Microstructure of polypropylene and active center in Ziegler-Natta catalyst: Effect of novel salicylate internal donor

Article abstract of DOI:10.1039/c6ra13819e

Five salicylates with different sizes of hydrocarbon substituents were firstly synthesized and employed as ecofriendly internal donors of the Ziegler-Natta catalyst for propylene polymerization. The influences of these salicylates and traditional, industrial, internal donor diisobutyl phthalates on the microstructure of polypropylene and active center in a Ziegler-Natta catalyst were studied. It was found that the catalyst activities of the catalysts containing salicylate internal donors with a proper volume were higher than the catalysts containing diisobutyl phthalate internal donors. GPC results showed that the molecular weights of polypropylene prepared by salicylate internal donors were lower than those prepared by diisobutyl phthalate, which indicated that the polypropylene chains produced by salicylate internal donors were easier to transfer than those prepared by diisobutyl phthalate internal donors. Deconvolution of the GPC curves exhibited that as the volume of the salicylate internal donor increased some of the active centers for low molecular weight transferred into the active centers for high molecular weight. The results of ¹³C-NMR and SSA both suggested that a salicylate internal donor with an appropriate catalyst size volume was beneficial for increasing the isotactic sequence length, isotacticity index and regular triads "mm" of polypropylene. However, further increasing the volume of the salicylate internal donor in a catalyst would lead to the polypropylene chain containing more stereo-defects. Moreover, the active centers with different stereospecificity parameters, p_iso, in the catalyst could explain the trend of stereo-defects in polypropylene chains when different internal donors were used. In addition, it was found that the isotactic sequence length and isotacticity index of polypropylene prepared by isobutyl 2-benzyloxy-3,5-isopropyl benzoate were close to that produced by a diisobutyl phthalate internal donor. Moreover, the lamella thickness distribution of the polypropylene produced by a salicylate internal donor was broad, which might have potential application for expanded polypropylene materials.

Full text of DOI:10.1039/c6ra13819e

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Microstructure of polypropylene and active center in Ziegler-
Natta catalyst: Effect of novel salicylate internal donor
Qian Zhou,a Tao Zheng,a Huayi Li,*, b Zhi Luo,a Ailian Wang,a Liaoyun Zhang,*,a and Youliang Hub
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Abstract: Five salicylates with different size of hydrocarbon substituents were firstly synthesized and employed as
ecofriendly internal donors of Ziegler-Natta catalyst for propylene polymerization. The influences of these salicylates and
traditional industrial internal donor diisobutyl phthalate on the microstructure of polypropylene and active center in
Ziegler-Natta catalyst were both studied. It was found that the catalyst activities of catalyst containing salicylate internal
donors with proper volume were higher than that catalyst containing diisobutyl phthalate internal donor. GPC results
showed that the molecular weights of polypropylene prepared by salicylate internal donors were all lower than that
prepared by diisobutyl phthalate, which indicated that polypropylene chains produced by salicylate internal donors were
easier to transfer than that prepared by diisobutyl phthalate internal donor. Deconvolution of GPC curves exhibited that as
increasing the volume of salicylate internal donor, some of active center for low molecular weight transferred into active
center for high molecular weight. The results of 13C-NMR and SSA both suggested that the salicylate internal donor with
appropriate size volume in catalyst was beneficial to increasing isotactic sequence length, isotacticity index and regular
triads ‘‘mm’’ of polypropylene, while further increasing the volume of salicylate internal donor in catalyst would lead to
the polypropylene chain containing more stereo-defects. Moreover, the active centers with different stereospecificity
parameter piso in catalyst could explain the trend of stereo-defects in polypropylene chain when different internal donors
were used. In addition, it was found that the isotactic sequence length and isotacticity index of polypropylene prepared by
isobutyl 2-benzyloxy-3,5-isopropyl benzoate were close to that produced by diisobutyl phthalate internal donor.
