Rigid chain ribbon-like metallopolymers

Article abstract of DOI:10.1002/pola.27255

A set of new copolymers is here reported in which the repeating units are connected each other through Cu(II) metal centers. The coordination link is based on the bis-chelating properties of salicylaldiminate groups of two different monomers. Due to their chemical structure, the two monomers afford, respectively, flexible and rigid repeating units in the metallocopolymers constitution upon coordination to copper centers. All the copolymers were soluble and easily processable. As shown by XRD analysis, rigid units' rich copolymers adopt a ribbon-like structure in solid state in which highly planar strands of polymer stack thanks to π-π interactions, similarly to the polymer composed exclusively by rigid units. This behavior can be justified assuming the existence of a partial block character in copolymer constitution where long sequences of rigid units are alternated to sequences of flexible units. This assumption is supported also by DSC and UV-Vis analysis.

Full text of DOI:10.1002/pola.27255

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Rigid Chain Ribbon-like Metallopolymers
1
1
2
1
Antonio Carella, Fabio Borbone, Giuseppina Roviello, Ugo Caruso,
2
2
1
3
Claudio Ferone, Laura Ricciotti, Beniamino Pirozzi, Paola Persico,
3
1
Alberto Giacometti Schieroni, Antonio Roviello
1
Chemical Sciences Department, University of Naples Federico II, Complesso Universitario Monte
Sant’Angelo via Cintia snc, Naples 80126, Italy
2
Department of Technology, University Parthenope of Naples, Faculty of Engineering, Centro Direzionale Isola C4,
Naples 80143, Italy
3
Institute for Macromolecular Studies (ISMAC), National Research Council of Italy, via Bassini 15, Milan 20133, Italy
Correspondence to: A. Carella (E-mail: antonio.carella@unina.it)
Received 9 March 2014; accepted 10 May 2014; published online 00 Month 2014
DOI: 10.1002/pola.27255
ABSTRACT: A set of new copolymers is here reported in which
the repeating units are connected each other through Cu(II)
metal centers. The coordination link is based on the bis-
chelating properties of salicylaldiminate groups of two differ-
ent monomers. Due to their chemical structure, the two mono-
mers afford, respectively, flexible and rigid repeating units in
the metallocopolymers constitution upon coordination to
copper centers. All the copolymers were soluble and easily
processable. As shown by XRD analysis, rigid units’ rich
copolymers adopt a ribbon-like structure in solid state in which
highly planar strands of polymer stack thanks to p2p
interactions, similarly to the polymer composed exclusively by
rigid units. This behavior can be justified assuming the exis-
tence of a partial block character in copolymer constitution
where long sequences of rigid units are alternated to sequen-
ces of flexible units. This assumption is supported also by DSC
and UV–vis analysis. V
2014 Wiley Periodicals, Inc. J. Polym.
C
Sci., Part A: Polym. Chem. 2014, 00, 000–000
KEYWORDS: copolymerization; differential scanning calorimetry;
metal-polymer complexes; UV–vis spectroscopy; X-ray
INTRODUCTION The search for novel organic materials able
to replace traditional inorganic ones in different high tech
applications is a general trend observed in the last decades.
A huge number of studies has been published in this period
which permitted the obtainment of materials with excellent
performance in different fields of electronics such as photo-
affect optical, electronic, and magnetic properties of the poly-
1
9,20
mers.
As already proposed, this kind of systems can be
considered the ultimate organic-inorganic hybrids, in which
2
1
the two components are mixed at a molecular level. They
have been deeply investigated for what concerns their elec-
2
2,23
trical properties (for application in photovoltaics
and
their
2
4,25
26,27
1
–4
5–11
sensors
), their electroluminescence activity,
voltaics,
field effect transistors,
and photonics, as for
by giving a better
2
8–30
31
1
2–16
NLO properties
and liquid crystalline behavior.
instance nonlinear optics (NLO),
understanding of the physical mechanism underlying the
activity of organic based materials. This interest stems first
of all from the possibility of processing organic materials
through solution techniques (spin-coating, inkjet printing, or
Regarding the nature of the organic ligands, Schiff-bases have
been widely investigated in the literature for the preparation
of interesting metallo type assemblies, mainly because of their
32–35
versatility, structural variety, and ease of preparation.
1
7,18
roll-to-roll techniques),
deeply reducing in this way the
cost of the device fabrication. The mechanical properties fea-
tured by organic materials, such as lightness and flexibility,
open the way to new application possibilities not affordable
for the heavy and rigid traditional inorganic ones. More
recently, a growing interest has been observed for metal con-
taining polymers with regard to their potential use in mate-
rial science: the presence of metal centers may significantly
In previous articles, the bis-chelating P12-8N ligand (whose
structure is reported in Chart 1) was used in the preparation
3
6,37
of linear metallopolymers.
