Synthesis and Reaction of Anthracene-Containing Polypropylene: A Promising Strategy for Facile, Efficient Functionalization of Isotactic Polypropylene

Article abstract of DOI:10.1021/acs.macromol.6b02550

A novel anthracene-containing isotactic polypropylene (An-iPP) with high molecular weight (>10 × 10⁴) and satisfying incorporation (5.7 mol %) was synthesized via direct copolymerization of propylene and 9-hexenylanthracene. The pendent anthryl group of the resulting An-iPP is quite active, and this provides a facile and efficient avenue to synthesize various functional iPPs. As a typical and important example, maleic anhydride (MA) functionalized polypropylene, was successfully prepared in a highly efficient, catalyst-free, byproduct-free, and controllable way via mild Diels-Alder (D-A) reaction between pendent anthryl groups and MA. More importantly, the D-A functionalization process did not sacrifice the original properties of the An-iPP, as no unfavorable degradation and cross-linking were detected in DSC and GPC analyses. Besides MA, several other dienophiles could also be conveniently used as functional reagents to prepare various functionalized iPPs with distinct properties. The unique fluorescent property of An-iPP was studied and could be used for functionalization process monitoring.

Full text of DOI:10.1021/acs.macromol.6b02550

Article
pubs.acs.org/Macromolecules
Synthesis and Reaction of Anthracene-Containing Polypropylene: A
Promising Strategy for Facile, Efficient Functionalization of Isotactic
Polypropylene
Deguang Zhang,† Li Pan,*,‡ Yanguo Li, Bin Wang, and Yuesheng Li
,§
†
‡
‡,∥
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
‡
Tianjin Key Lab of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin
3
00072, China
§
University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
∥
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
*
S Supporting Information
ABSTRACT: A novel anthracene-containing isotactic polypropylene (An-iPP)
4
with high molecular weight (>10 × 10 ) and satisfying incorporation (5.7 mol
%
) was synthesized via direct copolymerization of propylene and 9-
hexenylanthracene. The pendent anthryl group of the resulting An-iPP is
quite active, and this provides a facile and efficient avenue to synthesize various
functional iPPs. As a typical and important example, maleic anhydride (MA)
functionalized polypropylene, was successfully prepared in a highly efficient,
catalyst-free, byproduct-free, and controllable way via mild Diels−Alder (D−
A) reaction between pendent anthryl groups and MA. More importantly, the
D−A functionalization process did not sacrifice the original properties of the
An-iPP, as no unfavorable degradation and cross-linking were detected in DSC
and GPC analyses. Besides MA, several other dienophiles could also be
conveniently used as functional reagents to prepare various functionalized iPPs with distinct properties. The unique fluorescent
property of An-iPP was studied and could be used for functionalization process monitoring.
1
. INTRODUCTION
chains, etc., have been successfully obtained. All achievements
in recent years prompt us to further probe the synthesis of
more promising reactive intermediates and the following
functionalizations to develop novel functional polyolefin
As one of the most important polymers, isotactic polypropylene
iPP) has enjoyed wide application and commercial success
(
owing to its combination of outstanding physical property1,
materials with unique properties.
It is well-known that
19−30
excellent chemical resistance, and especially low cost.
the Diels−Alder (D−A) reaction, between a conjugated diene
and a substituted alkene, is particularly useful in synthetic
organic chemistry as a reliable method for forming six-
membered systems with good control over regio- and
stereochemical properties. A polyolefin containing reactive
groups such as (substituted) conjugated diene could be easily
transformed into various functional polymers via simple
reactions with different substituted alkenes. For instance,
anthracene is a relative stable and effective reactant which can
easily react with maleic anhydride (MA) via D−A reaction
under mild reaction conditions. Accordingly, polyolefins
containing pendent anthryl groups as the D−A grafting points
are expected to show ascendancy of conversion and reactivity as
well as byproducts-free and controllable reversibility. Yet at
present, applying this facile, efficient, and versatile route to
However, the intrinsic nonpolar property of iPP usually results
in low surface energies, poor dyeing properties, incompatibility
with polar polymers, and weak adhesion with inorganic
materials. These drawbacks of iPP greatly limit its further
application in some high value areas where interactive
2
properties are required. Therefore, developing functionalized
iPP containing polar groups or reactive functional groups has
2
−8
been a hot topic for researchers in the late several years.
In recent years, “reactive polyolefin intermediate” approach
pioneered by Chung’s group has attracted considerable
24
2
attention. Compared with postfunctionalization and direct
9
,10
copolymerization with polar comonomers approach,
the
“reactive polyolefin intermediate” approach does not need
particular catalyst bearing desirable polar group tolerance and
can conveniently prepare various well-defined functionalized
8
,11−23
polymer materials.
By taking advantage of the facile,
effective chemical transformation of the “reactive group”, well-
defined functional PEs or PPs with tunable amount of hydroxyl,
amino groups, ester groups, or poly(methyl methacrylate)
Received: November 25, 2016
Revised: February 28, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.6b02550
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Macromolecules
Article
2
5
functionalize polyolefin is rarely reported. In this contribu-
tion, a series of iPPs containing pendent anthryl groups were
efficiently synthesized by copolymerization of propylene and 9-
hexenylathracene (HA) for the first time. As expected, maleic
anhydride functionalized iPP (MAPP) and some other
functionalized iPPs were synthesized by D−A reaction between
the pendent anthryl groups of poly(propylene-co-HA) and
dienophiles without any side reaction. Unique fluorescent
properties brought by the pendent anthryl groups of iPP chains
were studied.
mL of anhydrous toluene was slowly added to the Grignard reagent
solution. The mixture was kept at room temperature for 8 h. After that
the mixture was acidified with hydrochloric acid (10 vol %), the
organic layer was separated, and the aqueous layer was extracted with
diethyl ether (2 × 100 mL). The combined organic layer was washed
with water and dried by MgSO , and the solvent was evaporated under
4
vacuum to give the crude product. 46 g of P O and 200 mL of
4
10
toluene were added to the crude product, and the mixture was stirred
for 6 h at room temperature. Then the solid residue was filtered off,
and the solvent was evaporated under vacuum. The crude product was
purified by column chromatography using hexane to give 33.4 g of
pure HA as yellow oily solid (yield 54%). IR (KBr): ν = 3047, 2945,
−
1 1
1
620, 1444, 729 cm . H NMR (400 MHz, δ, ppm, CDCl ): 8.32 (s,
3
2
. EXPERIMENTAL DETAILS
1
H), 8.26 (d, J = 8.7 Hz, 2H), 8.00 (t, J = 6.0 Hz, 2H), 7.52−7.42 (m,
2
.1. Materials and Instrumentation. All moisture and/or oxygen
4H), 5.85 (ddt, J = 16.9, 10.2, 6.7 Hz, 1H), 5.07−4.94 (m, 2H), 3.64−
3.58 (m, 2H), 2.18 (q, J = 7.1 Hz, 2H), 1.84 (ddd, J = 11.7, 9.4, 6.3 Hz,
sensitive manipulations were operated using standard Schlenk
techniques or in an MBraun glovebox under a dry nitrogen
atmosphere. Anhydrous solvents used in this work were purified by
a solvent purification system purchased from Mbraun. rac-Et-
13
2H), 1.73−1.64 (m, 2H). C NMR (101 MHz, δ, ppm, C Cl D ):
2 4 2
139.09 (s, 1C), 135.48 (d, J = 6.4 Hz, 1C), 131.69 (s, 2C), 129.61 (s,
2C), 129.37 (s, 2C), 125.71 (s, 3C), 125.17 (d, J = 6.2 Hz, 2C), 124.65
(s, 2 C), 114.93 (s, 1 C).
[
Ind] ZrCl2 and dimethyl(pyridylamido)hafnium precatalyst were
2
26,27
sysnthesized using the modified procedures.
Commercial
2.3. Typical Copolymerization Procedure. Copolymerizations
of propylene and HA were performed under atmospheric pressure in a
150 mL glass reactor equipped with a mechanical stirring bar. The
reactor was charged with prescribed volume of toluene and
propylene was used directly for polymerization without further
purification. Modified methylaluminoxane (MMAO, 10 wt % in
toluene), Al Bu , [Ph C][B(C F ) ], 6-bromo-1-hexene, P O ,
anthrone, methyl propiolate, vinylene carbonate, N-cyclohexyl-
maleimide, phenyl vinyl sulfone, fumaronitrile, 4-phenyl-1,2,4-triazo-
line-3,5-dione, and fullerene were purchased from Sigma-Aldrich and
used as received. 9-Vinylanthracene, 9-allylanthracene, and 9-
i
3
3
6
5
4
4
10
comonomer under a nitrogen atmosphere, and then the propylene
i
gas feed was started, followed by the addition of Al Bu
or MMAO.
3
After equilibration at the desired polymerization temperature for 5
min, the polymerization reaction was initiated by adding the prescribed
amount of catalyst and cocatalyst. After a desired period, the reactor
was vented. The resulting copolymers were precipitated from
hydrochloric acid/ethanol (2 vol %), filtered, washed three times
with ethanol, then marinated in acetone for 12 h to remove the
unreacted comonomer, and then dried under vacuum at 40 °C to
constant weight.