Meanwhile, the lamella thickness distribution of polypropylene produced by salicylate internal donor was broad, which
might have potential application as expanded polypropylene materials.
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group also uses aliphatic diester as ecofriendly internal donor15 and
obtains catalysts with high catalytic activity and polypropylene with
high isotacticity. However, the synthesis of aliphatic diester is hard
Introduction
Driven by the continuous developments in catalyst technology and
polymerization process, polypropylene has owned many excellent
properties and becomes one of the most widely used thermoplastic
resins.1-3 At present, the forth Ziegler-Natta catalyst is one of the
most widely used catalyst system for propylene polymerization,4
which always contains MgCl2-supported TiCl4, internal donor,
triethyl aluminium ( TEA ) and external donor.5 Since internal
donors are applied to Ziegler–Natta catalyst system, it has become
to be industrialized. Salicylic acid is known for its ability to ease
16
aches and pains, which is a composition of aspirin.
Moreover,
some salicylates are often used in cosmetics as a fragrance additive
and UV light absorber. However, it is not found any reports about
salicylate as internal donor for polypropylene polymerization. In
addition, it was found that internal donor could strongly impact on
the active center of Ziegler-Natta catalyst and the microstructure of
polypropylene15, 17, 18, which determine the property and industrial
a
key component for improving comprehensive property of
application of polypropylene. Successive self-nucleating and
polypropylene6-8. Many kinds of compounds9-14 are used as internal
donors of Ziegler–Natta catalyst for research. Now phthalate3 is the
most widely used internal donor for the industrial production of
polypropylene. However, it is found that phthalates is harmful to
human body, moreover, European Union and the United States
both have restricted the use of phthalates in certain plastics. Our
20
annealing ( SSA ) thermal fractionation19,
can give lamella
thickness and lamella thickness distribution of polypropylene, which
are related to the isotactic sequence length and isotactic sequence
distribution of polypropylene. In addition, 13C-NMR 21, 22can be used
not only to measure the stereo-defect distribution of polypropylene,
but also to analyze the stereoselectivity of active center in Ziegler-
Natta catalyst. Based on the results of 13C-NMR and DSC, a DSC
curve model23 could be used to analyze the active center with
different stereoselectivity in Ziegler-Natta catalyst. In our study, five
salicylates with different size of hydrocarbon substituents are firstly
synthesized and employed as ecofriendly internal donors of Ziegler-
Natta catalyst for propylene polymerization. It was found that the
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catalyst activity of catalyst containing SID internal donors with 1.14(d, 12H), 1.91 (m, 1H), 2.81 (m, 1H), 2.99 (m, 1H), 3.96 (d, 2H),
proper volume was higher than that catalyst containing DIBP 7.47 (s, 1H), 7.53 (t, 2H),7.66 (m, 2H) 8.16(d, 2H); SID-5: 0.88 (d,
internal donor. Deconvolution of GPC curves indicating that as 6H), 1.39(s, 18H), 1.78 (m, 1H), 3.90 (d, 2H), 7.54 (t, 2H), 7.63 (t,
increasing the volume of SID internal donor, some of active center 1H),7.93 (s, 2H) 8.25(d, 2H);
for low molecular weight transferred into active center for high
molecular weight. The results of 13C-NMR and SSA both suggested
that the SID internal donor with appropriate size volume in catalyst
was beneficial to increasing isotactic sequence length, isotacticity
index and regular triads ‘‘mm’’ of polypropylene, while further
increasing the volume of SID internal donor in catalyst would lead
to the polypropylene chain containing more stereo-defects. DSC
curve model was used to study the stereospecificity parameter piso
of active center in catalyst, which could be used to explain the trend
of isotactic sequence length of polypropylene prepared by different
internal donors. In addition, it was found that the isotactic
sequence length and isotacticity index of polypropylene prepared
by SID-4 were very close to that produced by DIBP internal donor.