P12-8N is characterized by
two salicylaldiminate end groups separated by a flexible
chain. Upon coordination with a metal a linear metallopoly-
mer could be obtained, characterized by liquid crystalline
properties due to the alternation, along the chain, of
Additional Supporting Information may be found in the online version of this article.
2014 Wiley Periodicals, Inc.
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CHART 1 Schematic representation of the two type of repeating units and of the way they are linked through copper metal cen-
ters; in copolymer constitution drawing x represents the molar fraction of the rigid unit.
mesogenic aromatic groups with flexible groups. A new bis-
chelating salicylaldiminate type ligand, DHTI and metallopol-
ymers based on it, are reported in this article. This ligand,
DHTI, is a diimine derivative of 2,5-dihydroxyterephthal-
deyde and is characterized by two salicylaldiminate func-
tional groups sitting on the same aromatic ring (see
Chart 1). In particular, a set of novel polymeric materials has
been prepared, constituted by two different repeating units,
deriving from P12-8N and DHTI respectively, and linked
together by copper (II) metal centers, in a main chain cova-
lent approach.
The prepared copolymers are hence characterized by flexible
repeating units (deriving from P12-8N) and rigid units
(deriving from DHTI). The insertion of DHTI type repeating
unit was thought to increase the overall planarity of the
polymeric systems, a desired feature for applications in fields
like organic electronics or spintronics. It is worth to point
out, in fact, that in the polymer containing exclusively rigid
units, the metal based linkage is expected to induce a highly
planar conformation of the polymer backbone as a conse-
quence of the square planar coordination of the ligands
around the copper centers (look for instance at the parent
CHART 2 Synthetic pathway for polymers and copolymers preparation.
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3
8
low mass compound reported by Carotenuto et al. where
the salicyl aldiminate group adopt a planar conformation
around the copper center). We prepared a set of metal
copolymers characterized by different molar ratio between
flexible and rigid units with the aim of modulating the proc-
essability of the materials. Also the two homopolymers based
exclusively on P12-8N ligand (already known in the litera-
ture but here presented with a slight modification in the
synthesis) or on DHTI, were prepared. All the synthesized
polymers and copolymers were carefully characterized for
what concerns their thermal, structural, and optical proper-
ties; as discussed in the following sections, the results
emerging from this combined analysis suggest a peculiar
partial block character in copolymers chemical constitution.
formed and the suspension was kept boiling for 5 min. Then
the precipitate was collected by suction filtration, solved in
chloroform (50 mL) and poured dropwise in 95% ethanol
(150 mL) containing 0.200 g of sodium acetate. The formed
solid was collected by suction filtration and washed twice
with 50 mL of boiling ethanol; 0.738 g of a violet powder
were recovered. Yield: 83%. IR (KBr, cm ): 2922 (aromatic
CAH stretching), 2852 (aliphatic CAH stretching), and 1610
(C@N stretching).
2
1
Synthesis of P0-Cu
P12-8N (0.480 g, 0.53 mmol) was solved upon heating in
30 mL of ethanol. After 10 min at gentle boiling, an ethanol
solution of Cu(CH COO) ꢀH O (0,116 g in 10 mL of solvent,
3
2
2
1
0% excess) was poured in this solution, followed by the
addition of an ethanol solution of NaCH COO (0.150 mg in
0 mL of 95% Ethanol). A light green precipitate formed
3
EXPERIMENTAL
1
and the suspension was kept boiling for 5 min. Then the pre-
cipitate was collected by suction filtration, solved in 50 mL
of chloroform, and poured dropwise in 95% ethanol
(150 mL) containing 0.200 g of sodium acetate. The formed
solid was collected by suction filtration and washed twice
with 50 mL of boiling ethanol; 0.399 g of a fibrous light
Synthesis
All the reagents and solvents were purchased from Sigma
Aldrich and Carlo Erba Reagenti and used without further
purification. P12-8N and 2,5-dimethoxyterephthaldeyde
were synthesized according to procedures already reported
in the literature. Two homopolymers and a set of copolymers
were prepared, starting from two different bis-chelating
monomers, one flexible (P12-8N) and one rigid (DHTI). The
general name attributed to the prepared polymers is Pn-Cu
3
1
39
2
1
green material were recovered. Yield: 78%. IR (KBr, cm ):
925 (aromatic CAH stretching), 2852 (aliphatic CAH
2
stretching), 1722 (C@O stretching), 1619 (C@N stretching),
and 1255 (esteric CAO stretching).
