Catenane-based mechanically-linked block copolymers

Article abstract of DOI:10.1039/c5cc09775d

An original strategy for the synthesis of diblock copolymers where the blocks are linked by a catenane junction is described. Starting from a functionalized catenane precursor, our strategy enables the preparation of a variety of copolymers by different techniques such as ROP, ATRP and CuAAC click reaction.

Full text of DOI:10.1039/c5cc09775d

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DOI: 10.1039/C5CC09775D
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Catenane-based Mechanically-linked Block Copolymers
B. Nisar Ahamed,a+ Roland Duchêne,a+ Koen Robeynsb and Charles-André Fustina*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An original strategy for the synthesis of diblock copolymers where influenced the properties (mechanical, crystallization, solution
the blocks are linked by a catenane junction is described. Starting behaviour…) of the polymers.6 More recently, we have
from a functionalized catenane precursor, our strategy enables demonstrated that a single catenane inserted in the middle of
the preparation of a variety of copolymers by different techniques a poly(ethylene oxide) chain is sufficient to affect the flexibility
of the latter.7
such as ROP, ATRP and CuAAC click reaction.
Rotaxane-based mechanically-linked block copolymers have
been previously reported3 but, to the best of our knowledge,
A mechanical, or topological, link is composed of separate
molecules that are linked together via their shape and spatial
organisation. This link is not covalent yet the components are
as strongly stuck together as in a covalent bond.1 The dynamic
nature of the topological bond and the possibility to control
the motions and relative positioning of the sub-components
have played a dominant role to turn this type of bond, and
particularly catenanes and rotaxanes, into smart materials and
synthetic molecular machines displaying remarkable
morphologies and properties which cannot be attained with
classical covalent systems.2
Mechanically-linked block copolymers are a new class of
macromolecules.3 Conceptually different from the well-studied
polycatenanes4 and polyrotaxanes5 where the mechanical links
are present in the repeating units, they contain a single
mechanical bond strategically located at the junction between
two polymer chains. These "articulated" block copolymers are
expected to give rise to unusual physical properties compared
to those of conventional copolymer chains. In particular,
mechanical, rheological, dynamic and self-assembly
behaviours, which critically depend on chain flexibility and
mobility, should be influenced by the presence of a defined
topological bond. In this respect we have previously shown
that a small amount (a few mole %) of a [2]catenane
incorporated into commodity thermoplastics strongly
no example of such copolymers based on a catenane junction
has been reported yet. In this contribution, we present an
original strategy in order to synthesize A-cat-B diblock
copolymers where the blocks are linked by a catenane junction
(Figure 1). The general strategy consists in synthesizing a
catenane precursor bearing appropriate functional groups to
allow the grafting of a polymer chain on each macrocycle. The
first chain is grafted onto one of the macrocycle through the
CuAAC
(Copper-catalysed
Azide-Alkyne
Cycloaddition)
reaction. This reaction was chosen because of its versatility
and efficiency even for polymer chain-ends. Moreover, many
different polymers can be easily end-functionalized with an
azide moiety. The second polymer chain can be grown directly
from an initiator function present on the other macrocycle,
either through atom transfer radical polymerization (ATRP) or
ring opening polymerization of lactones (ROP). A variety of
copolymers presenting different compositions are thus
accessible from this strategy. To illustrate this, we chose to
synthesize poly(ethylene oxide)-cat-poly(δ-valerolactone) and
CuAAC
ATRP
or ROP
=
a. Institute of Condensed Matter and Nanosciences (IMCN), Bio- and Soft Matter
division (BSMA), Université catholique de Louvain, Place Pasteur 1, 1348, Louvain-
la-Neuve (Belgium).
b. E-mail: charles-andre.fustin@uclouvain.be
c. IMCN, Molecules Solids and Reactivity division (MOST), Université catholique de
Louvain, Place Pasteur 1, 1348, Louvain-la-Neuve (Belgium).
+ Both authors contributed equally.