Scheme 1 Synthetic route of salicylate internal donors
Experimental
Materials
Salicylic acid, 3-methyl salicylic acid, 3,5- isopropyl salicylic acid, 3,5-
tert-butylation salicylic acid, benzoyl chloride, trimethylacetyl
chloride, sodium bicarbonate, chloroform, propylene, hydrogen,
triethylaluminium ( TEA ), diisopropyldimethoxysilane ( Donor-P )
and diisobutyl phthalate ( DIBP ) were purchased from Aladdin as a
reagent grade and used without further purification.
Scheme 2 Structures of salicylate internal donors with different
substituent groups
Preparation of catalyst
Synthesis of salicylate internal donor
The Ziegler-Natta catalysts with salicylate as internal donor were
prepared in a reactor under dry nitrogen condition. 5 g magnesium
ethoxide and 150 mL dry toluene were added to glass reactor under
nitrogen condition at room temperature. Then 50 mL TiCl4 was
added to reactor by funnel in half an hour at 5 ºC. 2 ml salicylate
internal donor was injected to reactor after dripping TiCl4. The
The synthesis method for the five salicylate internal donors was
according to literature.24 The purity of salicylate used for propylene
bulk polymerization was above 98%. The synthetic route of
salicylate and the structures of five salicylates were shown in
Scheme 1 and Scheme 2, respectively. As a typical example, the
synthesis process of isobutyl 2-benzyloxybenzoate ( SID-1 ) was
carried out as follows:
mixture was stirred at 110 ºC for
2 h. After reaction, the
supernatant was removed and the residue was washed by toluene.
Precipitated solid was treated again with a mixture of 150 ml
toluene and 50 ml titanium tetrachloride at 110 ºC for 2 h. After
discarding supernatant, the residue was washed with toluene and
hexane, respectively. A solid catalyst was obtained.
A 100 ml flask was placed with 40 ml isobutanol, 0.3 mol salicylic
acid, 0.5 ml concentrated sulfuric acid. The mixture was stirred at
o
120 C for 5 h. When water no longer appeared in water separator,
the heater was turned off. After the formation of isobutyl salicylate
was confirmed by gas chromatography, isobutyl salicylate was
purified by decompressing distillation. 0.05 mol isobutyl salicylate,
0.08 mol benzoyl chloride, 0.005 mol bismuth(III) oxychloride were
stirred at 40 oC for 6 h. After isobutyl 2-benzyloxybenzoate was
confirmed by gas chromatography, the mixture was stirred with
aqueous sodium hydrogen carbonate. Organic phase were
extracted from water phase, then a transparent and oily liquid 2-
benzyloxybenzoate was obtained by decompressing distillation.
The synthesis processes of the other salicylates including isobutyl 2-
benzyloxy-3-methyl benzoate ( SID-2 ), isobutyl 2-trimethyloxy-3-
Propylene bulk polymerization
Firstly, 800 g propylene gas and 0.3 MPa hydrogen gas were
charged with a reactor. The C( H2 )/C( propylene ) ratio was 2.59
mmol/mol. Then 20 mg Ziegler-Natta catalyst, 12 mmol TEA in n-
hexane solution and 0.39 mmol Donor-P were added to the system.
The reactor was heated to 70 oC and stirred at 300 r/min for an
hour. Then propylene was exhausted and polypropylene was
obtained as white powder.
UV–Vis spectra
methyl benzoate
( SID-3 ), isobutyl 2-benzyloxy-3,5-isopropyl
The content of Ti in catalyst was measured by UV–Vis spectra ( UV-
1800 ). Firstly, catalyst was dissolved in a dilute sulfuric acid
solution. Then titanium was converted to [TiO(H2O2]SO4 by the
addition of H2O2. Spectrophotometer was used to record UV–Vis
spectra of the resultant solution of complexes of catalyst.