(
see Chart 2), where n is a number indicating the percent
molar fraction of the rigid unit in the copolymer constitution.
We report here in details the synthetic procedure of the two
homopolymers, P0-Cu and P100-Cu. We also report the
details of the preparation of P50-Cu, as an example of
the copolymers synthesis; the procedure is the same for the
other copolymers of the series except that, according to the
desired composition, a different relative amount of rigid and
flexible monomers was used.
Synthesis of P50-Cu
2
,5-Dihydroxyterephthaldeyde (0.091 g, 0.55 mmol) was
solved upon heating in ethanol (20 mL); to this solution,
undecylamine (0.094 mg, 0.55 mmol) and 3-
butoxypropylamine (0.072 g, 0.55 mmol) were added. This
solution turned from yellow to orange and was then poured
in a boiling ethanol solution of P12-8N (0.498 g, 0.55 mmol,
in 20 mL of ethanol). After 10 min at gentle boiling, an etha-
nol solution of Cu(CH COO) ꢀH O (0.242 g in 10 mL of sol-
3
2
2
Synthesis of 2,5-Dihydroxyterephthaldeyde
vent, 10% excess) was poured in the first one, followed by
2,5-Dimethoxyterephtahldeyde (5.00 g, 22.1 mmol) were
the addition of an ethanol solution of NaCH COO (0.300 mg
3
refluxed in a solution composed of 48% HBr(aq) (200 mL)
and glacial acetic acid (250 mL). After 20 h, the system was
cooled to room temperature and poured in 400 mL of water.
A precipitate formed, that was collected by suction filtration
and purified by means of soxhlet extraction with benzene.
Upon cooling, the desired product crystallized in benzene
in 10 mL of 95% Ethanol). A violet precipitate formed and
the suspension was kept boiling for 5 min. Then the precipi-
tate was collected by suction filtration, solved in 50 mL of
chloroform, and poured dropwise in 95% ethanol (150 mL)
containing 0.200 g of sodium acetate. The formed solid was
collected by suction filtration and washed twice with 50 mL
of boiling ethanol; 0.337 g of a violet material were recovered.
and was then collected as gold crystals by filtration. Yield:
1
5
9%. H NMR (200 MHz, CDCl , d, ppm): 7.25 (d, 2H, J 5 4.0
3
21
Yield: 84%. IR (KBr, cm ): 2923 (aromatic CAH stretching),
Hz), 9.96 (s, 1H), 10.23 (s, 1H).
2
1
852 (aliphatic CAH stretching), 1729 (C@O stretching),
606 (C@N stretching), and 1254 (esteric CAO stretching).
Synthesis of P100-Cu
2,5-Dihydroxyterephthaldeyde (0.300 g, 1.80 mmol) was
Chemical Physical Characterization
solved upon heating in ethanol (30 mL); to this solution,
undecylamine (0.308 mg, 1.80 mmol) and 3-butoxypropyl-
amine (0.236 g, 1.80 mmol) were added. The solution turned
from yellow to orange. After 10 min at gentle boiling, an
All compounds and polymers were characterized by standard
techniques. DSC analyses were performed in dry ultrapure
nitrogen flow using a DSC7 PerkinElmer apparatus at a scan
rate of 10 K/min; DSC temperature calibration was made
using a Fluka indium sample having 99.999% purity.
ethanol solution of Cu(CH
3
COO)
2
ꢀH
2
O (0,396 g in 20 mL of
solvent, 10% excess) was poured in this solution, followed
by the addition of an ethanol solution of NaCH COO
X-ray diffraction analysis was carried out by the photo-
3
(
0.400 mg in 20 mL of 95% Ethanol). A dark precipitate
graphic method on a flat film camera, using CuKa Ni-filtered
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radiation. Phase transitions were observed with a Zeiss Axio-
scope polarizing microscope equipped with a Mettler micro-
furnace and an FP5 Mettler controller. The thermal stability
of the polymers was monitored by thermogravimetric analy-
ꢁ
21
sis (TGA) using a TA5000 apparatus at a 20 C min scan
rate in air atmosphere.
FTIR spectra were recorded with a JASCO FT/IR 430 spec-
trometer on KBr pellets; solution and thin film UV–vis spec-
tra were recorded on a spectrophotometer Jasco F-530 (scan
2
1
rate 200 nm min ). Thin film of polymers were prepared
2
1
by spin coating of a chloroform solution (10 mg mL ) on
quartz substrates, using a Laurell WS-650Mz-23NPP spin
processor.
The characterization of the “apparent” molecular weight dis-
tribution (MWD) of samples was performed by using a mul-
tiangle laser light scattering (MALS) absolute detector on-
line to a size exclusion chromatographic (SEC) system. The
SEC system consisted of an Alliance 2695 separation module
from Waters equipped with two on-line detectors: a MALS
Dawn DSP-F photometer from Wyatt Technology and a 2414
differential refractometer from Waters used as concentration
detector. Specifically, we define the obtained MWD
FIGURE 1 FTIR spectra of P0-Cu, P50-Cu, and P100-Cu; the
spectra are vertically displaced to allow a better comparison.
was obtained from demethylation reaction of 2,5-dimethoxy-
terefthaldeyde; DHTI was not isolated and directly used for
the next polymerization reaction step. The use of two differ-
ent amines was thought to increase the solubility of the
resulting polymers. The general procedure for polymers
preparation is reported in Chart 2: an ethanol solution of
DHTI prepared in situ was mixed with an ethanol solution of
P12-8N and kept at gentle boiling; then copper acetate in
ethanol was added, followed by sodium acetate.
“apparent” because the absolute MALS detector measure the
true molecular weight (M ) of the aggregate structures not
w
of the single separated molecules. The experimental method-
ology for a reliable use of the SEC-MALS system has been
4
0,41
described in literature.
The SEC-MALS experimental con-
The polymers so obtained immediately precipitated in the
reaction media and were then filtered and purified by dissolv-
ing in chloroform and re-precipitating in ethanol twice. All the
obtained polymers were soluble in chlorinated solvents.
ditions were the following: 50% Chloroform 1 50% Dichloro-
methane as mobile phase, 2 Styragel columns (500 and 100
ꢁ
21
Å) from Waters, 35 C of temperature, 0.6 mL min of flow
21
rate, 100 lL of injection volume, about 2 mg mL of sample
concentration.
FTIR analysis on KBr pellets were recorded on all the syn-
thesized metallo polymers. In Figure 1, the FTIR spectra of
the two homopolymers P0-Cu and P100-Cu, along with that
of P50-Cu, are reported. In all the presented spectra, it is
possible to identify the signals distinguishing the CAH
The inherent viscosities of the polymer solutions were meas-
ꢁ
ured in chloroform at 25.0 C at a concentration of 0.500 g
21
1
dL using an Ubbelohde viscometer. H NMR spectra were
recorded on a Varian Gemini XL 200-MHz apparatus.
2
1
stretching (at around 2920 and 2850 cm ) and the imine
2
1
C@N stretching (at around 1610 cm ); in the spectra of P0-
RESULTS AND DISCUSSION
Cu and P50-Cu, moreover, other two significant signals are
21
observed: in particular, carbonyl stretching at 1720 cm
The synthesized polymers and copolymers are constituted
by two different repeating units, P12-8N and DHTI, linked
together through copper(II) metal centers, as schematically
sketched in Chart 1. P12-8NCu was previously prepared by
direct condensation of bis[N-[2,4-dihydroxyphenyl]-octyli-
21
and ester CAO stretching at 1255 cm , associable to the
flexible repeating units deriving from P12-8N, appear, while
they are absent in the spectra of the fully rigid P100-Cu.
To assess their purity, the prepared polymers were investi-
gated by means of TGA: the samples were heated in air flow
0
mino copper (II) with 4,4 -(dodecane-1,12-diylbis(oxy))di-
3
6
benzoyl- chloride.
The so obtained polymer can be
ꢁ
up to a temperature of 700 C, at which the polymers are
depolymerized in a HCl /CHCl mixture to give P8–12N
aq
3
transformed in copper (II) oxide.
3
1
with high purity. In the present work, the so obtained
highly pure P8–12N was used for the synthesis of P0-Cu
As shown in Table 1, the measured residues are in good
accord with the expected theoretical values, thus confirming
the validity of the used synthetic methodology. The thermog-
ravimetric graphs for all polymers are reported in the Sup-
porting Information (Fig. S1).
(
and of the other copolymers) by reaction with Cu(OAc) as
2
described in Experimental Section.
DHTI was the monomer affording the rigid repeating unit in
copolymers and has been formed in situ by adding two dif-
ferent amines, 1-undecylamine and 3-butoxypropylamine, in
a 1:1 molar ratio, to 2,5-dihydroxyterephtaldeyde. The latter
The thermal stability of the prepared polymers depends upon
composition and in particular increases with the increase of
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TABLE 1 Thermal and Molecular Properties of the Synthesized Polymers
%
CuO
%CuO
T
d
ꢁ
g
inh
M
w
T
g
ꢁ
T
m
ꢁ
T
i
ꢁ
T
r
ꢁ
a
b
21 c
)
21 d
)
d
e
e
e
Polymer
calc.
exp.
( C)
(dL g
(kg mol
M /M
w n
( C)
( C)
( C)
( C)
P0-Cu
8.2%
8.2%
300
300
300
289
281
266
252
252
252
260
0.43
0.43
0.48
0.51
0.57
0.59
0.72
0.84
1.03
–
55.7
64.3
87.4
106.7
176.4
300.2
294.3
308.9
521.0
–
2.3
2.4
2.4
2.0
3.0
3.1
3.1
3.1
2.3
–
48
47
47
43
–
167
–
202.0
119
–
P20-Cu
P30-Cu
P40-Cu
P50-Cu
P60-Cu
P75-Cu
P87-Cu
P100-Cu
P’100-Cu
9.1%
9.3%
191.0
9.6%
9.9%
–
188.8
–
10.2%
10.9%
11.6%
13.0%
14.3%
16.1%
18.8%
10.4%
11.2%
11.8%
12.8%
14.1%
16.0%
18.5%
–
182.6
198
210
218
225
225
–
–
–
–
–
–
–
–
–
–
–
–
–
f
–
–
250
g
–
–
–
a
f
TGA run was performed in air atmosphere at a scan rate of 10 K
For P100-Cu, the recrystallization process overlaps in the last part with
polymer decomposition.
P’100 was synthesized with the same synthetic procedure used for
21
min
b
.
g
Determined as the temperature corresponding to a weight loss of 5%.
c
ꢁ
Inherent viscosity, determined by a Ubbelhode viscosimeter at 25
C
P100-Cu,
with
the
difference
that
1-octylamine
and
2-
in chloroform solution at a concentration of 0.500 g/dL.
propyloxyethylamine were used as amines in the formation of the
diimine.
d
Measured by SEC-MALS.
e
Determined from DSC experiment performed in nitrogen atmosphere
21
at a 10 K min scan rate.
ꢁ
flexible units molar fraction. It ranges from 252 C, decompo-
sition temperature of P100-Cu, P87-Cu, and P75-Cu, to 300
C, for P0-Cu, P20-Cu, and P30-Cu (see Table 1).
nematic liquid crystalline phase upon melting. Two endo-
thermic peaks are in fact observed in the DSC trace of P0-
ꢁ
ꢁ
Cu, one related to the melting of the polymer (T 5 167 C)
m
and the other to the isotropization of the liquid crystal phase
The apparent whole MWD, the weight average of the Mw)
and the polydispersity index (M /M where M denotes the
ꢁ
ꢁ
(T 5 200 C). An exothermic process occurs at about 120 C,
i
w
n
n
assignable to crystallization of amorphous zones of the
polymer.
numeric average) of the prepared polymers and copolymers
were determined by using the absolute on-line SEC-MALS
method. The inherent viscosity were measured off-line
The phase behavior of P0-Cu was moreover observed
through optical microscope analysis carried out in tempera-
ture and confirmed the interpretation given to DSC results.
ꢁ
through a Ubbelhode viscometer at 25 C of temperature in
w
chloroform solvent. Very interestingly, the M and inherent
ꢁ
viscosity values reported in Table 1 are consistent between
them. Figure 2 shows the comparison of the differential
whole MWD of all prepared polymers and copolymers.
Upon melting at 167 C P0-Cu afforded a nematic liquid
crystal phase characterized by the typical Schlieren textures,
which were preserved when the polymer was rapidly cooled
down to room temperature; this liquid crystalline phase is
The analysis of the molecular results (see Table 1 and Fig. 2)
clearly shows that, on average, a rise of the apparent Mw
and of inherent viscosity occurs as the molar percentage of
rigid units in the copolymeric systems increases. This behav-
ior is not surprising: in P100-Cu, the polymer backbone con-
formation is highly planar because of the coordination
geometry of the ligand DHTI on metal copper centers. In
copolymers characterized by large molar fraction of rigid
units, it can be expected that large segments of polymer
backbone adopt a highly planar conformation thus favoring
aggregation phenomena that result eventually in a inherent
viscosity and apparent Mw increase.
The thermal behavior of the synthesized polymers was
checked by DSC analysis and the results are summarized in
Table 1. The DSC thermograms of the homopolymers P0-Cu
and P100-Cu are reported in Figure 3(a). Polymer P0-Cu
FIGURE 2 Comparison of differential MWD of polymers and
copolymers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
ꢁ
shows a glass transition temperature (T ) value of 48 C. As
g
already known from the literature, P0-Cu gives rise to a
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solution of this polymer afforded instead
a liotropic
mesophase as deduced from the observation of shear bands
on films obtained from shearing and fast evaporation of the
solvent.
