Catalytic synthesis of styryl-capped isotactic polypropylenes

Article abstract of DOI:10.1002/pola.24139

Bis-styrenic molecules, 1,4-divinylbenzene (DVB) and 1,2-bis(4-vinylphenyl) ethane (BVPE), were successfully combined with hydrogen (H₂) to form consecutive chain transfer complexes in propylene polymerization mediated by an isospecific metallocene catalyst (i.e., rac-dimethylsilylbis(2-methyl-4- phenylindenyl)zirconium dichloride, I) activated with methylaluminoxane (MAO), rendering a catalytic access to styryl-capped isotactic polypropylenes (i-PP). The chain transfer reaction took place in a unique way where prior to the ultimate chain transfer DVB/H₂ or BVPE/H₂ caused a copolymerizationlike reaction leading to the formation of main chain benzene rings. A preemptive polymer chain reinsertion was deduced after the consecutive actions of DVB/H₂ or BVPE/H₂, which gave the styryl-terminated polymer chain alongside a metal-hydride active species. It was confirmed that the chain reinsertion occurred in a regio-irregular 1,2-fashion, which contrasted with a normal 2,1-insertion of styrene monomer and ensured subsequent continuous propylene insertions, directing the polymerization to repeated DVB or BVPE incorporations inside polymer chain. Only as a competitive reaction, the insertion of propylene into metal-hydride site broke the chain propagation resumption process while completed the chain transfer process by releasing the styryl-terminated polymer chain. BVPE was found with much higher chain transfer efficiency than DVB, which was attributed to its non-conjugated structure with much divided styrene moieties resulting in higher polymerization reactivity but lower chain reinsertion tendency.

Full text of DOI:10.1002/pola.24139

Catalytic Synthesis of Styryl-Capped Isotactic Polypropylenes
HUAHUA HUANG, CHENGANG CAO, HUI NIU, JIN-YONG DONG
CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Received 2 March 2010; accepted 19 May 2010
DOI: 10.1002/pola.24139
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Bis-styrenic molecules, 1,4-divinylbenzene (DVB)
and 1,2-bis(4-vinylphenyl)ethane (BVPE), were successfully
combined with hydrogen (H2) to form consecutive chain
transfer complexes in propylene polymerization mediated by
an isospecific metallocene catalyst (i.e., rac-dimethylsilylbis(2-
methyl-4-phenylindenyl)zirconium dichloride, I) activated with
methylaluminoxane (MAO), rendering a catalytic access to sty-
ryl-capped isotactic polypropylenes (i-PP). The chain transfer
reaction took place in a unique way where prior to the ultimate
chain transfer DVB/H2 or BVPE/H2 caused a copolymerization-
like reaction leading to the formation of main chain benzene
rings. A preemptive polymer chain reinsertion was deduced af-
ter the consecutive actions of DVB/H2 or BVPE/H2, which gave
the styryl-terminated polymer chain alongside a metal-hydride
active species. It was confirmed that the chain reinsertion
occurred in a regio-irregular 1,2-fashion, which contrasted with
a normal 2,1-insertion of styrene monomer and ensured subse-
quent continuous propylene insertions, directing the polymer-
ization to repeated DVB or BVPE incorporations inside polymer
chain. Only as a competitive reaction, the insertion of propyl-
ene into metal-hydride site broke the chain propagation
resumption process while completed the chain transfer process
by releasing the styryl-terminated polymer chain. BVPE was
found with much higher chain transfer efficiency than DVB,
which was attributed to its non-conjugated structure with
much divided styrene moieties resulting in higher polymeriza-
C
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tion reactivity but lower chain reinsertion tendency.
2010
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
3709–3713, 2010
KEYWORDS: chain transfer; functionalization of polymers; metal-
locene catalysts; propylene; Ziegler-Natta polymerization
INTRODUCTION Polypropylene (usually isotactic, i-PP) is val-
ued by a supreme performance-cost ratio, which has made it
one of the most widely used polymeric materials worldwide.
To further boost its applicability, one current development of
i-PP is to introduce reactive/functional groups onto its inert
macromolecular chains in pursuit of controlled chemical
reactivity.1–3
metric ansa-metallocene catalysts.19–21 Styrene insertion (in
2,1-fashion)22–25 induces active site congestion (because of
the bonding of benzene ring to the side of transition metal),
which triggers selective hydrogen chain transfer. This resem-
bles a consecutive chain transfer process, first to styrene
then to hydrogen, resulting in a selective incorporation of
styrene (or its derivative) unit at i-PP chain end. Thus far,
the styrene/hydrogen consecutive chain transfer reaction has
been tested with various para-substituted styrene derivatives
and been authenticated as a unique approach particularly
effective for introducing a phenyl chain end to i-PP. As such,
considering 1,4-divinylbenzene (DVB, Chart 1) as a vinyl-sub-
stituted styrene, an intriguing question of whether DVB
could combine with hydrogen to induce an analogous reac-
tion is raised. If viable, this would represent a catalytic way
of delivering a non-heteroatom but chemically versatile moi-
ety, styryl group,26,27 to the chain end of i-PP (i-PP-t-St).
However, a preliminary investigation into DVB/H2 as chain
transfer complex in rac-dimethylsilylbis(2-methyl-4-phenylin-
denyl)zirconium dichloride (I)-MAO-catalyzed propylene poly-
merization has revealed that, although a consecutive chain
transfer reaction does take place, the resulting styryl-termi-
nated i-PP chain tends to reinsert into the simultaneously
generated metal-H species (in regio-irregular 1,2-fashion) to
resume chain propagation.28 This brings up the uncertainty
One important access to reactive/functional polyolefins is by
using a chain transfer agent to introduce reactive/functional
groups at chain termini.4 In this regard, the recent progress
has been largely made by heteroatom chain transfer agents,
including alanes,5–7 silanes,8–10 boranes,11–13 phosphines,14,15
amines,16 and organozincs.17,18 Because of a rich availability
of catalysts, considerable successes have been achieved in
synthesis of heteroatom group-terminated polyethylenes. In
regard to i-PP analogues, however, more efforts are needed
to conquer such difficulties as simultaneously controlling
both a stereo-specific polymerization and chain transfer reac-
tion, especially when an electron-rich heteroatom chain
transfer agent is employed.14–16
Previously, a unique chain transfer reaction employing sty-
rene (or its derivatives) and hydrogen (H2) together as a bi-
component chain transfer complex was discovered in propyl-
ene polymerization mediated by some conventional C2-sym-
Correspondence to: J.-Y. Dong (E-mail: jydong@iccas.ac.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3709–3713 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
fate, diethyl ether, petroleum ether, and methylenechroride
were purchased from Sinopharm Chemical Reagent Beijing
Co., China and used without further purification.
Preparation of BVPE
An oven-dried round-bottomed flask equipped with a mag-
netic stirrer, a reflux condenser and a dropping funnel was
charged with 1.55 g (0.064 mol) of magnesium and 0.10 g
(0.8 mmol) of iodine in 50 mL of THF. To this suspension,
18 mL (0.128 mol) of 4-vinylbenzyl chroride in 80 mL of
THF was added dropwise at room temperature. The mixture
was further stirred at reflux for 12 h. After cooling down,
ice-water (200 mL) was added and the reaction mixture was
extracted with diethyl ether (200 mLꢁ3). The combined or-
ganic layer was dried over anhydrous sodium sulfate. The
solvent was removed by rotary evaporation and then the res-
idue was purified by column chromatography on silica gel
using petroleum ether: methylenechroride (3:1) as eluent to
give BVPE as a white solid with about 85% yield. 1H NMR
(400MHz, CDCl3, solvent reference at 7.26 ppm), dH 7.32–
7.34 (d, 4H), 7.13–7.14 (d, 4H), 6.66–6.73 (dd, 2H), 5.69–
5.74 (d, 2H), 5.19–5.22 (d, 2H), 2.90 (s, 4H) ppm.
CHART 1 Structures of the bis-styrenic molecules, DVB and
BVPE.
of whether i-PP-t-St could really be produced by this simple
process. However, as we report here, we have found that in
situ i-PP-t-St chain reinsertion is only to build up the poly-
mer chain, which eventually will still be terminated by a
round of DVB/H2 chain transfer, only that this time the
monomer (propylene) plays an indispensable role. Moreover,
with a designed bis-styrenic molecule, 1,2-bis(4-vinylphenyl)-
ethane (BVPE, Chart 1), that loosens the tight conjugation of
DVB and widens the distance between two double bond moi-
eties, both the chain termination efficiency increases and the
disturbing chain re-insertion tendency discouraged. Efficient
catalytic synthesis of i-PP-t-St is then expected.
Polymerization
EXPERIMENTAL
In a typical reaction (run D-1 in Table 1), a Parr 450-mL
stainless steel autoclave reactor equipped with a mechanical
stirrer was charged with mixed gases of 0.12 bar hydrogen
and 0.88 bar nitrogen. (In polymerizations without hydrogen,
pressure was initially brought to 1 bar nitrogen only.) Fifty
milliliters of toluene, 4.3 mL of MAO (1.40 M in toluene),
and 1 mL of 1,4-DVB (1.75 M in toluene) were injected into
Materials and Instruments
All room- and high-temperature 1H and 13C NMR spectra
were recorded on a Bruker AM-300 instrument. The mea-
surement was carried out at 110 ꢀC using either 1,1,2,2-tet-
rachloroethane-d2 or o-dichlorobenzene-d4 as solvent. The
melting temperature of the polymers was measured by dif-
ferential scanning calorimetry (DSC) using a Perkin-Elmer
DSC-7 instrument controller with a heating rate of 10 ꢀC/
min. The molecular weight and molecular weight distribution
of the polymers were determined by gel permeation chroma-
tography (GPC) using a Waters Alliance GPC 2000 instru-
ment equipped with a refractive index (RI) detector and a
set of u-Styragel HT columns of 106, 105, 104, and 103 pore
size in series. The measurement was performed at 150 ꢀC
with 1,2,4-trichlorobenzene as the eluent with a flow rate of
1.0 mL/min. Narrow-molecular-weight PS samples were
used as standards for calibration.
ꢀ
the reactor and stirred vigorously for 5 min at 30 C. About
2 lmol of rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 catalyst in tolu-
ene solution was then added to the rapidly stirring solution,
and 4 bar propylene was fed to initiate the polymerization
reaction. Propylene was fed continuously into the reactor to
maintain a constant pressure (5 bar) during the ꢀcourse of
the polymerization. After 15 min of reaction at 30 C, the re-
actor pressure was released. The reaction solution was dis-
charged from the reactor and quenched with acidified etha-
nol. The resulting polymer was filtered and washed
extensively with tetrahydrofunan (THF) to remove excess
1,4-DVB, and finally dried under vacuum at 50 ꢀC for 12 h.
About 3.55 g of polymer product was obtained.
All O2- and moisture-sensitive manipulations were carried
out inside a nitrogen-filled vacuum atmosphere dry-box
equipped with a dry train. Tetrahydrofuran and toluene were
deoxygenated by nitrogen purge before refluxing for 48 h
and were then distilled over sodium. Methylaluminoxane
(MAO, 1.4 M in toluene), which was kindly supplied by CNPC
Lanzhou Chemical Company, was dried under vacuum to
remove trimethylaluminum (TMA), and the resulting TMA-
free MAO was diluted in toluene before used. rac-Me2Si[2-
Me-4-Ph(Ind)]2ZrCl2 was synthesized according to litera-
ture.29 Polymerization-grade propylene was supplied by Yan-
shan Petrochemical Co., China. High-purity grade hydrogen,
deuterium (D2), and nitrogen were used as received. Pure
1,4-DVB was isolated from a commercially available divinyl-
benzene mixture (Aldrich) according to a published proce-
dure.30 4-Vinylbenzyl chloride (Alfa) was dried over CaH2
before distillation. Magnesium, iodine, anhydrous sodium sul-
RESULTS AND DISCUSSION
The structures of DVB and BVPE are shown in Chart 1. Pro-
pylene polymerization was conducted using a typical i-PP-
type metallocene complex, that is, rac-dimethylsilylbis(2-
methyl-4-phenylindenyl)zirconium dichloride (1)-MAO.31 The
polymerization results are summarized in Table 1. Both DVB
and BVPE rendered efficient chain termination, which was
evidenced by the zero-yielding blank reactions of runs D-0
and B-0. In both sets of reactions with DVB (runs D-1 to
D-4) and BVPE (runs B-1 to B-5), the introduction of appro-
priate dosages of hydrogen according to the engaged DVB or
BVPE concentrations effectively enabled polymerization, with
the catalyst activities in most cases maintaining >106 g/mol
Zr.h. With the increase of DVB or BVPE concentration,
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TABLE 1 Results of Propylene Polymerization with 1-MAO in the Presence of DVB (D) or BVPE (B) and Hydrogena
e
f
Run
D (B) (mol/L)
H2 (bar)
Ab
[D(B)]c (mol %)
[Stl]d (mol %)
Mn
PDI
3.1
Tm (ꢀC)
0
0
13.0
ꢂ0
7.1
7.3
6.0
3.3
ꢂ0
5.9
3.7
2.1
1.0
1.0
46.8
158
D-0
D-1
D-2
D-3
D-4
B-0
B-1
B-2
B-3
B-4
B-5g
a
0.035
0.035
0.070
0.127
0.226
0.025
0.025
0.033
0.061
0.085
0.064
0
0.12
0.35
0.50
0.50
0
0.68
0.74
1.18
2.64
0.08
0.17
0.33
0.54
8.8
4.1
2.8
1.9
2.9
2.6
2.1
1.8
148
142
135
127
0.08
0.12
0.40
0.35
0.40
0.56
0.60
1.53
1.76
1.80
d
0.17
0.34
0.61
0.75
0.83
3.2
2.5
1.7
1.1
0.9
1.8
1.6
1.8
1.7
2.1
149
145
142
137
135
Polymerization conditions: 50 mL of toluene, propylene, 4 bar (3.22
mol/L); 1, 2 lmol; Al/Zr (mol/mol), 3000; temperature, 30 ꢀC; time, 15
min.
Terminal styryl group concentration.
ꢁ104 g/mol.
e
f
Determined by DSC.
b
g
Catalyst activity, ꢁ106g of PP/(molZr.h).
Polymerization conditions: propylene, 1 bar; 30 min.
c
Total DVB or BVPE incorporation.
molecular masses of the resultant polymers showed a clear
decreasing trend, but their polydispersity indexes (PDI) kept
narrow. All the polymers are completely soluble in 1,2,4-tri-
chlorobenzene and 1,1,2,2-tetrachloroethane at elevated
temperature.
species. Although seemingly, it is also possible to saturate
the bis-styrenic molecules by a double 2,1-insertion, the
structural analytical results have largely ruled it out. The
successive 2,1-/1,2-double insertions of DVB or BVPE can
only happen when a consecutive chain transfer to DVB/H2
or BVPE/H2 is followed by an in situ reinsertion of the
released styryl-terminated polymer chain into the newly
born M-H species. In fact, similar chain reinsertions have
Typical 1H NMR spectra of the polymers obtained (runs D-4
and B-4) are shown in Figure 1. The styryl moiety signals
are weak but clear. In association with the typical styrene/
H2 chain transfer polymerization behaviors, the styryl moiety
can be concluded from the chain end DVB or BVPE unit. In
contrast, at d ¼ 7.05 ppm, there appears a single aromatic
proton resonance signal with a much stronger intensity. This
signal should not be expected in polymers obtained with
DVB/H2 or with such a strong intensity in polymers with
BVPE/H2, which indicates double enchainment of DVB or
BVPE. The corresponding 13C NMR spectra (aliphatic section)
are shown in Figure 2. n-Propyl group corresponding with
styryl at the other chain end also exhibits weak resonances.
To the contrary, the 5-phenyl-2-methylpentyl sequence (d ¼
28.3, 30.5, 36.0, and 36.9 ppm) representing the enchain-
ment of DVB or BVPE displays noticeably stronger signals,
the situation being more pronounced with DVB. At the same
time, three resonances are observed at d ¼ 30.5, 33.0, and
38.8 ppm, which correspond to a 2-methyl-4-phenylbutyl
sequence that roughly closes the intensity gap between
5-phenyl-2-methylpentyl and n-propyl.
The above results have made it clear that although DVB and
BVPE can also combine with hydrogen to form chain transfer
complexes, they lead a different chain transfer reaction
(Scheme 1), which not only introduces a DVB or BVPE unit
to chain end but also incorporates more inside the chain in a
unique double enchainment mode. Based on the structure
analysis results, the double enchainment of DVB or BVPE
must have proceeded via first a 2,1-insertion into metal-poly-
mer bond and second 1,2-insertion into metal-hydride (M-H)
FIGURE 1 1H NMR spectra of polymers obtained with (A) DVB/
H2 (run D-4) and (B) BVPE/H2 (run B-4) chain transfer com-
plexes (solvent: 1,1,2,2-tetrachloroethane-d2; temp.: 110 ꢀC).
SYNTHESIS OF ISOTACTIC POLYPROPYLENES, HUANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 (a) Mn vs. [propylene]/[DVB] and (b) Mn vs. [propyl-
ene]/[BVPE] plots.
Obviously, the higher the ratio of rates ii/i, the more DVB or
BVPE being incorporated in-chain rather than at chain end. In
this regard, BVPE, with two methylene spacers intervening
between the two styrene moieties, would result in a much
detached styryl double bond from the M-H species in the tran-
sient (IV) compared to DVB, which would decrease rate ii
while increase rate i. This explains the higher chain end incor-
poration percentages of BVPE (30–57%) compared with DVB
(11–28%). In addition, BVPE is more reactive in polymeriza-
tion than DVB due to a released structural conjugation. The
combination has rendered BVPE with much higher chain
transfer efficiency than DVB, which is confirmed by the Mn vs.
[propylene]/[DVB (BVPE)] plots shown in Figure 3.
FIGURE 2 13C NMR spectra of polymers obtained with (A) DVB/
H2 (run D-4) and (B) BVPE/H2 (run B-4) chain transfer com-
plexes (solvent: o-dichlorobenzene-d4; temp.: 110 ꢀC).
been reported elsewhere.32,33 The double bond at chain ter-
minal is favored by the adjacent M-H due presumably to a
preemptive coordination.24 Intriguingly, the chain reinsertion
(rate ii) proceeds in a steric-favored regio-irregular 1,2- fash-
ion, which actually resumes chain propagation. A primary
styrene insertion into an M-H species of C2-symmetric ansa-
metallocene catalyst is no surprising at all since it has been
previously observed by Oliva and coworkers.22 At the same
time, although weak, competition for insertion into M-H by
propylene monomer (rate i) exists, with which a true chain
transfer process is completed and the styryl-terminated poly-
mer chain really released.
To further confirm the deduced chain transfer mechanism,
D2 was used to replace H2 in combination with DVB in a po-
lymerization reaction conducted under otherwise identical
conditions as run D-4. 13C NMR spectrum of thus obtained
polymer is compared with that of the prototypical D-4 in
Figure 4. Clearly, the resonances at d ¼ 33.0 and 36.0 ppm
corresponding to the two benzyl carbons transform to weak
multiplet signals, indicating a bonding with D2. At the same
time, a more careful examination of the spectrum reveals
that the rather weak resonance at d ¼ 20.0 ppm represent-
ing the n-propyl terminal weakens to an undetectable level.
All these results indicate that the majority of H2(D2) went to
the two benzyl groups of different chain sequences whereas
only a minor portion went to the n-propyl terminal, which is
in good agreement with the proposed reaction mechanism.
CONCLUSION
In conclusion, bis-styrenic molecules, DVB and BVPE, are
able to combine with H2 to induce a new chain transfer reac-
tion during metallocene-mediated isospecific propylene poly-
merization, which enables a catalytic synthesis of styryl-
capped i-PP. The chain transfer reaction takes place in a
unique way where, prior to the ultimate chain transfer, DVB/
H2 or BVPE/H2 cause a copolymerization-like reaction lead-
ing to the formation of main chain benzene rings. This is
SCHEME 1 Catalytic synthesis of styryl-capped i-PP.
attributed to
a
preemptive polymer chain reinsertion
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