Gas chromatography
benzoate ( SID-4 ) and isobutyl 2-benzyloxy-3,5-tert-butyl benzoate
( SID-5 ) were similar to that of SID-1. 1H NMR (DMSO) δ (ppm): SID-
1: 0.81 (d, 6H), 1.73(m, 1H), 3.92 (d, 2H), 7.25 (t, 2H), 7.60 (m, 3H),
7.72 (d, 1H), 8.07 (d, 1H), 8.19 (d, 2H); SID-2: 0.82 (d, 6H), 1.21(s,
9H), 1.92 (m, 1H), 2.13 (s, 3H), 4.01 (d, 2H), 7.08 (t, 1H), 7.38 (d, 1H),
7.80 (d, 1H); SID-3: 0.85 (d, 6H), 1.80(m, 1H), 3.95 (d, 2H), 7.42 (d,
2H), 7.60 (t, 3H), 7.75 (d, 1H), 8.25 (d, 2H) ; SID-4: 0.85 (d, 6H),
2 | J. Name., 2012, 00, 1-3
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The ester content of the catalysts was determined on gas Ziegler-Natta catalyst. As shown in Table 1, the Ti contents and
chromatograph ( clarus 580 ), manufactured by PE in the US.
ester contents in SID catalysts were both lower than that in DIBP
catalyst, indicated that the Ti and ester contents in catalyst could be
affected by types of internal donors. Besides, comparing Cat-1, Cat-
3 and Cat-4, the ester content of catalysts increased from Cat-1 to
Cat-4, while their Ti content decreased from Cat-1 to Cat-4. It was
reported that ester could coordinate with Mg atoms on the (110)
facets of MgCl2 crystallites27, which could block the surface sites on
MgCl2 support and lead to Ti hardly coordinating with support.
Therefore, the trend of relative content of Ti and ester was contrary.
Previous research found that the increase of ester content in
catalyst was beneficial to improving catalyst activity of Ziegler-Natta
catalyst28. Table 1 showed that catalyst activity increased from
Differential scanning calorimetry
Mettler 822e differential scanning calorimeter was used to measure
the thermal properties of polypropylene. 2～4 mg polypropylene
was put in an aluminium pan. For erasure of thermal history, the
pan with polypropylene was heated from 50 oC to 200 oC, the
o
heating rate was 50 C/min, and it was held at 200 oC for 5 min in
nitrogen atmosphere. Then polypropylene in pan was cooled to 50
oC and held at 50 oC for 5 min. Finally, polypropylene was heated to
200 oC at 10 oC/min.
Gel permeation chromatography
PL-GPC 220 high-temperature gel permeation chromatography ( 6.6×105 gPP/g.Ti to 24.1×105 gPP/g.Ti, as the ester content of
Polymer Laboratories Ltd ) was used to measure molecular weights catalyst increased from 1.1% to 2.2%. The trend of catalyst activity
( Mn and Mw ) and the polydispersity index ( PI ) of polypropylene. and ester content in catalyst also matched previous conclusion. In
The measuring temperature was 413.15 K. 1,2,4-trichlorobenzene addition, it was found that catalyst activities of SID catalyst (except
was solvent of polypropylene. The injection volume was 100 ml and for SID-1) were much higher than that of DIBP catalyst, which
the flow rate was 1.0 ml/min. Calibration was made by linear indicated that SID internal donor with proper volume were more
polystyrene as the standard sample.
beneficial to improving the catalyst activity than DIBP internal
donor in catalyst.
Successive self-nucleation and annealing
Successive self-nucleation and annealing ( SSA ) technology has
been widely used to analyze lamellar thickness and isotactic Table 1 Components and catalytic activity of Ziegler-Natta catalysts
sequence length of polymer25. The method included several heating containing different internal donor
and cooling steps, which finished by DSC.