2.4. Diels−Alder Functionalization of Poly(propylene-co-
HA). In a typical reaction, 0.2 g of poly(propylene-co-HA) copolymer
was dissolved in 20 mL of toluene at 100 °C under nitrogen in a 100
mL flask with a magnetic stirrer. After cooling the system to 80 °C, an
appropriate amount of functional dienophile was added to the reactor,
and the reaction was stirred for another 6 h at the temperature before
precipitating the reaction mixture in 200 mL of acetone. The resulting
grafting copolymer was isolated by filtration, purified by two
dissolving−precipitating cycles, and dried under vacuum at 40 °C
for 24 h.
2
8
butenylanthracene were synthesized according to the literature.
Maleic anhydride was purchased from Sigma-Aldrich and purified by
sublimation before use. All high-temperature NMR spectra were
recorded on a Bruker AM-400 instrument in 1,1,2,2-tetrachloroethane-
d2 at 120 °C. The melting and crystallization temperatures of the
polymers were measured by differential scanning calorimetry (DSC,
TA Instruments, Model Q2000) with a heating and cooling rate of 10
°
C/min. Thermogravimetric analysis (TGA) was carried out under a
nitrogen atmosphere using thermogravimetric analyzer (TA Instru-
ments, Model Q50) with a heat rate of 20 °C/min. The molecular
weights and molecular weight distributions of the polymer samples
were determined at 150 °C by a PL-GPC 220 type high-temperature
gel permeation chromatograph (GPC). 1,2,4-Trichlorobenzene
TCB) was employed as elute solvent at a flow rate of 1.0 mL/min,
and the calibration was made by polystyrene standard Easi-Cal PS-1
PL Ltd.). The FT-IR spectra were recorded on a Bio-Rad FTS-135
(
(
spectrophotometer using polymer thin films. Polarizing optical
microscopy (POM) observation was done on an Olympus BX51
polarizing optical microscopy equipped with a LTS 350 hot stage and
a TMS 94 temperature programmer (Linkam). The water contact
angel experiment was conducted on a Kruess G10/DSA10 contact
angle analyzer. UV−vis spectroscopy was conducted with a
SHIMADZU UV-1600PC. A slit of 2 nm was applied, and the
absorption was recorded by 0.1 nm at a scan rate of 200 nm/min.
Excitation spectra and emission spectra were obtained on a
PerkinElmer LS 50B luminescence spectrometer with xenon discharge
lamp excitation. Cyclic voltammetry was performed on a CHI600E
electrochemical workstation in a standard three-electrode cell under a
blanket of argon in a 0.1 M solution of 0.1 M n-Bu NPF at room
3
. RESULTS AND DISCUSSION
.1. Synthesis of Poly(propylene-co-HA). Since the
3
dimethyl(pyridylamido)hafnium complex (Cat. Hf) and the
classical metallocene catalyst rac-Et[Ind] ZrCl2 (Cat. Zr)
displayed excellent performance in the copolymerization of
propylene with 4-phenyl-1-butene and p-(3-butenyl)styrene,
2
18
these two catalysts were both adopted for the copolymeriza-
tions. To achieve an effective incorporation of anthracene-
containing comonomer and avoid depression of catalytic
activity caused by the bulky and conjugated functional group,
selecting a promising comonomer is also a matter of great
concern. To find out an optimal comonomer candidate, as
shown in Scheme 1, a series of 9-substituted anthracene
comonomers with different olefin substitutions, including 9-
vinylanthracene (VA), 9-allylanthracene (AA), 9-butenyl-
anthracene (BA), and 9-hexenylanthracene (HA), were
synthesized according to the similar synthetic route reported
4
6
temperature. Measurement was carried out with platinum working and
platinum counter electrodes and an Ag/AgCl reference electrode; the
reduction potential was recorded at a scan rate of 100 mV/s and
reported with reference to the ferrocene/ferrocenium redox couple.
2
.2. Synthesis of 9-Hexenylanthracene (HA). Freshly cut
magnesium pieces (7.36 g) were suspended in 200 mL of dry
tetrahydrofuran. A few drops of 6-bromo-1-hexene and a pellet of
iodine were added into the flask and stirred for 10 min to initiate the
reaction. After the color of iodine faded, a total amount of 49 g of 6-
bromo-1-hexene was added dropwise into the flask at 0 °C. The
mixture was stirred at room temperature for 4 h. Then the system was
heated to reflux for another 4 h. Subsequently, the system was cooled
to 0 °C again, and a solution of anthrone (38.9 g, 200 mmol) in 100
28
in the literature and being screened in the copolymerizations.
Homopolymerizations of all these synthesized anthracene-
containing monomers were tried first but no polymer or
oligomer can be obtained. As observed, both Cat. Hf and Cat.