For what concerns the copolymers series, depending on their
composition, they feature a behavior typical of P0-Cu or
P100-Cu. Up to a rigid units percentage of 40%, copolymers
show a glass transition occurring at temperatures similar to
the T of P0-Cu. We assume the presence of long sequences
g
of flexible units in these copolymers that gives rise to ther-
mal behavior similar to that of the fully flexible homopoly-
mer P0-Cu. This assumption is also validated by the
ꢁ
observation of an endothermic peak at around 190 C in the
DSC analysis [see Fig. 3(b)], corresponding roughly to the
isotropization temperature of P0-Cu: hence, the long sequen-
ces of flexible units in P20-Cu, P30-Cu, and P40-Cu do not
crystallize but adopt the liquid crystalline arrangement typi-
ꢁ
cal of P0-Cu and isotropize at 190 C.
The thermal behavior of copolymers with lower content of
rigid units was then observed at the optical microscope.
ꢁ
P20-Cu was completely fluid at 170 C and shear bands, typ-
ical of a nematic phase, could be induced (see Fig. 4).
Beyond the isotropization temperature determined from
DSC, all the sample became isotropic. Similar behavior was
shown by P30-Cu even if, after the isotropization tempera-
ture, a slightly anisotropic field was observed. For P40-Cu, it
was not possible to induce shear bands and largely aniso-
tropic zones were observed above the Ti.
As far as the copolymers with higher content of rigid units
are concerned, also in this case, it is possible to interpret
their thermal behavior of these copolymers by assuming the
presence of long sequences of all flexible units and of all
rigid units.
The glass transition associable to the flexible units, is no
more observed, probably because the rigid units domains act
as physical crosslinks that hinder the chain mobility: we
FIGURE 3 DSC traces for all the synthesized polymers.
stable up to ꢂ200 ꢁC above which the melt sample turns
isotropic. By shearing the liquid crystal phase, a well-defined
band structure was produced.
No glass transition or melting is instead observed for the
rigid P100-Cu. The thermogram of P100-Cu, shown in
Figure 3(a), is characterized by an exothermic phenomenon,
associable to the crystallization of amorphous phase of the
ꢁ
polymer, having its onset around 200 C and reaching its
ꢁ
maximum at around 250 C; considering the fact that this
ꢁ
polymer has a T of 255 C, as determined by TGA, it can be
d
assumed the recrystallization process overlaps, in the last
part, with the decomposition process of the polymer. When
analyzed at the optical microscope, no phase transitions
could be observed for P100-Cu; a concentrated chloroform
FIGURE 4 Optical microscope photograph for P20-Cu: shear
bands induced on the melt polymers and preserved ad room
temperature.
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TABLE 2 d Spacing of the Synthesized Polymers Determined by XRD Analysis
˚
˚
˚
˚
˚
˚
2
h/d
1
(A)
2h/d
2
(A)
2h/d
3
(A)
2h/d
4
(A)
2h/d
5
(A)
2h/d
6
(A)
a
b
b
b
b
c
c
c
c
c
d
P0-Cu
4.79/18.46
4.79/20.23
4.31/20.50
4.31/20.50
4.31/20.50
4.31/20.50
4.31/20.50
4.31/20.50
4.31/20.50
5.26/16.79
–
–
–
–
20.65/4.30
20.29/4.39
20.65/4.30
20.65/4.30
20.23/4.39
18.74/4.73
18.52/4.79
18.52/4.79
18.73/4.73
18.52/4.79
–
–
–
–
–
–
–
–
–
a
a
a
a
a
e
e
P20-Cu
P30-Cu
P40-Cu
P50-Cu
P60-Cu
P75-Cu
P87-Cu
d
d
d
d
d
d
9.04/9.78
9.04/9.78
13.20/6.71
13.19/6.70
13.20/6.71
12.97/6.82
12.51/7.07
12.51/7.07
12.97/6.82
13.42/6.59
d
23.93/3.72
24.33/3.66
23.93/3.72
23.73/3.75
23.93/3.72
23.53/3.78
23.53/3.78
d
d
8.57/10.31
8.57/10.31
8.57/10.31
8.57/10.31
8.57/10.31
10.21/8.66
32.13/2.78
32.13/2.78
32.13/2.78
32.13/2.78
e
P100-Cu
e
P’100-Cu
a
ꢁ
d
As a fiber, annealed at 100 C for 1 h.
Four spots on the first layer.
Strongly polarized line.
b
c
e
ꢁ
Cold pressed powder, annealed at 220 C for 1 h.
Strongly equatorially polarized halo.
observed instead the exothermic phenomenon typical of P100-
Cu and associable to recrystallization of rigid units rich amor-
phous zones (this phenomenon was actually already observed
for P40-Cu, just after the isotropization) and it is shown in Fig-
ure 3(c). The recrystallization temperature depends upon the
composition and is lower for lower content of rigid units. This
is expected, because copolymers with higher content of flexible
units experience a higher mobility at a given temperature and
thus structuration processes are favored.
20.5 Å, d , can be associated to the aliphatic side chain of
the polymer backbone (and hence, to the width of the rib-
bon), admitting their intercalation or a tilting respect to the
1
The simultaneous DSC-optical microscope analysis suggests
that the synthesized copolymers have a partial block charac-
ter in their constitution: phase behavior typical of P0-Cu are
in fact observed for copolymers containing higher amount of
flexible units (up to 40%). This constitutional feature could
arise from the different reactivity of the two bis-imines used
in the synthesis: in P12-8N the two hydroxylimine chelating
moieties sit well separated from each other while in DHTI
these chelating functionalities are on the same ring resulting
in a totally different electronic environment giving rise to a
different reactivity.
XRD spectra of the synthesized polymers were recorded to
better investigate their structural features; in Table 2 these
features are summarized. In Figure 5, the diffraction spec-
trum of a fiber of P0-Cu and of P100-Cu are reported. The
spectrum in Figure 5(a) is referred to an annealed fiber of
P0-Cu. The four spots observed in the low angle region of
the spectrum (2h 5 4.79 Å) are indicative of the cybotactic
nature of this phase. The diffraction images of an annealed
P100-Cu sample, reported in Figure 5(b), features six differ-
ent reflections, corresponding to interplanar spacings of
1 2 3 4 5
20.50 Å (d ), 10.31 Å(d ), 6.82 Å(d ), 4.73 Å(d ), 3.78 Å(d ),
and 2.78 Å(d ), the latter being very weak. Following the
6
experimental evidences arisen from the DSC analysis [see
ꢁ
Fig. 3(a)], the chosen annealing temperature was 220 C.
FIGURE 5 (a) X-rays diffraction spectrum of P0-Cu fiber
ꢁ
A ribbon like crystal structure can be assumed for this poly-
mer. The reflection occurring at interplanar spacing of
annealed 1 h at 100 C; (b) X-rays diffraction spectra of P100-
ꢁ
Cu cold pressed and annealed 1 h at 220 C.
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reported. Both the spectra present a slight equatorial polar-
ization of the second and third reflections (hence confirming
that these reflections belong to the same family of planes,
being the second and third order reflection of that at 20.5
Å). The main difference between the two spectra is that in
P50-Cu spectrum the reflection corresponding to d is some-
3
how broader because of the contribution of the dodeca-
methylene chain in the polymer backbone coming from 50%
of flexible units. With higher content in flexible units, this
signal becomes even broader and turns actually in to a halo.
XRD analysis, consistently with the results of the calorimetric
analysis, suggests that large blocks made up of rigid units
characterize the series of copolymers, affording eventually
structural features typical of P100-Cu. The optical properties
of all the synthesized polymers were investigated through
UV–vis spectroscopic analysis, both in solution and in film
phase. In Figure 7, the spectra of the homopolymers in chlo-
roform solution are reported. P0-Cu in characterized by two
main absorption regions, centered at 278 and at 360 nm,
respectively. The first could be associated to the aromatic
part of the polymer while the second involves ligands-metal
interaction (LMCT band).
For what concerns P100-Cu, the spectrum is once again
characterized by two main absorptions, red shifted as com-
pared to P0-Cu [see Fig. 7(a)]. The first absorption peak is
centered at around 311 nm while the second absorption
FIGURE 6 X-rays diffraction spectra of: (a) P87-Cu cold pressed
ꢁ
and annealed at 220 C 1 h; (b) P50-Cu, in fiber shape,
ꢁ
annealed at 100 C.
backbone direction, or both these effects. The interplanar spac-
ings d and d can be considered as second and third order
2
3
reflections of d . The fourth reflection (d 5 4.73 Å), for its
1
4
position and intensity, should indicate a tight packing of the
aliphatic side chain. The fifth reflection (d 53.78 Å) can be
5
associated to p-p stacking of the macromolecules backbone,
characterized by high planarity because of the square planar
coordination of the rigid aromatic groups around the metal
center. The sixth reflection cannot find an easy assignation.
To obtain a further confirmation of the proposed model, the
homopolymer P’100-Cu, characterized by shorter aliphatic
side chain respect to P100-Cu, was synthesized (in particular,
in the synthesis octylamine and 3-ethoxypropylamine were
used instead of undecylamine and 3-butoxypropylamine). As
expected, for P’100-Cu the spacing corresponding to ribbon
width, 16.8 Å, was lower than that found for P100-Cu (20.8
Å, see Table 2).