( 1 ) heating
Catalytic Activity
Ti a
( wt % )
2.5
Ester b
( wt % )
1.1
Samples
Donor
o
o
( ×105 g PP/ g.Ti )
polypropylene to 200 C at 50 C/min and then held for 5 min; this
step was for erasure of crystalline thermal history of polypropylene;
( 2 ) Cooling polypropylene to 50 oC at 20 oC/min and held for 5 min;
( 3 ) Then heating polypropylene from 50 oC to a partial melting
temperature denoted Ts at 20 oC/min; ( 4 ) kept polypropylene at Ts
Cat-1
Cat-2
Cat-3
Cat-4
Cat-5
Cat-D
SID-1
SID-2
SID-3
SID-4
SID-5
DIBP
6.6
13.7
10.3
24.1
23.7
8.3
1.7
1.9
1.8
1.7
0.9
2.2
o
for 10 min; ( 5 )Then cooling polypropylene from Ts to 50 C at 20
0.7
1.9
oC/min; ( 6 ) Repeat step ‘c’ to ‘e’ at increasingly a new lower Ts,
2.7
6.8
which were varied from 164 to 144 oC at 5 oC intervals for a total
a the weight percent of Ti in catalyst.
b the weight percent of ester in catalyst.
five self-nucleation/ annealing steps.
( 7 ) Finally, heating
polypropylene from 50 oC to 200 oC at 10 oC/min. Then the multiple
melting curve ( SSA curve ) was obtained. Pijpers gave the method
of looking for the optimum Ts temperatures of polypropylene in
literature.26
Molecular Weight and Polydispersity Index of Polypropylene
Table 2 Molecular weight and polydispersity index of polypropylene
prepared by different internal donor
Nuclear magnetic resonance spectroscopy
DMX 300M ( Bruker ) was used to test 13C-NMR spectra of
polypropylene. 80 mg~100 mg polypropylene was dissolved in 0.5
ml of deuterated o-dichlorobenzene at 383 K. The o-
dichlorobenzene solvent was used to provide the internal lock
signal with its highest peak at 132.700 ppm. The number of pulses
was more than 5,000; Pulse angle was 30; Spectrum width was
25,000 Hz, and relaxation delay was 7 s. All spectra were completely
proton decoupled.
Donor
Mn×10-4 (g/mol)
Mw×10-4 (g/mol)
Mw/Mn
SID-1
SID-2
SID-3
SID-4
SID-5
DIBP
4.53
5.13
4.25
4.99
3.83
7.54
26.39
26.45
27.24
27.38
27.55
43.26
5.82
5.15
6.40
5.48
7.19
5.74
Table 2 listed the molecular weight and polydispersity index of
polypropylene prepared by different internal donors. It was shown
that the polydispersity index of polypropylene prepared by SID
internal donor ranged from 5.15 to 7.19, which was similar with
that prepared by DIBP internal donor. Table 2 also showed that the
molecular weights of polypropylene prepared by SID internal
donors were all lower than that prepared by DIBP, which indicated
that polypropylene chain produced by SID internal donor was easier
to transfer than that prepared by DIBP internal donor. Furthermore,
Results and discussion
Components and catalytic activity of Ziegler-Natta catalyst
Five salicylates with different hydrocarbyl substituents on phenyl
group were firstly prepared, as Scheme 2 showed. Then the five
salicylates ( SID ) and di-butyl phthalate ( DIBP ) were used as
internal donors to prepare six Ziegler-Natta catalysts, as Table 1
listed, which also displayed the Ti content and ester content in
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*
*
it was also found that the weight-average molecular weight Mw of donor in catalyst some of CC*, CD and CE* transferred into CA and
*
polypropylene successively increased from SID-1 to SID-5. In order CB
. In addition, it was found that the molecular weight of
to deeply analyze the trend of molecular weight of polypropylene polypropylene prepared by each kind of active center in DIBP
prepared by different internal donors, GPC curves were catalyst were obviously different with that in SID catalyst, which
deconvoluted by Schulz–Flory distribution functions.
indicated that the active centers in SID catalyst were different with
that in DIBP catalyst. The molecular weight of polypropylene mainly
depended on hydrogen response, owning that chain transfer to
hydrogen was more sensitive than the chain transfer to monomer or
to TEA27. And the sensitivity of chain transfer to hydrogen was
primarily dependent on regioselectivity of active center, which had
great relationship with proportion of secondary (2,1-) monomer
insertion30. Taking into account of the same addition amount of
hydrogen for SID and DIBP systems, the different molecular weight
of polypropylene prepared by active center in SID and DIBP catalysts
was ascribed to the different internal donor in catalyst. Although it
was reported that some of ester internal donor could be removed
by TEA, internal donor that strongly coordinated with support could
not be removed31, which could influence on the active center in
catalyst. It was suggested that the different type of internal donor
remained in catalyst might have different impacts on regioselectivity
of active center in Ziegler-Natta catalyst, which led to SID and DIBP
catalysts producing polypropylene with different molecular weight.
The stereo-defect in polypropylene chain
Fig. 1 Resolution of GPC curves into five Flory components for
polypropylene
The stereo-defect of molecular chain could significantly impact on
the thermal and mechanical properties of polypropylene, therefore,
detailed characterization of the stereo-defect in molecular chain
was important to obtain polypropylene with better properties. The
fraction of heptane-insoluble crystalline polypropylene was defined
as the isotactic index ( I.I.) of polypropylene, which could be used to
roughly analyze the stereo-defect in polypropylene chain. Table 4
showed the isotactic index of polypropylene prepared by different
internal donors, it was found that the isotactic index of
polypropylene increased from 96.3% to 98.6% as internal donor
successively changing from SID-1 to SID-4, which indicated that the
volume of SID internal donor could impact on the stereoregularity
of polypropylene. In addition, the isotactic index of polypropylene
produced by SID-4 was close to that prepared by DIBP internal
donor. In order to obtain more information of stereo-defect in
molecular chain, successive self-nucleation and annealing (SSA) and
nuclear magnetic resonance spectroscopy (13C-NMR) were both
used to study the microstructure of polypropylene. 32, 33
Active centers based on the molecular weight of polypropylene
The Mw/Mn ratio of polypropylene prepared by single active center
catalyst was narrow (2~3), and the wider distribution of molecular
weight indicated the catalyst containing multiple active centers.
Judging by the polydispersity index of polypropylene, catalysts with
SID and DIBP internal donors both contained several types of active
centers. Fig. 1 showed that GPC curves of polypropylene prepared
by SID and DIBP internal donors were both deconvoluted into five
kinds of Flory compounds with small polydispersity index, which
could correspond to different active centers in catalyst, as
literatures did29. Compared the active centers in catalyst with
different SID internal donors, it was found that each type of active
center in different SID catalyst produced polypropylene with similar
molecular weight, however, the relative content of each kind of
active center in five SID catalysts changed regularly. The relative
*
contents of CA and CB* increased from Cat-1 to Cat-5, while the
relative contents of CC*, CD* and CE* all decreased from Cat-1 to Cat-
5. It was indicated that as increasing the volume of SID internal
Table 3 Results of GPC Deconvolution by Flory Components
*
*
*
CC
*
*
CA
CB
CD
CE
Samples
b
Mw
Fra
Fr
Mw
Fr
Mw
8.2
8.0
7.7
8.1
7.3
17.9
Fr
Mw
2.9
3.0
2.9
3.3
2.5
6.7
Fr
Mw
0.9
1.2
1.1
1.2
0.7
2.1
Cat-1
Cat-2
Cat-3
Cat-4
Cat-5
Cat-D
16.3
18.1
18.3
19.0
19.8
8.2
56.4
56.5
57.9
57.6
58.4
138.9
33.9
35.0
35.7
35.9
37.9
30.7
19.7
19.8
19.8
19.2
18.5
51.2
32.3
31.1
30.7
30.1
29.3
38.8
12.9
11.6
11.4
11.3
10.2
18.0
4.4
3.9
3.6
3.5
2.7
4.3
a Fr was the weight percentage of the fraction produced by a certain active center in catalyst.
b Weight average molecular weight, in 104 g/mol.
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Table 4 SSA Results of Polypropylene Prepared by different internal donors
a
c
ΔHm
I.I.b
Tm1
Tm2
( oC )
Tm3
( oC )
Tm4
( oC )
160.6
160.3
160.5
160.3
160.0
--
Tm5
( oC )
154.8
154.7
155.0
153.5
154.0
--
Donor
( J/g )
104.2
99.1
( % )
96.3
96.9
98.0
98.6
97.7
98.5
( oC )
175.9
175.9
176.0
176.3
176.1
171.4
SID-1
SID-2
SID-3
SID-4
SID-5
DIBP
170.2
169.4
169.9
169.9
169.8
166.0
165.8
165.1
165.6
165.5
165.2
160.2
109.9
118.1
106.2
121.0
a the isotactic index ( I.I.) was tested by extraction with boiling n-heptane for 6 h.
b The value of the endothermic enthalpy was determined from SSA curve.
c The melting temperature was determined from the peak value in the SSA curves.
As an effective method in features of microstructure of The lamellar thicknesses of polypropylene after SSA thermal
polypropylene, SSA could give lamellar thicknesses and thickness fractionation were calculated and listed in Table 5. It was showed
distribution of polypropylene, which could be connected with the that polypropylene prepared by SID internal donor had five kinds of
isotactic sequence length of polymer. The melting curves of lamellar thicknesses but polypropylene prepared by DIBP internal
polypropylene prepared by different internal donor after SSA donor owned only three kinds of lamellar thicknesses, which
thermal fractionation were shown in Fig. 2 and the melting enthalpy indicated that the distribution of lamellar thickness in
ΔHm of each polypropylene were listed in Table 4. Comparing the polypropylene produced by SID internal donor might be different
melting enthalpy ΔHm of polypropylene prepared by SID-1, SID-3 with that prepared by DIBP internal donor. It was also found that all
and SID-4, it was found that the melting enthalpy ΔHm of of SID internal donor produced polypropylene with similar lamellar
polypropylene increased from SID-1 to SID-4, while the melting thickness, which differed from those prepared by DIBP internal
enthalpy ΔHm of polypropylene prepared by SID-5 was lower than donor, revealing that the type of internal donor could affect the
those prepared by SID-4, which reflected the trend of crystallinity thickness of each kind of lamellae in polypropylene.
degree of polypropylene.
Table 5. The lamellar thicknesses of polypropylene prepared by
different internal donor
Donor
SID-1
SID-2
SID-3
SID-4
SID-5
DIBP
L1
L2
L3
L4
L5
22.65
22.65
22.68
22.79
22.71
16.04
13.81
13.57
13.66
13.66
13.62
11.89
11.78
11.52
11.71
11.67
11.56
9.30
9.45
9.34
9.42
9.34
9.23
--
7.74
7.71
7.82
7.44
7.55
--
Moreover, it was found that the isotactic sequence length of
polypropylene was closely related with lamellar thickness. The
following equations38 were used to calculated average lamellar
thickness, thickness distribution (arithmetic average Ln, weighted
average Lw and broadness index I) and average isotactic sequence
length(arithmetic average MSLn, weighted average MSLw). ni is the
content of each fraction on SSA curve, and the Li is the thickness of
lamella for each fraction, Lhelix=0.65 nm.
Fig. 2 SSA melting curves of polypropylene samples prepared by
different internal donor
ꢒꢓꢐꢓ ꢔ ꢒꢕꢐꢕ ꢔ ꢒꢖꢐꢖ ꢔ ꢒꢗꢐꢗ ꢔ ⋯ ꢒꢘꢐꢘ
Each melting peak of polypropylene in SSA melting curves was
related to different thickness of lamellar crystallites, which formed
and annealed at each self-nucleation temperature.34 Thomson–
Gibbs equation35 could give the information about the thickness of
lamellar of polypropylene:
ꢐꢑ
ꢂ
ꢂ
ꢂ ꢙ ꢚ ꢐꢛ
ꢛ
ꢒꢓ ꢔ ꢒꢕ ꢔ ꢒꢖ ꢔ ꢒꢗ ꢔ ⋯ ꢒꢘ
ꢒꢓꢐꢕꢓ ꢔ ꢒꢕꢐꢕꢕ ꢔ ꢒꢖꢐꢕꢖ ꢔ ꢒꢗꢐꢕꢗ ꢔ ⋯ ꢔ ꢒꢘꢐꢘꢕ
ꢒꢓꢐꢓ ꢔ ꢒꢕꢐꢕ ꢔ ꢒꢖꢐꢖ ꢔ ꢒꢗꢐꢗ ꢔ ⋯ ꢔ ꢒꢘꢐꢘ
ꢕ
∑
∑
ꢚ ꢐ
ꢛ
ꢛ
ꢐꢜ
ꢂ
ꢚ ꢐꢛ
ꢛ
ꢃ ꢄ
ꢌ
ꢀꢁ ꢂ ꢀꢁ ꢅ ꢆ ꢇꢈ/∆ꢉꢃꢊꢋ
⁄
I ꢂ ꢐꢜ ꢐꢑ
ꢝꢞꢐ ꢂ 3ꢐ/ꢐꢟꢠꢡꢛꢢ
equilibrium melting temperature ꢍꢎꢏ = 460 K 36, ΔH0 = 184×106 J/m3,
the surface energy
=0.0496 J/m2 and Li is the lamellar thickness.37
σ
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1
1
1
ꢣꢣꢣꢣ ꢔ 3 ꢤ ꢤ ꢣꢥꢥꢥ ꢔ 2ꢥꢣꢣꢥ ꢔ ꢥꢣꢥꢣ ꢔ ꢥꢣꢥꢥ
2
2
2
ꢝꢞꢐ ꢂ
Table
Table 6 Lamellar thickness statistical parameters and isotactic
sequence length of polypropylene prepared by different internal
donor
1
2
1
2
1
2
ꢣꢣꢣꢥ ꢔ ꢥꢣꢣꢥ ꢔ ꢥꢣꢥꢣ ꢔ ꢥꢣꢥꢥ
7
showed the average isotactic sequence length MSL
calculated from the results of 13C-NMR. Comparing polypropylene
prepared by SID-1, SID-3, SID-4, it was found that the average meso
sequence length MSL calculated from 13C-NMR increased from SID-
1 to SID-4, however, SID-5 give polypropylene with lower meso
sequence length than SID-4, which was in good coincident with the
trend of MSLn calculated from SSA. Moreover, the trend of the
average isotactic sequence length was also similar with the rule of
isotacticity index and regular triads ‘‘mm’’ of polypropylene
prepared by different SID internal donors, which was suggested that
the appropriate size of volume of SID internal donor in catalyst
would benefit to decrease the content of stereo-defect in
polypropylene chain, while further increasing the volume of SID
internal donor in catalyst might lead to polypropylene chain
containing more stereo-defect. It was supposed that SID internal
donor with different volume might have different effect on the
stereoselectivity of catalyst, which led to produce polypropylene
with different stereo-defect.
Donor
SID-1
SID-2
SID-3
SID-4
SID-5
DIBP
Ln(nm)
12.36
12.77
12.62
12.96
12.66
13.11
Lw(nm)
13.27
13.37
13.39
13.52
13.48
13.76
MSLn
57.04
58.93
58.24
59.82
58.43
60.50
MSLw
61.25
61.71
61.80
62.40
62.22
63.09
I
1.074
1.057
1.061
1.056
1.065
1.043
Comparing the average isotactic sequence length MSLn of
polypropylene prepared by different internal donors in Table 6, the
isotactic sequence length MSLn of polypropylene prepared by SID
internal donor ranged from 57.04 to 59.82, which were close to that
produced by DIBP internal donor. