Metallocene-mediated synthesis of chain-end functionalized polypropylene and application in PP/clay nanocomposites

Article abstract of DOI:10.1016/j.jorganchem.2005.04.031

This paper summarizes our research in the preparation of chain end functionalized isotactic polypropylene (PP) having a terminal functional group, such as Cl, OH, and NH₂. The chemistry involves metallocene-mediated propylene polymerization usi

Full text of DOI:10.1016/j.jorganchem.2005.04.031

Journal of Organometallic Chemistry 690 (2005) 6292–6299
www.elsevier.com/locate/jorganchem
Metallocene-mediated synthesis of chain-end functionalized
polypropylene and application in PP/clay nanocomposites
T.C. Chung *
Department of Materials Science and Engineering, The Pennsylvania State University, 325 Steidle Building, University Park, PA 16802, USA
Received 10 January 2005; revised 12 February 2005; accepted 18 April 2005
Available online 8 June 2005
Abstract
This paper summarizes our research in the preparation of chain end functionalized isotactic polypropylene (PP) having a terminal
functional group, such as Cl, OH, and NH . The chemistry involves metallocene-mediated propylene polymerization using rac-
2
Me
transfer agents. The molecular weight of the resulting PP polymers with a terminal Cl, OH and NH
2
Si[2-Me-4-Ph(Ind)]
2
ZrCl
2
/MAO complex in the presence of styrene derivatives (St-f) and hydrogen, which serve as the chain
group (i.e., PP-t-Cl, PP-t-OH
2
and PP-t-NH ) are inversely proportional to the molar ratio of [St-f]/[propylene]. Despite the extremely low concentration of func-
2
þ
tional group, the high molecular weight chain end functionalized PP-t-OH and PP-t-NH exhibit a distinctive advantage over other
þ
3
functional PP polymers containing side chain functional groups or long functional blocks. The terminal hydrophilic OH and NH
3
+
+
cations, with good mobility and reactivity, effectively hydrogen bond and ion-exchange the cations (Li , Na , etc.) located between
the clay interlayers, respectively. Such interactions anchor the PP chain to the clay surfaces. On the other hand, the remaining rest of
the unperturbed end-tethered high molecular weight PP tail exfoliates the clay layers. This exfoliated structure is maintained even
after further mixing of the PP-bearing platelets with pure neat PP polymers.
ꢀ
2005 Elsevier B.V. All rights reserved.
Keywords: Functionalization; Polypropylene; Metallocene; Nanocomposite; PP/clay composite
1
. Introduction
properties (such as melting temperature, crystallinity,
glass transition temperature, melt flow, etc.) that are al-
most the same as those of the pure polymer. Neverthe-
less, the terminal reactive group at the polymer chain
end has good mobility and can provide a reactive site
for many applications, such as adhesion to the sub-
strates [2] and reactive blending [3] to improve the com-
patibility of two dissimilar polymer blends. This chain
end functionalized polymer structure is particularly use-
ful in polyolefin, whose lack of functionality and poor
compatibility with other materials has imposed limita-
tions on its applications [4].
In chemistry, the preparation of chain end functional-
ized polymers has been largely limited to living
polymerization with the use of an initiator bearing a
functional group [5] or a control termination (function-
alization) reaction [6] of the living polymer chain end.
Ever since the discovery of Ziegler–Natta catalysts in
the early 1950s the functionalization of polyolefin (i.e.
PE, PP, EP, s-PS, etc.) has long been a scientifically
interesting and technologically important subject [1].
The lack of functionality in polyolefins, resulting in poor
compatibiltiy with other materials, has been a major
drawback in many applications, particularly those in
which adhesion, dyeability, printability, or compatibility
with other polymers is paramount.
The chain-end functionalized polymer is a very
attractive polymer structure that possesses an unper-
turbed polymer chain structure and desirable physical
*
Tel.: +1 814 863 1394; fax: +1 814 865 2917.
E-mail address: chung@ems.psu.edu.
0
022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.031

T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
6293
Unfortunately, there are only a few Ziegler–Natta type
St-Cl
St-OSi
CH2=CH
St-NSi
CH =CH
2
CH =CH
2
transition metal coordination catalysts that exhibit
living polymerization behavior, and most of them are
limited to the preparation of polyethylene and poly-
2
O
(CH2)2
Cl
(
1-hexene) cases [7]. Furthermore, living polymerization
SiR3
N
only produces one polymer chain per initiator, which
presents a relatively low rate of catalyst activity in the
typical polyolefin preparation.
SiR3 SiR3
Scheme 1.