As can be seen from Table 2, XRD spectra of copolymers
present close similarity to P100-Cu spectrum when content
of the rigid units is ꢃ 30%; all copolymers from P30-Cu to
P87-Cu feature in fact the five characteristic reflections of
P100-Cu. In Figure 6, the spectra of P87-Cu and P50-Cu are
FIGURE 7 UV–vis spectra in chloroform solution of P0-Cu and
P100-Cu (a); concentration dependence of LMCT band position
and shape for P100-Cu (b). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
8
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band position is strongly related to the solution concentra-
tion. In particular, as shown in Figure 7(b), as the concentra-
tion increases, the maximum of this second absorption shifts
from ꢂ479 to 530 nm. This phenomenon is related to the
establishing of aggregates that, because of strong p-p interac-
tions between the highly planar strands of polymers, occurs
even in very diluted solution as already observed in the pre-
viously discussed SEC experiments (see Table 1).
In Figure 8, the spectra of some of the synthesized copoly-
mers are shown: in these spectra, the signals typical of the
parent homopolymers to which they are related are clearly
visible, in ratios depending upon the composition of the
copolymers.
Also for copolymers rich in rigid units, a strong dependence
of the absorption involving metal-ligand interaction upon
concentration was observed, as for P100-Cu. In Figure 7(b),
the spectrum of P50-Cu, at different solution concentrations,
is presented. Once again, this behavior is consistent with the
results of SEC experiments (see Table 1).
All the prepared materials were moreover processed as thin
film by spin coating of a chloroform solution. The UV–vis
spectra of these films are presented in Figure 9. The optical
behavior of P0-Cu film closely resembles that of P0 in
FIGURE 9 Normalized UV–vis spectra of thin film of (a) P0-Cu and
P100-Cu; (b) P40-Cu, P50-Cu, and P75-Cu. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
solution; for what concerns instead P100-Cu, the film spec-
trum is similar to that of a concentrated solution with the
difference that the lower energy electronic transition is
largely red shifted (ꢂ30 nm). This red shift is clearly related
to the film structuration, which results in a well ordered
stacking of the planar macromolecular strands. Also in film
phase, copolymers present the same peaks shown by the
related homopolymers: it is worth noting that the presence
of an absorption band at ꢂ550 nm in copolymer films is
indicative of long sequences of rigid units that can pack
together leading to absorption in the same range of the
P100-Cu film. The copolymer film optical behavior is in line
with the previous analysis that indicate a partial block
arrangement of copolymer constitution.
CONCLUSIONS
In conclusion, we prepared a novel set of metallopolymers in
which repeating units are connected through metal centers.
The repeating units were endowed with two salycilaldimi-
nate end groups that work as bischelating groups in the
polymer assembling. Two kinds of repeating units were
used: one flexible, in which the chelating groups were sepa-
rated by a long and flexible chain and the other rigid, where
the chelating groups sit on the same phenyl ring. Several
copolymers with rigid units contents in the range 0–100%
were synthesized and characterized. All the polymers show a
FIGURE 8 UV–vis spectra in chloroform solution of P40-Cu, P50-
Cu, and P75-Cu (a); concentration dependence of LMCT band
position and shape for P50-Cu (b). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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good processability and solubility in chlorinated solvents. A
combined molecular, thermal, structural and optical analysis
revealed a peculiar and very interesting feature for the syn-
thesized copolymers: a partial block character in their consti-
tution seems to emerge, with long sequences of rigid units
alternating to sequences of flexible units. Copolymers with
content in rigid units ꢃ than 50% presented in fact XRD spec-
tra very similar to that of the polymer P100-Cu, based exclu-
sively on rigid units. This behavior has been associated to the
existence of long sequences of rigid repeating units in these
copolymers that can interact forming crystalline domains. The
analysis of the XRD reflections is consistent with a ribbon-like
arrangement in which highly planar strands of polymer stack
thanks to p-p interactions. UV–vis absorption measurement in
solid phase showed for the more rigid copolymers optical fea-
tures common to P100-Cu: once again, this occurrence has
been interpreted by assuming the existence of long sequences
of rigid units and to their packing. The synthesized class of
copolymers can be thus considered extremely appealing: the
possibility of enhancing the processability of P100-Cu, by
inserting in its main chain flexibile units, while preserving its
ability of giving highly planar and well structured packing,
make these materials interesting for applications in different
field, like for instance, organic electronics and spintronics.
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