Electronic Supplementary Information (ESI) available: Full details on the synthesis
and characterization of all compounds. See DOI: 10.1039/x0xx00000x
)
Fig. 1 Synthetic strategy toward mechanically-linked A-cat-B block copolymers
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DOI: 10.1039/C5CC09775D
Chemical Communications
Si
Tips
O
HO
Tips
O
O
O
O
O
O
O
O
O
O
O
O
Br
O
N
Pd
N
O
O
O
Br
O
N
N
Acetic$Acid
TBAF,$THF
2nd$gen.$Grubbs$catalyst
CH Cl
NEt3
CH Cl
N
₂
₂
₂
₂
O
O
+
O
O
N
O
O
O
O
O
O
N
Pd
N
N
N
75%
85%
73%
54%
N
Pd
N
N
N
N
Pd
N
N
N
Pd
N
O
O
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
OH
OH
O
O
Br
L1
L2.Pd.CH CN
Precatenane1L1[Pd]L2
CatTIPS
Cat1
Cat2
₃
Scheme 1 Synthesis of precatenane L1[Pd]L2 and of catenanes CatTIPS, Cat1 and Cat2
poly(ethylene oxide)-cat-poly(n-butyl acrylate) copolymers as further broaden the accessible copolymer compositions, the
examples. alcohol moiety was reacted with bromoisobutyryl bromide,
The first step toward A-cat-B block copolymers is thus the yielding catenane Cat2, which can now be used as an initiator
synthesis of the catenane precursor, which relies on a for ATRP. The two synthesized catenanes Cat1 and Cat2 can
template based on a square planar palladium(II) complex. This thus be the precursors of a broad range of mechanically-linked
efficient template, already used for the synthesis of several block copolymers of varied compositions. As an illustration we
catenanes and rotaxanes,8 has many advantages: the complex synthesized PEO-cat-PVL block copolymers from Cat1 and PEO-
is neutral, allowing its easy dissolution in organic solvents, and cat-PnBA copolymers from Cat2.
is robust enough to withstand chemical reactions performed The synthesis of both copolymers starts with the grafting of an
on other parts of the molecule. The catenane synthesis follows azide functionalized PEO (Mn = 2000 gmol-1, Đ = 1.05) chain on
a 3+1 strategy developed by the group of Leigh,8a-e and the macrocycle bearing the propargyl moiety by the CuAAC
consists in assembling a tridentate ligand, pre-complexed with click reaction. The CuAAC was performed in DCM using a
a Pd2+ ion, with a monodentate ligand (Scheme 1). The first CuBr/Tris(benzyltriazolylmethyl)amine (TBTA) complex as
building block is the monodentate macrocyclic ligand L1, catalyst, and yielded quantitative functionalization for both
bearing a protected alkyne group (see Electronic Supporting catenane Cat1 and Cat2 (see ESI). Two macroinitiators, one for
Information for the full synthetic details). The second ATRP and one for ROP, were obtained in this way.
component is the tridentate pyridine 2,6-dicarboxamide ligand
L2 bearing an alcohol moiety, which is subsequently
complexed with Pd2+ to give L2.Pd.CH3CN. These two building
blocks are then assembled to give the precatenane L1[Pd]L2,
which is converted into catenane CatTIPS by ring-closing olefin PEO$Cat1
metathesis (RCM). A crystal structure of the precatenane was
O
O
O
O
N
OH
O
N
Pd
N
N
N
O
44
O
N
N
obtained by X-ray diffraction and confirmed the favourable
O
O
orientation of the two terminal alkene groups for RCM (see ESI
Fig. S1). The next step was to deprotect the alkyne moiety by
cleaving the triisopropylsilyl (TIPS) group to give catenane
Cat1. The protection of the propargyl group was necessary to
prevent its participation during the olefin metathesis step.
Early trials have indeed shown that the propargyl group reacts
during the RCM step if unprotected.
O
O
Diphenyl)phosphate
Valerolactone
CH Cl
₂
₂
O
O
PEO$cat$PVL
O
O
O
The propargyl group on catenane Cat1 enables the
attachment of a polymer chain by CuAAC click reaction,
whereas the presence of the alcohol moiety on the other
macrocycle can be used as initiator for ring opening
polymerization of lactones. The X-ray crystal structure of Cat1
(see ESI Fig. S3) showed that these two functions are
favourably orientated toward the outside of the catenane, and
should thus allow the easy grafting of polymer chains. To
H
n
N
O
O
O
N Pd
N
N
N
O
O
N
44
N
O
O
O
O
Scheme
macroinitiator.
2 Synthesis of PEO-cat-PVL copolymers from the PEO-Cat1
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1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
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PEO-OH
DOI: 10.1039/C5CC09775D
PEO-Cat2
PEO-OH
PEO-Cat1
PEO-cat-PVL
PEO-cat-PnBA
17
18
19
21
Retention Time (minute)20
17
18
19
20
21
Retention Time (minute)
Fig. 2 SEC traces showing the increase of the molar mass after each step of the
synthesis of a PEO-cat-PVL copolymer.
Fig. 3 SEC traces showing the increase of the molar mass after each step of the
synthesis of a PEO-cat-PnBA copolymer.
PEO-cat-PVL block copolymers were then prepared by the ring
opening polymerization of δ-valerolactone from the alcohol
present on the PEO-Cat1 macroinitiator (Scheme 2). We
performed ROP catalysed by the weakly acidic diphenyl
phosphate acid (DPP) since this polymerization proceeds under
mild conditions and was previously successfully used to grow
analysis revealed that the PVL block synthesized in this way
has a number-average molar mass (Mn) of 21000 gmol-1.
The second family of block copolymers, PEO-cat-PnBA, was
synthesized by ATRP of n-butylacrylate using PEO-Cat2 as
macroinitiator (Scheme 3). ATRP was performed in 2-butanone
with
a
CuBr/pentamethyldiethylenetriamine (PMDETA)
PVL from
a
sensitive pseudo-rotaxane initiator.3b-c
complex as catalyst. Polymerization for 5 h at 60°C yielded a
copolymer with a PnBA molar mass of 8960 gmol-1 as
Polymerization for 3 hours at room temperature with 2 eq. of
DPP yielded a well-defined PEO-cat-PVL copolymer (dispersity,
Đ, of 1.11). The pure copolymer was recovered by simple
precipitation in methanol. The size-exclusion chromatography
(SEC) trace of the PEO-cat-PVL mechanically-linked block
copolymer clearly shows the increase in molar mass upon ROP
compared to the starting PEO-Cat1 polymer (Figure 2). 1H NMR
1
determined by H NMR. SEC traces of PEO-Cat2 and of the
PEO-cat-PnBA mechanically-linked copolymer are shown in
Figure 3. They clearly show that well-defined copolymer (Đ =
1.11) has been obtained under these conditions.
An additional advantage of the palladium-templated
catenanes is the possibility to tune the mobility of the rings.
When palladium is present in the catenane, the rings are
essentially locked due to the metal complex involving both
rings. If the palladium is removed, the rings are free to move
as previously demonstrated.8b-d We thus removed the
palladium ion from the catenane junction of one copolymer by
O
O
O
O
O
PEO$Cat2
N
Br
O
N
N
exposing it to potassium cyanide (see ESI for details),8b-d
a
strong competing ligand, to free the rings and thus obtain a
copolymer with a flexible junction.
O
N Pd
N
O
O
44
N
N
O
O
O
In conclusion, we have described the synthesis of
functionalized catenanes as precursors toward mechanically-
linked block copolymers. The proposed strategy is highly
versatile and gives easy access to a variety of novel copolymers
by combining different techniques such as ROP, ATRP and
CuAAC click reaction. This will allow us to prepare a range of
materials with different composition, chain length, rigidity,
amphiphilic character,… and to study the influence of the
mechanical link on the properties of the block copolymers.
Moreover, the impact of different degrees of mobility of the
catenane rings (with and without palladium) on those
properties will be of particular interest.
O
CuBr/PMDETA
n-butyl2acrylate
Butanone
O
O
PEO$cat$PnBA
O
O
O
HN
Br
x
O
O
N
N
N
O
H
N
O
O
N
44
O
N
O
O
O
O
Scheme
3 Synthesis of PEO-cat-PnBA copolymers from the PEO-Cat2
macroinitiator.
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CAF is Research Associate of the FRS-FNRS. CAF thanks FRS-
FNRS for Mandat d'Impulsion Scientifique F.4506.12. RD
thanks FRIA for PhD fellowship.
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DOI: 10.1039/C5CC09775D
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