In our previous papers [8], we have reported a new
chemical route to prepare polyolefin (including PE,
PP, s-PS, etc.) with a terminal reactive group and obtain
high catalyst activity. The chemistry is based on a chain
transfer reaction involving a reactive chain transfer
especially benefits from the very small quantity of the
chain transfer agent needed in the preparation of high
polymers. Therefore, the additional protection–depro-
tection step causes almost no change in the polymeriza-
tion conditions and procedures.
agent, including dialkylborane (H-BR ) [9] and p-methyl-
2
styrene/hydrogen (p-MS/H ) [10], during metallocene-
2
Eq. (1) illustrates the general reaction scheme. During
the course of propylene 1,2- insertion, the propagating
mediated olefin polymerization. All polymers formed
contain a terminal borane or p-MS group and have
a relatively narrow molecular weight distribution
+
M –C site (II) eventually reacts with a St-NSi unit
2
(
k ) (via 2,1-insertion) to form a St-NSi capped propa-
12 2
(
M /M ꢀ 2). The polymer molecular weight (from a
w
n
gating site (III) with an adjacent phenyl group interact-
ing with metal cation. The new propagating site (III) is
incapable of continuing the insertion of St-NSi (k )
few thousand to a hundred thousand) was basically
controlled by the mole ratio of [chain transfer agent]/
2
22
[
olefin]. Both the terminal borane and p-MS groups
or propylene (k ) due to the steric jamming. However,
2
1
are very versatile, and can serve as the reactive sites
for subsequent functionalization reactions or conversion
to living initiators for chain extension reactions.
it can react with hydrogen to complete the chain transfer
reaction. This consecutive reaction with St-NSi and
2
hydrogen results in a PP-t-St-NSi polymer chain (IV)
2
and a regenerated Zr-H species (I) reinitiates the poly-
merization of propylene and continues the polymeriza-
tion cycles. After completing the polymerization, the
desirable NH terminal group in PP-t-NH (V) can be
easily recovered during the sample work-up step using
HCl aqueous solution. Theoretically, the PP-t-NH2
molecular weight should be linearly proportion to the
molar ratio of [propylene]/[St-NSi2].
2
. Synthesis of PP-t-Cl, PP-t-OH and PP-t-NH2
2
2
It is very desirable to extend this chemical route to di-
rectly prepare polyolefin with a desirable terminal func-
tional group, such as Cl, OH, and NH . In other words,
2
the ideal reaction is a one-pot in situ polymerization
process [11], and no chain end functionalization would
be needed after the polymerization reaction. The basic
idea in the direct preparation of the chain functionalized
PP was to use a functionalized styrenic chain transfer
agent (St-f) as the chain transfer agent that could engage
in similar metallocene-mediated propylene polymeriza-
tion/chain transfer reaction under some reaction condi-
tions. Three St-f molecules were investigated, including
p-chlorostyrene (St-Cl), silane-protected p-vinylphenol
L
CH3
Zr-(CH -CH)
+
-
A
CH2=CH
2
x
L
(II)
CH3
x CH =CH
2
k12
(CH2)
2
k11
2
,1-insertion
N
SiR3 SiR3
L
+
Zr
L
+
CH3
-
A
H
(I)
-
A
Zr-CH-CH2-(CH2-CH)x
L
L
Cat: {SiMe2[2-Me-5-Ph(Ind)]2}ZrCl2/MAO
(III)
H2
(CH2)2
N
(
(
St-OSi) and silane-protected p-ethylaminostyrene
St-NSi ), as illustrated in Scheme 1. The silane groups
k2, hydrogen
2
SiR3 SiR3
provide effective protection for both the OH and NH2
functional groups during the metallocene catalysis, and
they can be completely deprotected by aqueous HCl
solution during the sample work-up procedure.
CH3
SiR3
SiR3
(IV)
(CH-CH ) -CH -CH -
(CH ) -N
2 2
2
x
2
2
HCl (in work-up step)
No external protection agent is needed for St-Cl in
metallocene/MAO systems. However, the metallocene
cationic site is very sensitive to the OH and NH func-
CH3
CH-CH ) -CH -CH -
(
(CH ) -NH
2 2 2
(V)
2
x
2
2
2
ð1Þ
Table 1 summarize the experimental results involving
tional groups. The silane groups not only provide effec-
tive protection for both the OH and NH functional
2
groups during the metallocene catalysis, but also can
be completely deprotected by aqueous HCl solution dur-
ing the sample work-up procedure. The overall reaction
St-Cl/H , St-OSi/H , and St-NSi /H , respectively, in
2
2
2
2
rac-Me Si[2-Me-4-Ph(Ind)] ZrCl /MAO catalyzed poly-
2
2
2
merization of propylene. In all control reactions, a small

6294
T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
Table 1
a
2
A summary of PP-t-Cl, PP-t-St-OH and PP-t-St-NH polymers
b
St-f (mol/l)
c
ꢁ3
)
Run
H
2
(psi)
Yield (g)
Catalyst activity
Cl, OH or NH
2
in PP (mol%)
M
n
(·10
PDI (M
w
n
/M )
T
m
(ꢁC)
A-1
A-2
A-3
A-4
B-1
B-2
B-3
B-4
C-1
C-2
C-3
C-4
D-1
D-2
D-3
D-4
E-1
E-2
E-3
E-4
F-1
F-2
F-3
a
0.144
0.144
0.144
0.144
0.289
0.289
0.289
0.289
0.198
0.198
0.198
0.198
0.396
0.396
0.396
0.396
0.125
0.125
0.125
0.125
0.250
0.250
0.250
0
ꢀ0
ꢀ0
4402
24192
70435
ꢀ0
–
–
–
–
6
12
20
0
1.36
7.56
21.8
ꢀ0
0.13
0.13
0.12
–
45.3
47.5
46.1
–
1.9
2.0
2.1
–
159.0
158.2
158.1
–
6
1.24
4.44
12.91
ꢀ0
4388
15712
45673
ꢀ0
0.23
0.23
0.22
–
18.5
19.1
18.7
–
2.2
2.1
2.1
–
156.8
157.1
156.9
–
14
20
0
6
0.36
2.36
8.34
ꢀ0
1165
7637
26995
ꢀ0
0.09
0.08
0.10
–
52.1
53.4
52.5
–
2.3
2.4
2.2
–
159.1
158.4
158.2
–
12
20
0
6
ꢀ0
ꢀ0
–
–
–
–
12
20
0
0.63
2.57
ꢀ0
2039
8318
ꢀ0
0.18
0.20
–
23.4
22.0
–
2.5
2.0
–
157.4
156.3
–
6
0.41
2.62
9.78
ꢀ0
1327
8480
31655
ꢀ0
0.07
0.06
0.08
–
56.3
55.4
58.9
–
2.1
2.5
2.3
–
159.1
158.4
158.2
–
12
20
0
12
20
0.81
3.11
2622
10066
0.18
0.19
25.3
24.2
2.2
2.3
156.3
155.9
ꢁ6
Reaction conditions: 50 ml toluene; catalyst: rac-Me
2
Si[2-Me-4-Ph(Ind)]
2
ZrCl
2
/MAO catalyst, [Zr] = 1.25 · 10 mol, [MAO]/[Zr] = 3000, pro-
pylene = 100 psi, temperature = 30 ꢁC, time = 15 min.
St-f: St-Cl for runs A and B; St-OSi for runs C and D, St-NSi
b
2
for runs E and F.
c
Catalyst activity = kg of PP/mol of catalyst Æ h.
7
0000
amount of any styrene derivative (St-f) effectively stops
the polymerization of propylene. The introduction of
hydrogen gradually restores the catalyst activity. Over-
all, the PP molecular weight is governed by the chain
transfer agent – the higher the concentration of the
St-f, the lower the molecular weight of the resulting
polymer.
60000
50000
c
b
a
40000
Fig. 1 shows the plot of the polymer molecular weight
(
ing three St-Cl/H , St-OSi/H , and St-NSi /H chain
M ) versus the molar ratio of [propylene]/[St-f], includ-
n
30000
2
2
2
2
transfer agents. In general, the polymer molecular
weight and molar ratio of [propylene]/[St-f] are linearly
proportional. It is clear that the chain transfer reaction
2
0000
10000
to St-f (with rate constant k ) is the dominant termina-
tr
tion process, and that it competes with the propagating
reaction (with rate constant k ). The degree of polymer-
0
p
0
10
20
30
[propylene] / [St-f]
mol / mol)
40
50
60
ization (X ) follows a simple comparative equation
n
(
X = k [olefin]/k [p-MS] with a chain transfer constant
p
n
tr
k /k of 1/21 for St-Cl/H , 1/48 for St-OSi/H , and 1/
tr
p
2
2
Fig. 1. The plots of number average molecular weights (M ) of PE-t-St-f
n
3
k /k values are significantly lower than those seen in
4 for St-NSi /H , respectively. It is intriguing that the
polymers vs. the mole ratio of [propylene]/[St-f] using: (a) St-Cl,
2
2
2
(b) St-OSi, and (c) St-NSi , respectively.
tr
p
styrene and p-MS cases [11] under similar reaction con-
ditions. The bulky protected functional groups may
reduce the frequency of chain transfer reaction. Poly-
mers with very low molecular weights (a few thousand)
have been obtained, and the molecular weight distribu-
tions are quite narrow, which are generally consistent
with single site polymerization processes. Some low
molecular weight polymers with exceptionally low Mw/
M < 2 (despite extra efforts to recover all polymers)
may be associated with the sensitivity of GPC at very
low molecular range.
The terminal functional group at the polymer chain
end provides direct evidence for the chain transfer
n

T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
6295
1
reaction. Fig. 2 shows the H NMR spectra (with inset
of magnified region and chemical shift assigments) of
PP-t-St-OSi polymer and the corresponding PP-t-St-
OH. In addition to three major peaks (d = 0.95, 1.35,
and 1.65 ppm) for the CH CH , and CH groups in
3
,
2
the PP backbone, there are three minor chemical shifts
at 0.25, 2.61, and 6.75–7.18 ppm (with an intensity ratio
near 6/2/4) shown in Fig. 2(a), corresponding to
–
OSi(CH ) (t-Bu), –CH -/, and –CH –C H –OSi, res-
3 2 2 2 6 4
pectively. The chemical shift for the silane protecting
group completely disappears in Fig. 2(b), indicating
the occurrence of a very effective deprotection reaction.
The equally split chemical shifts for the phenyl protons,
combined with no detectable side product, further indi-
cate the terminal p-alkylphenol moiety.
1
Fig. 3 compares the H NMR spectra of PP-t-St-NSi
2
3
polymer (M = 24.2 · 10 ; M /M = 2.3) and the corres-
n
w
n
ponding PP-t-St-NH . In addition to the chemical shifts
2
for PP polymer, Fig. 3(a) shows all of the chemical shifts
associated with the protecting bis(trimethylsily)amino
terminal group that is connected to the symmetrical p-
dialkylbenzene moiety. In fact, all four phenyl protons
merge into a single chemical shift at 7.22 ppm. Fig.
3(b) shows an almost identical spectrum, except for
the disappearance of the silane protecting group at
0
.24 ppm.
1
Fig. 3. H NMR spectra of (a) a PP-t-St-NSi
corresponding PP-t-St-NH (M
vent: C Cl
; temperature: 110 ꢁC).
2
polymer and (b) its
2
n w n
= 24,200 g/mol, M /M = 2.3) (sol-
2
D
2
4
The existence of a terminal functional group in PP
was further evidenced by chain extension reaction using
the terminal functional group as the reaction site for
coupling reaction with polyester and polyamide. Over-
all, the combined experimental results strongly indicate
a clean and effective reaction scheme, as illustrated in
Eq. (1). The combination of the facile in situ chain trans-
fer to St-f/H during the catalytic polymerization of pro-
2
pylene and the subsequent complete deprotection
reaction during the sample work-up step affords a very
interesting reaction scheme in the preparation of the
chain end functionalized i-PP with a Cl, OH, and NH2
terminal group via a one-pot reaction process.
3. PP/clay nanocomposites
The availability of a broad range of well-defined
functionalized PP polymers provides us with a great
advantage in evaluating their applications in PP/clay
nanocomposites [12]. The most desirable ammonium
þ
ꢁ
1
Fig. 2. H NMR spectra of (a) a PP-t-St-OSi polymer and (b) its
group terminated i-PP ðPP-t-NH Cl Þ polymers were
3
prepared by simple mixing between PP-t-NH and HCl.
2
+
Both pristine Na -montmorillonite clay (Na -mmt), with
corresponding PP-t-St-OH. (M
n
= 22000 g/mol, M
w
/M
n
= 2.0) (sol-
+
vent: C Cl
2
D
2
4
; temperature: 110 ꢁC).

6296
T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
an ion-exchange capacity of ca. 0.95 mequiv/g, and a di-
octadecylammonium-modified montmorillonite organo-
philic clay (2C18-mmt) were obtained from Southern
Clay Products. Static melt intercalation was employed
to prepare all PP/clay nanocomposites.
posite under various application conditions. The similar
results were also observed in other chain functionalized
polyolefin cases. The featureless XRD pattern of the
melt blending sample clearly shows the exfoliated PP-
t-OH/2C18-mmt nanocomposite structure, implying
that PP-t-OH is also a good interfacial agent for exfoli-
Fig. 4 compares the X-ray diffraction (XRD) patterns
before and after static annealing of a physical mixture
ating the clay interlayers.
þ
(
(
90/10 weight ratio) of an ammonium terminated i-PP
ꢁ
The binary PP-t-NH =mmt hybrid was subsequently
further mixed/blended (at a 50/50 weight ratio) with a
3
þ
PP-t-NH Cl ; M = 58900 and M = 135500 g/mol;
n
3
w
+
T = 158.2 ꢁC) and a pristine Na -mmt clay. Namely,
neat –unfunctionalized– i-PP (M = 110000 and M =
n
m
w
þ
ꢁ
simple mixing of dried PP-t-NH Cl powder and
250000 g/mol). Fig. 5 shows the XRD patterns of (a)
the physical mixture and (b) the melt blending, of the
3
+
Na -mmt, ground together by mortar and pestle at
þ
ambient temperature, creates the XRD pattern in Fig.
(a), with a (001) peak at 2h @ 7ꢁ, corresponding to
exfoliated PP-t-NH =mmt structures with neat i-PP.
The exfoliated structure is maintained after further mix-
ing with i-PP, which is compatible (co-crystallizable)
3
4
+
the characteristic Na -mmt d-spacing of ca. 1.26 nm.
The mixed powder was then heated – annealed under
þ
with the backbone of the largely-isotactic PP-t-NH3
polymer, as also directly observed in the TEM image
static conditions – at 190 ꢁC for 2 h under vacuum.
þ
The resulting PP-t-NH =mmt hybrid shows a feature-
(inset in Fig. 5). Apparently, the i-PP polymer chains
þ
3
less XRD pattern in Fig. 4(b), indicating the formation
of an exfoliated clay structure, which corresponds to the
thermodynamically stable state, as the ammonium-
largely serve as diluents in the ternary PP-t-NH =
mmt=i-PP system, with the thermodynamic stable
3
þ
PP-t-NH =mmt exfoliated structure dispersed in the
i-PP matrix [13].
3
+
terminated PP exchange the alkali (Na ) cations at the
mmt surfaces.
For comparison, several functionalized PP polymers,
containing randomly distributed functional groups in
the side chains or lumped together in a block-copolymer
microstructure, were also evaluated in PP/montmorillon-
ite nanocomposites. Similar static melt-intercalation
procedures were followed, except for employing alkyl-
Similar featureless patterns were also observed in the
ꢁ
þ
þ
PP-t-NH Cl =2C H -2CH -N -mmt nanocomposite
18 37
3
3
cases, suggesting that the dioctadecylammonium cations
in organophilic clay may also be ion-exchanged by
þ
þ
PP-t-NH3 to form a similar PP-t-NH =mmt structure.
It is clear that an organic surfactant is not needed to
3
þ
ꢁ
promote compatibility between PP-t-NH Cl and pris-
+
tine Na -mmt clay. Beyond any economic benefits, such
3
an elimination of the mmtꢀs organic surfactant, this also
offers some significant materials advantages. For exam-
ple, it eliminates two major concerns relating to the ther-
mal stability of the surfactant during high temperature
melt processing and to the long-term stability of the
small organic surfactant in the polymer/clay nanocom-
Fig. 5. X-ray diffraction patterns of the 50/50 mixture by weight of
þ
exfoliated PP-t-NH =mmt structure (90/10 weight ratio) and neat
3
–
physical mixture of PP-t-NH =mmt and i-PP and (b) the same mixture
unfunctionalized– i-PP. The XRD traces shown correspond to (a) the
þ
þ
ꢁ
þ
Fig. 4. X-ray diffraction patterns of PP-t-NH Cl =Na -mmt (90/10
weight ratio) (a) physical mixture by simple powder mixing at ambient
3
3
after static melt-intercalation. Bright-field TEM image of the melt-
þ
temperature and (b) the same mixture after static melt-intercalation
þ
mixed ternary PP-t-NH =mmt=PP system ((b) in the XRD) shows the
exfoliated montmorillonite structure.
3
(
PP-t-NH =mmt hybrid).
3

T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
6297
ammonium-modified montmorillonites (2C18-mmt for
all random copolymers, and C18-mmt for the block
copolymer). XRD patterns are shown of four nanocom-
posites made with 6 wt% of 2C18-mmt clay and 94 wt%
block copolymer PPs form multiple contacts with each
of the clay surfaces, as illustrated in Fig. 6(b), which
not only results in aligning the polymer chains parallel
to the clay surfaces, but can also bridge consecutive clay
platelets promoting intercalated structures, especially
for the higher lateral size montmorillonites.
of three side-chain functionalized PPs (M = 95000 and
n
M = 210000 g/mol) containing: (a) 1 mol% p-methyl-
w
styrene, (b) 0.5 mol% maleic anhydride, and (c)
0
.5 mol% hydroxy side groups, respectively, and the ori-
ginal 2C18-mmt clay. All the functionalized PPs were
derived from the same random PP copolymer synthe-
sized by metallocene catalysis, which contained 1 mol%
p-methylstyrene (p-MS) comonomer. Subsequently, the
p-MSꢀs were selectively functionalized towards hydroxy
4. Experimental
All manipulations were carried out under inert atmo-
1
sphere or by using standard Schlenk technique.
H
NMR was measured on a Bruker AM-300 or WM-360
spectrometer with DISNMR software. The measure-
(
OH) and maleic anhydride, respectively, without chang-
ing the PP backbone. In addition, we also compared the
XRD pattern of PP–b-PMMA block copolymer/6 wt%
C18-mmt which contains a 5 mol% methyl-methacrylate
block. These XRD patterns clearly show that there is a
definite intercalated structure for all the side chain
functionalized PP cases, manifesting itself through an
interlayer d-spacing increase of about 1 nm compared
to that of the parent alkyl-ammonium-mmt. TEM was
also employed to examine the degree of exfoliated (disor-
dered) structure, and the quantitative image analysis
indicates only between 20% and 40% exfoliated layers
in all four systems [14].
ments were usually taken at 120 ꢁC using d -1,1,2,2-tet-
2
rachloroethane as solvent. The molecular weight and
molecular weight distribution of the polymers were
determined by gel permeation chromatography (GPC)
using a Waters 150 C with a refractive index (RI) detec-
tor. The measurements were taken at 140 ꢁC using 1,2,4-
trichlorobenzene (TCB) as solvent and a mobile phase
of 0.7 ml/min flow rate. Broad molecular weight PE
samples were used as standards for calibration. Differen-
tial scanning calorimetry (DSC) was measured on a Per-
kin–Elmer DSC-7 instrument controller. The DSC
curves were recorded during the second heating cycle
from 30ꢁ to 180 ꢁC with a heating rate of 20 ꢁC/min.
XRD data were collected on a diffractometer using Cu
Ka radiation (k = 0.1506 nm). Braggꢀs law (k = 2dsinh)
was used to calculate the spacing. Direct observation
of the PP/clay nanocomposite structure was realized
by bright field TEM of nanocomposite films under
strain in a JEOL ꢁ1200EX operating at 120 kV. Ultra-
thin sections of the PP/clay with a thickness of approx-
imately 50 nm were prepared at room temperature using
an ultramicrotome equipped with a diamond knife. The
sections were transferred dry to carbon-coated Cu grids
of 200 mesh.
The experimental results clearly demonstrate the
þ
advantage of chain-end-functionalized PP ðPP-t-NH Þ
3
that seems to adopt a unique molecular structure atop
of the clay surfaces and results in an exfoliated montmo-
rillonite structure (Fig. 6(a)). The terminal hydrophilic
þ
NH functional group anchors the PP chains (via ion-
3
exchange) on the inorganic surfaces, and the hydropho-
bic high molecular weight and semi-crystalline PP ‘‘tail’’
effectively exfoliates the clay platelets, an exfoliated
structure which is maintained even after further mixing
with neat PP. In contrast, side-chain functionalized or
Me Si(2-methyl-4-phenyl-indenyl) ZrCl
2
(Boulder
2
2
Scientific Co.) was used as received. MAO (Albe-
marle) in 30 wt% of solution in toluene was isolated
by vacuum removal of toluene and re-compounded
to 2.5 M toluene solution prior to use. CP grade pro-
pylene gas (MG Industries) was dried over CaH prior
2
to use. CP grade toluene, hexane, and THF were
deoxygenated by argon purge before refluxing for
48 h and then distilled over sodium benzophenone
right before use.
4
(
.1. Synthesis of 4-(t-butyldimethylsilyloxy)styrene
St-OSi)
4-(t-Butyldimethylsilyloxy)styrene was synthesized in
two steps. In a 500 ml flask equipped with a magnetic
stirring bar, 70.4 g (1.02 mol) of imidazole was mixed
with 52.4 g (0.42 mol) of 4-hydroxybenzaldehyde and
Fig. 6. Illustration of the molecular structures of (a) chain end
functionalized and (b) side chain functionalized polyolefin located
between clay interlayers.

6298
T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
7
7.4 g (0.52 mol) of t-butyldimethylsilyl chloride in THF
Ph(Ind)] ZrCl /MAO catalyst and p-NSi -St and H as
2 2 2 2
solution. The mixture was stirred at ambient temperature
for 4 h before being poured into cold water. The organic
layer was separated and extracted with ether, then dried
with magnesium sulfate. About 94 g of 4-(t-butyldimeth-
ylsilyloxy)benzaldehyde (90% yield) was obtained after
evaporating the solvent. The second reaction step was
performed under a nitrogen atmosphere. In a 500 ml
flask equipped with a magnetic stirring bar, 123.6 g
the chain transfer agents. A systematic study was
conducted to evaluate the effects of p-NSi -St and H con-
centrations on the catalyst activity and polymer molecu-
2
2
lar weight. The PP-t-NH polymers were then prepared
2
from PP-t-St-NSi polymers by treating them with hydro-
2
gen chloride, which can be accomplished during the sam-
ple work-up step. Alternatively, the isolated PP-t-St-NSi2
(2 g) was suspended in 50 ml of THF at 50 ꢁC before add-
ing dropwise 2 N methanolic hydrogen chloride solution.
The mixture was stirred for 4 h at 50 ꢁC, and then poured
(0.38 mol) of methyltriphenylphosphonium bromide sus-
pended in THF was treated with 149.6 ml (0.19 mol) of
n-butyllithium(2.5 M in hexane). After 1 h, 80.0 g
(
into 1 N methanolic NaOH solution. The PP-t-NH poly-
2
0.34 mol) of 4-(t-butyldimethylsilyloxy)benzaldehyde
mer was collected by filtration and washed with 1 M aque-
ous ammonia and water under a nitrogen atmosphere.
The polymer was dried overnight at 50 ꢁC under vacuum.
was introduced dropwise into the red solution. The mix-
ture was stirred overnight at room temperature and then
poured into cold water. The organic layer was separated
by ether extraction and dried with magnesium sulfate.
Further purification was performed by distillation under
vacuum (10 Torr) at elevated temperature (80 ꢁC). 65 g
of 4-(t-butyldimethylsilyloxy)styrene was obtained with
a yield of more than 90%.
The PP-t-NH polymer yield was quantitative. Overall,
2
the PP-t-NH molecular weight is governed by the chain
2
transfer agent – the higher the concentration of the
p-NSi -St, the lower the molecular weight of the resulting
2
polymer. It is clear that the chain transfer reaction to
p-NSi -St (with rate constant k ) is the dominant termi-
2
tr
nation process, and that it competes with the propagating
reaction (with rate constant k ). The degree of polymeri-
4.2. Synthesis of 4-{2-[N,N-bis(trimethylsilyl)amino]-
ethyl}styrene (p-NSi -St)
p
zation (X ) follows a simple comparative equation
n
2
X = k [olefin]/k [St-NSi ] with a chain transfer constant
2
n
p
tr
1
A silane protected chain transfer agent p-NSi -St was
2
of k /k = 1/34. H NMR spectra of PP-t-St-NSi poly-
tr p 2
prepared in two steps. In a 500 ml flask equipped with a
magnetic stirring bar, 100 g of lithium bis(trimethylsi-
lyl)amide dissolved in 200 ml of THF was slowly added
into a mixture of 50 ml (0.658 mol) of chloromethyl
methyl ether and 50 ml of THF at 0 ꢁC under a nitrogen
atmosphere. After completing the addition, the solution
was allowed to warm up to room temperature for 2 h be-
fore evaporating the excess chloromethyl methyl ether
and THF solvent. N,N-Bis(trimethylsilys)methoxymeth-
ylamine (80% yield) was isolated by distillation. In the
mer and the corresponding PP-t-St-NH2 show clean
deprotection reaction with the complete disappearance
of the silane protecting group at 0.24 ppm.
þ
ꢁ
þ
4.4. Preparation of PP=PP-t-NH Cl =Na -
montmorillonite clay nanocomposite
3
+
+
Na -montmorillonite clay (Na -mmt) with an ion-
exchange capacity of ca. 95 mequiv/100 g (WM) was
þ
ꢁ
obtained from Southern Clay Product. PP-t-NH Cl
(T = 158.2 ꢁC; M = 58,900 and M = 135,500 g/mol)
3
second step, p-NSi -St was prepared by treating 4-
2
m
n
w
vinylbenzylmagnesium chloride with N,N-bis(trimeth-
ylsilys)methoxymethylamine. In a 500 ml flask equipped
with a magnetic stirring bar and a condenser, 15.2 g of
magnesium was suspended in 50 ml dry ether, and
was prepared using excess HCl reagent. Static melt inter-
ꢁ
þ
þ
calation was employed to prepare PP-t-NH Cl =Na -
ꢁ
3
þ
montmorillonite nanocomposite. PP-t-NH Cl dried
3
+
powder and Na -mmt with 90/10 weight ratio were first
8
0 ml of 4-vinylbenzyl chloride diluted with 50 ml dry
mixed and ground together in a mortar and pestle at
ambient temperature. The XRD pattern of this simple
mixture shows a (001) peak at 2h ꢀ 7, corresponding
ether was then introduced dropwise through the con-
denser. The solution was refluxed for 4 h before the
addition of 117 g of N,N-bis(trimethylsilys)methoxy-
methylamine over a period of 2 h. The reaction was al-
lowed to proceed at room temperature for another 2 h
before adding 100 ml of aqueous NaOH solution
+
to Na -mmt interlayer structure with a d-spacing of
1.45 nm. The mixed powder was then heated at 190 ꢁC
for 2 h under nitrogen condition. The resulting
þ
ꢁ
þ
PP-t-NH Cl =Na -mmt nanocomposite shows a fea-
tureless XRD pattern, indicating the formation of an
exfoliated clay structure.
3
(30%). The organic layer was separated and dried, and
the residual was then distilled over CaH2 to obtain
þ
ꢁ
þ
p-NSi -St with 70% yield.
2
The binary PP-t-NH Cl =Na -mmt exfoliated nano-
composite was further melt mixed (50/50 weight ratio)
3
4
.3. Synthesis of NH group terminated PP (PP-t-NH )
2
with commercial neat i-PP (M = 110000 and M =
n
2
w
þ
ꢁ
þ
2
50000 g/mol). First, the PP-t-NH Cl =Na -mmt exfo-
3
The p-NSi -St terminated PP polymers (PP-t-St-NSi )
2
liated nanocomposite and neat i-PP with 50/50 weight
ratio were ground together in a mortar and pestle at
2
were prepared by the combination of rac-Me Si[2-Me-4-
2

T.C. Chung / Journal of Organometallic Chemistry 690 (2005) 6292–6299
6299
ambient temperature. This simple mixture shows a fea-
tureless XRD pattern. The mixed powder was then
heated at 190 ꢁC for 2 h under nitrogen condition. The
[2] (a) D. Napper, Polymeric stabilization of colloidal dispersions,
Academic Press, London, 1983;
(
(
b) A. Gast, L. Leibler, Macromolecules 19 (1986) 686;
c) L.H. Lee, Adhesion and adsorption of polymers, Plenum
þ
ꢁ
þ
resulting ternary PP=PP-t-NH Cl =Na -mmt nano-
3
Press, New York, 1980.
composite also shows a featureless XRD pattern, indi-
cating that the stable exfoliated structure in the binary
[3] (a) C. Koning, M. Van Duin, C. Pagnoulle, R. Jerome, Prog.
Polym. Sci. 23 (1998) 707;
(b) J. Noolandi, K.M. Hong, Macromolecules 15 (1982) 482;
þ
ꢁ
þ
PP-t-NH Cl =Na -mmt exfoliated nanocomposite is
clearly maintained after further mixing with PP that is
3
(c) S.T. Milner, H. Xi, J. Rheol. 40 (1996) 663.
4] T.C. Chung, ChemTech 27 (1991) 496.
[
þ
ꢁ
compatible with the backbone of PP-t-NH Cl .
3
[5] (a) C.H. Bamford, A.D. Jenkins, R.P. Wayne, Trans. Faraday
Soc. 56 (1960) 932;
(b) H.C. Haas, N.W. Schuler, H.S. Kolesinski, J. Polym. Sci. Part
A1 5 (1967) 2964.
6] (a) H. Brody, D.H. Richards, M. Szwarc, Chem. Ind. (1958)
5
. Conclusion
[
[
1
473;
b) K. Hayashi, C.S. Marvel, J. Polym. Sci. Part A1 2 (1974)
571;
We have shown a one-pot process to prepare chain
(
end functionalized PP polymers, involving metallo-
cene-mediated propylene polymerization/chain transfer
reaction. With the proper choice of reaction conditions,
chain transfer agents, and catalyst system, it is very con-
venient and efficient (with high catalyst activity) to pre-
pare i-PP with a terminal styrenic unit containing either
2
(c) M. Morton, L.J. Fetters, J. Inomater, D.C. Rubio, R.N.
Young, Rubber Chem. Technol. 49 (1976) 303.
7] (a) H. Yasuda, M. Furo, H. Yamamoto, A. Nakamura, S. Miyake,
N. Kibino, Macromolecules 25 (1992) 5115;
(b) M. Brookhart, J.M. Desimone, B.E. Grant, M.J. Tanner,
Macromolecules 28 (1995) 5378;
an OH or NH group. This reaction scheme is especially
2
(c) K.J. Shea, J.W. Walker, H. Zhu, M. Paz, J. Greaves, J. Am.
Chem. Soc. 911 (1997) 9049;
useful in the preparation of chain-end functionalized PP
with high molecular weight. Almost the same reaction
procedures used in regular PP polymerization can be di-
rectly applied, except for the addition of a small amount
of chain transfer agent. In turn, the PP polymer with a
reactive functional group is a useful interfacial material
that can be used as polymeric surfactant to prepare the
exfoliated PP/clay nanocomposites.
(
(
d) G. Desurmont, T. Tokimitsu, H. Yasuda, Macromolecules 33
2000) 7679.
[
[
8] T.C. Chung, Prog. Polym. Sci. 27 (2002) 39.
9] (a) G. Xu, T.C. Chung, J. Am. Chem. Soc. 121 (1999) 6763;
(b) G. Xu, T.C. Chung, Macromolecules 32 (1999) 8689;
(c) T.C. Chung, G. Xu, Yingying Lu, Youliang Hu, Macromol-
ecules 34 (2001) 8040.
[
[
10] (a) T.C. Chung, J.Y. Dong, J. Am. Chem. Soc. 123 (2001)
871;
b) J.Y. Dong, T.C. Chung, Macromolecules 35 (2002) 1622.
11] (a) J.Y. Dong, Z.M. Wang, H. Han, T.C. Chung, Macromolecules
5 (2002) 9352;
b) T.C. Chung, J.Y. Dong, U.S. Patent 6,479,600 (2002).
4
(
Acknowledgments
3
(
[
[
12] Z.M. Wang, H. Nakajima, E. Manias, T.C. Chung, Macromol-
ecules 36 (2003) 8919.
13] (a) R. Krishnamoorti, E.P. Giannelis, Langmuir 17 (2001)
The authors would like to thank the Office of Naval
Research and the National Institute of Standards and
Technology for their financial support.
1
448;
b) R. Krishnamoorti, J.X. Ren, A.S. Silva, J. Chem. Phys.
14 (2001) 4968;
c) J.X. Ren, B.F. Casanueva, C.A. Mitchell, R. Krishnamoorti,
Macromolecules 36 (2003) 4188.
(
1
References
(
[
1] T.C. Chung, Functionalization of Polyolefins, Academic Press,
London, 2002.
[14] E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T.C.
Chung, Chem. Mater. 13 (2001) 3516.