Zr failed to efficiently incorporate the anthryl comonomers
B
DOI: 10.1021/acs.macromol.6b02550
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Scheme 1. Synthetic Route of Functionalized iPP
The representative results of propylene/HA copolymeriza-
tions are summarized in Table 1. One can note that Cat. Zr
displayed high catalytic activity in the copolymerzations (up to
0 × 105 gpolymer molCat.
−1
h , runs 4−7). The HA
−1
4
incorporation is not sensitive to reaction temperature but can
be remarkably enhanced by HA dosage in feed. As shown in
Table 1, a 2-fold increase in HA content is achieved when a 2-
fold increase in HA dosage applied (run 5 vs run 7). However,
the molecular weights (MWs) of the resultant copolymers are
4
below 1.8 × 10 (runs 4−7). On the contrary, surprisingly,
besides a high catalytic activity, Cat. Hf can also produce
copolymers with much higher MWs (up to more than 10-fold
that from Cat. Zr) and HA incorporations (up to 5.7 mol %,
runs 10−16). Moreover, the addition of varying amount of
i
Al Bu in the reaction system helps to tune the MWs of the
3
i
copolymers, as the Al Bu serves as an efficient scavenger and
3
also a chain transfer agent. The unimodal distributed MW of
the copolymer demonstrates a single site catalyzed copoly-
merization system. Nonetheless, the copolymers containing 5.7
mol % and higher amount of pendent anthryl groups failed to
fully dissolve during GPC tests. The poor solubility might be
caused by the intensive intermolecular interaction originated
from enhanced π−π stacking between the pendent anthryl
with short spacers between anthrancene and the aliphatic
double bond. For example, no VA and AA incorporation can be
detected in the copolymers synthesized by Cat. Zr (runs 1 and
2
in Table 1), while only 0.2 mol % of AA incorporation can be
observed in those prepared by Cat. Hf (run 8 in Table 1). The
comonomer with longer spacer is easily incorporated; i.e.,
acceptable BA incorporations (around 1.1 mol %) can be
achieved by either catalysts (runs 3 and 9 in Table 1). Further
increase of the aliphatic bridge length between double bond
and anthryl functional group is helpful to obtain copolymers
with even higher comonomer incorporations. Incorporations of
HA are increased to 1.3 mol % by Cat. Zr and 1.8 mol % by
Cat. Hf under similar conditions, respectively (runs 4 and 10,
Table 1). These observations strongly indicate that the aliphatic
chain length between the double bond and the functional group
plays a crucial role in determining the incorporation of the
31−33
groups.
To estimate the copolymerization reactivity ratio of
propylene and HA catalyzed by Cat. Hf, a series of
copolymerization with low monomer conversion (<10%)
were carried out by varying monomer feed ratio, and the
resulting copolymer composition were compared (see Table S1
in Supporting Information for more details). The reactivity
ratio values of propylene (r ) and HA (r ) were determined
1
2
3
4
according to the Fineman−Ross equation. The intercept and
slope of Fineman−Ross plot show r = 1.6, r = 0.46, and r
= 0.74. These results clearly demonstrate that copolymers
×
1
1
2
r
2
2
9,30
comonomer.
Therefore, HA was chosen as the optimal
obtained by using Cat. Hf are random copolymers with
dominant continuous propylene sequence and isolated HA
incorporation.
comonomer to prepare anthracene-containing iPPs (An-iPPs)
possessing high comonomer incorporations.
a
Table 1. Summary of Propylene/Substituted Anthracene Copolymerizations
b
c
Mw [104] PDI
c
d
d
e
e
run
Cat.
[Al]/[Cat.] comonomer [mmol] temp [°C] act.
incorp [mol %] incorp [wt %] Tm [°C] Tc [°C]
1
2
3
4
5
6
7
8
9
Cat. Zr
Cat. Zr
Cat. Zr
Cat. Zr
Cat. Zr
Cat. Zr
Cat. Zr
Cat. Hf
Cat. Hf
Cat. Hf
Cat. Hf
Cat. Hf
Cat. Hf
Cat. Hf
Cat. Hf
Cat. Hf
2000
2000
2000
2000
2000
2000
2000
100
VA/2.00
AA/1.56
BA/1.19
HA/1.19
HA/1.19
HA/1.19
HA/3.57
AA/3.12
BA/1.19
HA/1.19
HA/1.19
HA/1.19
HA/0.60
HA/2.38
HA/3.57
HA/4.76
30
30
30
30
45
60
45
25
25
25
25
25
25
25
25
25
37
3.4
4.0
1.5
1.8
1.5
1.8
1.5
2.0
1.9
2.1
2.0
2.0
1.9
2.0
0
0
22
34
38
36
40
35
7
0
0
1.1
1.3
1.3
1.6
3.9
0.2
1.2
1.8
1.6
2.2
0.8
3.8
4.5
5.7
6.3
7.5
111
106
93
65
7.5
59
9.2
48
f
f
22.4
1.1
n.d.
162
138
137
137
135
140
121
n.d.
137
84
88
89
82
89
70
100
30
24
28
20
30
24
26
30
11.0
18.1
12.0
6.5
2.1
2.0
1.9
1.8
1.9
1.8
2.0
6.5
1
1
1
1
1
1
1
0
1
2
3
4
5
6
100
10.2
9.2
150
200
12.2
4.5
100
24.2
17.5
24.0
100
19.6
23.0
27.3
f
f
100
n.d.
n.d.
n.d.
n.d.
g
g
f
f
100
−
−
a
Reaction conditions: propylene pressure = 1 atm, runs 1−7: Cat. Zr = 3 μmol, [Al] = MMAO, V = 30 mL, reaction for 5 min; runs 8−16: Cat.
−1
total
i
b
5
Hf = 5 μmol, cocatalyst = [Ph C][B(C F ) ] = 10 μmol, [Al] = Al Bu , V = 40 mL, reaction for 10 min. Catalytic activity: 10 g
h . MW determined via GPC at 150 °C in TCB vs narrowly distributed polystyrene standards. Comonomer incorporations (mol % or wt %) were
established by H NMR spectra. Melting temperature and crystallization temperature were determined by DSC. Not detected. Partial gelation
mol
Cat.
3
6
5
4
3
total
polymer
−1
c
d
1
e
f
g
occurred when dissolving sample at 150 °C.
C
DOI: 10.1021/acs.macromol.6b02550
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As we have proved previously, both Cat. Zr and Cat. Hf
could promote propylene polymerization and produce iPP; the
4
former gave low MW (3.3 × 10 ) iPP with a T of 138 °C,
m
4
while the latter yielded high MW (6.3 × 10 ) iPP with a T of
60 °C. In this context, thermal properties of the propylene/
m
1
8
1
HA copolymers were studied by DSC measurement, and
representative DSC curves are shown in Figure S3. The
incorporation of a small amount of bulky HA remarkably
reduced the regularity of the polymer chain, and accordingly
the Tm value of the copolymers dramatically decreased. For
example, when the HA incorporation was enhanced from 1.3 to
3.9 mol %, the copolymer prepared by Cat. Zr became
amorphous from semicrystalline (run 7, Table 1). The
copolymers prepared by Cat. Hf displayed higher Tm than
those by Cat. Zr, which should be ascribed to the contribution
of higher MW. When HA incorporation reached 4.5 mol % or
even higher, melting and crystallization peaks were no longer
observed during the second heating stage and first cooling
stage. The copolymers containing more than 4.5 mol % HA
were also insoluble in hot toluene after being hot pressed at 180
°C, suggesting a possible cross-linking or strong intramolecular
interaction might have happened during the melting process.
The cross-linking might be ascribed to the noncovalent π−π
stacking, since the first heating stage of DSC gave the chains
enough conformational flexibility to bring the anthryl groups
3
1,32
closer in melt.
Though the noncovalent π−π stacking could
be reversed or weakened when heated, the partially remained
linkages were enough to disturb the sufficient folding of
Figure 1. (a) H and (b) 13C NMR spectra of poly(propylene-co-HA)
with HA incorporation of 3.8 mol % (run 14 in Table 1).
1
3
5
polymer segments and crystallization of the polymer.
.2. Structure Characterization of Poly(propylene-co-
3
HA). Previous achieved anthracene-containing polymers were
synthesized from radical and ionic polymerization; their
microstructures were somewhat complicated and ambiguous
because of the rearrangement and complicated insertion
spectrum are assigned unambiguously and shown in Figure 1b.
All these analyses indicated that the active anthryl groups
successfully remained pending at the end of the side chain.
Moreover, consecutive HA structure has not been detected for
all the copolymers, further proving the isolated distribution of
HA in the copolymers. To the best of our knowledge, this is the
first example of successful synthesis of well-defined HA-
containing iPPs with clearly identified structure and tunable
HA content.
36−39
behavior of the olefin-substituted conjugated structure.
1
13
In a sharp contrast, H NMR and C NMR spectra analyses
demonstrate that the propylene/HA copolymerizations pro-
moted by Cat. Zr and Cat. Hf via coordination copolymeriza-
tions can yield well-defined isotactic poly(propylene-co-HA)s
containing some pendent anthryl groups. Figure 1a represents
3
.3. Functionalization of Poly(propylene-co-HA). With
1
the H NMR spectrum of poly(propylene-co-HA) with HA
the well-defined An-iPP at hand, we then turned our attention
to the effective functionalization of the HA-containing reactive
intermediates via D−A reaction. Because of the low cost of
production and the high polarity and reactivity stemming from
the succinic anhydride moieties in the polymer, MAPP has
become one of the most representative and commercially
content of 3.8 mol %. The spectrum reveals that HA is inserted
into the PP chain exclusively via the hexenyl double bond, as no
signals within the range of 5.0−6.0 ppm corresponding to
terminal double bonds can be found. The reactive anthryl
groups are completely located at the end of the side chains. The
resonances at 7.40, 7.54, 8.08, and 8.40 ppm are attributed to
the protons of the pendent anthryl groups. The HA
40
important functionalized iPPs. MAPP is widely used in
anticorrosive coating, glass fiber reinforced PP, and compatibil-
1
incorporations can be calculated from H NMR spectra by eq
41−43
izer for various polymer blends.
However, at present, most
1:
of the MAPP is prepared on the basis of free-radical grafting
MA of iPP chains via melt reaction at higher temperature
HA mol % = [2I8
.40−8.31
/(2I
8.40−8.31
+ I1.79−0.85 − 9)] × 100%
44
(
around 200 °C). The free radical reaction is usually
(
1)
accompanied by harsh effects, which will result in unfavorable
where I8.40−8.31 and I1.79−0.85 are the peak areas of protons at
defects, such as cross-linking, serious MW reduction, stained
45
8
.40−8.31 and 1.79−0.85 ppm, respectively. The incorporation
wt % can also be calculated from mole incorporation by using
eq 2:
deep color, and structure complexity. Many efforts have been
devoted to developing highly controllable approaches to
overcome all the disadvantages. Nonetheless, how to obtain
high-MW MAPP with satisfactory MA content (higher than 1
wt %) and well-controlled molecular structure is still one of the
biggest challenges. Thus, facile and effective functionalization of
HA wt % = [1.3HA mol %/(21 + 1.09HA mol %)] × 100%
(
2)
The exclusive insertion behavior of HA can also be supported
1
3
13
1
13
11,46
by C NMR (Figure 1b), C DEPT (Figure S4), and H− C
1
3
HSQC spectrum (Figure S5). Resonances in the C NMR
D
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containing intermediate. As expected, the MA-functionalized
iPP synthesized from this new approach can greatly avoid the
above-mentioned undesirable but unavoidable side reactions.
The D−A reaction between the pendent anthryl groups with
MA achieved a completed conversion in 45 min at 100 °C.
After precipitation in acetone and further purification, white
polymer could be obtained without any cross-linking or
ascribed to the MA detachment via the retro-D−A reaction
because the control sample (An-iPP) did not decompose at
such temperature.
The MA content after the quantitative conversion of pendent
anthryl groups can also be confirmed by FT-IR spectroscopy of
the copolymer films (2a in Figure S8). New absorption bands
around 1784 cm− of carbonyl stretch and the absence of bands
1
1
−1
degradation. As revealed by the H NMR spectrum of the
corresponding MA-functionalized product (2a in Figure 2), the
around 740 cm of the anthryl C−H out-of-plane vibration
clearly indicate the successful introduction of MA groups. The
absorption of these specific bonds was used for the calculation
of MA content in the copolymer by using the equation reported
47
by Chung and co-workers. According to the data listed in
Table 2, the experimental value of MA contents is very close to
the corresponding calculated contents based on complete
functional groups conversion. The MA content of the high-MW
copolymer can be easily tuned by adjusting the HA
incorporation and easily reach a relative high level of 6.9 wt
%
%
, which is far superior to the commercial products (ca.1 wt
). This endows the copolymers with improved polarity, as
evidenced by that the water contact angle of hydrophobic
polypropylene material decreases from around 116° to 91.2°.
More importantly, the undesirable cross-linking and serious
degradation can be effectively avoided via this new function-
alization avenue by comparison of the GPC and DSC results of
the copolymers before and after the modification (Figures S9
and S10). One can find that after the D−A reaction the newly
obtained copolymer displays very similar MW and molecular
weight distribution. By comparing the data in Table 2, it is
interesting to see that despite the fact that the T , melting
m
enthalpy, and crystallization enthalpy are nearly unchanged
after MA functionalization, about 10 °C of increase in
crystallization temperature (T ) compared to the unfunction-
c
alized copolymers can be observed. The higher crystallization
temperature and the improved crystallization ability might be
Figure 2. 1H NMR spectra of functionalized iPP. Samples were
prepared from An-iPP containing 1.8 mol % HA. Asterisk represents
the signals of solvent.
mainly caused by the promoted heterogeneous nucleation
48,49
origining from the introduction of MA.
The POM pictures
of An-iPP and its MA-functionalized product (Figure S11)
clearly indicated the impoved nucleation of MAPP compared to
An-iPP.
resonances in the region of 8.5−7 ppm relative to the
conjugated anthryl groups disappeared, while new signals of
As aforementioned, D−A reaction allowed the efficient and
clean syntheses of numerous chemicals with well-designed
structure by selecting the reactants, in more detail, by tuning
the substituted group of conjugated diene and the alkene.
Consequently, the An-iPP provides us a very versatile pathway
to produce well-defined functional iPPs through the D−A
reaction, which are very difficult to achieve by conventional
methods. With the aim of testing the possibility of synthesizing
new functional iPPs, as presented in Scheme 1, besides MA
[
4 + 2] adducts structure appeared in the region around 7.4
ppm. Meanwhile, the resonances of corresponding carbons also
1
3
shifted from 122 to 134 ppm in C NMR spectra (see
Supporting Information, Figure S6 (2a)). These results
indicated that the active pendent groups in the copolymer
were completely transformed into [4 + 2] adduct structures
after the D−A cycloaddition. Additionally, the TGA analysis
shown in Figure S7 revealed that the first stage of
decomposition of MAPP started at 200 °C, which might be
Table 2. Summary of MA Functionalization of An-iPP
a
b
c
run
HANMR [mol %]
MACal [wt %]
MAIR [wt %]
Tm [°C]
ΔHm [J/g]
Tc [°C]
ΔH [J/g]
water contact angle [deg]
c
d
e
1
0.8
0.8
1.4
1.4
3.8
3.8
0
0
143
143
140
139
121
117
44
45
56
54
45
47
91
102
89
60
56
71
70
56
62
n.d.
2
1.6
0
1.6
0
98.6
109.1
95.1
d
3
4
3.0
0
2.4
0
98
d
e
5
70
n.d.
6
6.9
7.8
79
91.2
a
1
b
c
HA contents estimated from H NMR spectra. MA contents were calculated from HA content (mol %) based on 100% functionalization. MA
contents estimated from FT-IR spectra usingthe equation MA (wt %) = K(A1780/d), where A1780 is the absorbance of the carbonyl group at 1780
−1
cm , d is the thickness (mm) of the film, and K is a constant (0.25) that is determined by a calibration curve of several commercial PP-g-MA
47
d
e
polymers with known MA contents.
An-iPPs before MA functionalization. Not determined.
E
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Macromolecules
1a), the pendent anthryl group also were tried to react with
Article
(
various substituted alkenes, such as methyl propiolate (1b),
vinylene carbonate (1c), N-cyclohexylmaleimide (1d), phenyl
vinyl sulfone (1e), fumaronitrile (1f), 4-phenyl-1,2,4-triazoline-
3
,5-dione (1g), and fullerene (1h). A variety of functional iPPs
can be obtained by simply heating up the versatile An-iPP
together with these functional reagents in solution in high
1
efficiency. Figure 2 shows the H NMR spectra of
1
3
corresponding functionalized iPPs (2a−2h). C NMR and
FT-IR spectra of some represented functionalized iPPs are
given in the Supporting Information (Figures S6 and S8). The
full conversion of all the functionalization reactions is assumed
based on the complete disappearance of signals relative to the
1
reactive anthryl group in H NMR spectra. Undoubtedly, there
are much more potential substituted alkenes that can react with
the reactive An-iPP intermediate and give various novel
functional iPPs. By choosing a proper dienophile with unique
property such as fluorescent property, electric property,
hydrophilicity, biocompatibility, etc., iPPs possessing corre-
sponding desired functional properties could be easily obtained.
Figure 4. Time dependence of MA functionalization process
calculated from UV−vis spectra. Inset: comparison of UV−vis spectra
of MA functionalization of An-iPP. The reaction was carried out in
toluene at 100 °C. Samples were taken after certain reaction time,
diluted to 0.3 mg/mL, and measured.
3
.4. Fluorescent Properties of Poly(propylene-co-HA).
The pendent anthryl groups not only effectively bring reactive
reaction sites into iPP but also endow some unique properties
to the resultant iPP copolymers. The solution of An-iPP had
strong absorbance in UV−vis spectra and emitted blue
fluorescence under UV light (365 nm) excitation. As shown
in Figure 3, the major excitation signals appear at 341, 361, and
iPP solution display three strong absorbance bands in the
region of 340−400 nm, while MAPP showed no absorbance in
the same region. The plot in Figure 4 reveals the decrease in
the absorption of pendent anthryl groups and time relation of
MA functionalization reaction of the An-iPP. According to the
slope, the reaction rate decreases greatly after 17 min, by the
time when 80% of the functionalization was completed. After
4
5 min, no absorbance corresponding to anthryl groups can be
detected, suggesting that the MA functionalization reaction has
finished and this process is very fast and efficient.
A similar fluorescent property was also observed for the An-
iPP films. Anthracene derivatives are also known to undergo the
light-induced [4 + 4] cycloaddition to form cyclooctane-type
52
dimers. Once the [4 + 4] cycloaddition occurred, the
fluorescent properties of the copolymer were influenced as well.
To test the film’s fluorescent property, a piece of the copolymer
film was exposed under sunlight for 12 h, and the certain region
of the film was covered to avoid being irradiated by sunlight.
After the treatment, the whole film emitted purple fluorescence
under 365 nm UV light, yet the exposed region showed a
different visual fluorescence with lighter color and weaker
intensity (Figure S13). Compared to An-iPP, the light exposed
An-iPP film showed a decline of elongation at break in the
tensile test (Figures S14 and S17), while both Tm and Tc
Figure 3. Fluorescence excitation and emission spectra of poly-
(
propylene-co-HA) solution before (black lines) and after MA
functionalization (red line). Inset: an illumination photograph of
poly(propylene-co-HA) solution before (left) and after MA function-
alization (right) under 365 nm UV light.
(
Figures S15, S16, S18, and S19) slightly dropped according to
DSC results shown in the Supporting Information.
4
. CONCLUSIONS
3
4
81 nm, and the major emission signals are observed at 401,
18, and 444 nm. The shapes of the polymer’s excitation and
The synthesis of novel anthracene-containing iPP (An-iPP) via
copolymerization of 9-hexenylanthracene (HA) and propylene
by dimethyl(pyridylamido)hafnium catalyst is described here, in
emission spectra are similar to that of reported anthracene, and
50
5
−1 −1
the fluorescence intensity stays at a high level. The pendent
fluorescent anthryl groups can be converted into non-
fluorescent D−A adducts after the cycloaddition of MA. As
observed, after the MA functionalization, no fluorescence can
be detected from the emission spectra. Based on this property,
such functional polymers can be utilized in materials’ crack
sensoring. Additionally, the absorption change of fluorescence
spectra and UV−vis spectra after D−A reaction of the pendent
anthryl groups makes it possible to monitor the functionaliza-
tion process of An-iPPs by the decrease of anthryl absorbance.
It was also observed from the UV spectra (Figure 4) that An-
which the catalytic activity (>24 × 10 g
molCat. h ),
polymer
4
molecular weight (>10 × 10 ), and incorporation ratio (5.7 mol
%) have reached a satisfying level simultaneously. The pendent
anthryl groups in poly(propylene-co-HA) display high Diels−
Alder (D−A) reactivity toward numerous functionalization
reagents under mild conditions. Based on this reactive
polyolefin intermediate, various functional iPPs including
maleic anhydride functionalized iPP with high molecular weight
and high functional group content were obtained in a facile
way. The employed D−A functionalization strategy possess
remarkable features such as operational simplicity, high
5
1
F
DOI: 10.1021/acs.macromol.6b02550
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
efficiency, ready availability of reagents at low cost, and no
damage to iPPs’ excellent performance. The fluorescent
properties were also introduced into An-iPPs by pendent
anthryl groups and can be used in monitoring the
functionalization process. The herein reported efficient
conversion of anthracene-containing “reactive polyolefin
intermediates” under mild conditions has great potential in
modifying and improving iPP’s properties. More studies on the
design and synthesis of unique functional polymers with
unprecedented performances are in progress.
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ASSOCIATED CONTENT
Supporting Information
Combination of Metallocene Catalysts and Reactive Comonomers.
■
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*
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The Supporting Information is available free of charge on the
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AUTHOR INFORMATION
Corresponding Author
E-mail lilypan@tju.edu.cn (L.P.).
ORCID
Li Pan: 0000-0002-9463-6856
enaminoketonato)titanium Catalysts and Their Applications in
Preparing Polyolefin-graft-poly(ε-polycaprolactone). Polymer 2010,
5
■
1, 3636−3643.
*
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Notes
(19) Zheng, Y.; Pan, L.; Li, Y.; Li, Y. Synthesis and Characterisation
The authors declare no competing financial interest.
of Novel Functional Polyolefin Containing Sulfonic Acid Groups. Eur.
Polym. J. 2008, 44, 475−482.
ACKNOWLEDGMENTS
The authors are grateful for financial support by the National
Natural Science Foundation of China (No. 21234006).
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