarkably,
this daughter strand is found to possess a narrow molecular weight
distribution and a chain length nearly equivalent to that of the parent
template. This is in contrast to nontemplated polymerization, or
polymerization with an incorrect template, which gives a short
polymer with a broad molecular weight distribution. Thus, nucleo-
base-templated polymerization is a useful tool in polymer synthesis,
in this case allowing the faithful transfer of chain length and
polydispersity from parent template to a daughter conjugated
polymer strand.
a Conditions: (i) (a) CH3(CH2)3N+F-, THF; (b) Pd(PPh3)2Cl2, CuI, PPh3,
Et3N.
Initial work focused on the synthesis of monomers 1a and 1b,
containing adenine and thymine units, and solubilizing alkyl chains
(Scheme 1).18,20 Nontemplated Sonogashira coupling polymeriza-
tion of 1a or 1b was carried out in THF (Scheme 1b). Polymers
4nt and 5nt were characterized by 1H NMR, UV-vis, fluorescence
spectroscopy, and gel permeation chromatography (GPC, Table 1).20
The degree of polymerization in these polymers was low (|DP|4nt
) 9.8 and |DP|5nt ) 12.1), and their molecular weight (MW)
distributions were broad (polydispersity index (PDI) ) Mw/Mn )
2.77 for 4nt and 2.89 for 5nt), consistent with their generation via
a step polymerization mechanism.
The templated creation of adenine-conjugated polymer 4t was
then examined. For this, thymine diblock copolymer template 2
was synthesized by living ROMP, to give a polymer with a narrow
MW distribution (PDI ) 1.07) and an average |DP| of 20 for the
thymine block.17 Sonogashira polymerization of 1a was carried out
in the presence of complementary template 2 (Scheme 1a).11
Interestingly, the resulting polymer 4t now exhibits a significantly
narrower MW distribution (PDI ) 1.2), a higher MW (Mn )
14800), and degree of polymerization (|DP| ) 30.7) (Table 1).20
When templated polymerization was repeated using a slight excess
(1.5 equiv) of monomer 1a as compared to the template, a polymer
with low PDI (1.22) and an even closer chain length to the template
were observed (|DP| ) 25).12 The standard used for GPC analysis
is a relatively flexible poly(methyl methacrylate),13 which can
overestimate the MW of rigid conjugated polymers by GPC. Thus,
the |DP| ≈ 25 obtained is, within error, comparable to the |DP|
of the thymine block of template 2 of 20 units. MALDI MS
confirmed the |DP| of this polymer.20 Polymer 4t shows red-shifted
emission and absorption maxima, corresponding to π-π* transitions
along the conjugated backbone, as compared to nontemplated
polymer 4nt. 4t and 4nt have equally short fluorescence lifetimes.
Thus, this red shift is likely the result of an increase in the length
9
4182 J. AM. CHEM. SOC. 2009, 131, 4182–4183
10.1021/ja809613n CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
Table 1. GPC and Photophysical Data of Conjugated Polymers and Templates in CHCl3
a
λem
max
Mn
Mw
PDI
|DP|
λabs
max
2
18 000
18 600
4700
14 800
12 500
3200
18 700
19 900
13 000
17 700
15 300
7200
1.04
1.07
2.77
1.2
1.22
2.25
40(C4), 20(THY)
40 (C4), 20(DAP)
9.8
30.7
25.3
6.6
265
0
0
449
458
458
450
3
293
408
427
427
295
4nt
4t
4t
4it
b
a Excitation at 400 nm. b 1.5 equiv of 1a.20
of polymer 4t by the template effect, rather than aggregation and
excimer formation. To the best of our knowledge, this is the first
example of transfer of the controlled molecular weight and narrow
molecular weight distribution from a polymer generated by living
methods to a conjugated polymer and is one of very few examples
of the generation of conjugated polymers with controlled molecular
weight distribution.14-16
In a control experiment, monomer 1a was polymerized with the
noncomplementary, incorrect template 3 (Scheme 1c). The resulting
polymer 4it showed no improvement in properties, as compared to
nontemplated 4nt, with low molecular weight, high PDI, and similar
absorption/emission spectra (Table 1). Thus, the templated synthesis
of 4t occurs by selective molecular recognition of complementary
nucleobases.19 Similar improvement in chain length, PDI, and red
shifts in absorption/emission maxima were observed for the
templated polymerization of thymine monomer 1b on diamidopy-
rimidine (DAP) template 3 to generate 5t (Scheme 1).20,21
Hydrogen-bonding of monomers to templates was confirmed by
1H NMR, with downfield shifts of the NH resonances of adenine
or thymine units of 1a and 1b upon binding to complementary
polymers (e.g., 8.33 to 8.46 ppm for thymine).20 As well, we
previously showed that copolymer 3 forms spherical aggregates in
CHCl3, held together by weak DAP-DAP interactions (Figure
1a).17 Addition of monomer 1b resulted in the complete disap-
In summary, we presented a new method that uses nucleobase
recognition to read out and efficiently copy the controlled chain
length and narrow molecular weight distribution of a polymer
template generated by living polymerization, into a daughter
conjugated polymer. Nontemplated polymerization or polymeriza-
tion with the incorrect template generates a short conjugated
oligomer with a significantly broader molecular weight distribution.
This opens the door to the use of a large number of polymers
generated by living methods, such as anionic polymerization,
controlled radical polymerizations, and other mechanisms to
program the structure, length, and molecular weight distribution
of polymers normally generated by step polymerization methods
and significantly enhance their properties. Efforts to explore the
transfer of a nucleobase sequence from template to daughter strands,
to ultimately mimic DNA-templated polymerization with fully
synthetic polymers, are ongoing in our laboratory.
Acknowledgment. We thank NSERC, Xerox, CFI, CSACS, and
CIFAR. H.F.S. is a Cottrell Scholar of the Research Corporation.
P.K.L. thanks CIHR for a Chemical Biology Scholarship.
Supporting Information Available: Synthetic procedures, photo-
physical, NMR, GPC, MALDI MS, and molecular modeling. This
material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 1. Transmission electron microscopy (TEM) images of (a) template
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(19) Molecular models for the H-bond alignment of template 2 with daughter
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(20) See Supporting Information.
(21) We chose a DAP- rather than an adenine-polymer template because adenine-
polymers self-associate, rendering their adenine units inaccessible for
H-bonding.9 DAP units self-associate only weakly.
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DAP-DAP interchain interactions, and replacement by the far
stronger DAP-thymine interactions of 1b to polymer 3 (Figure
1b).17 On the other hand, noncomplementary adenine monomer 1a
does not deaggregate the micelles from 3 (Figure 1c). This is
consistent with hydrogen-bonding of template 2 and 3 with
complementary monomers, correctly aligning them on these strands
for efficient template polymerization.
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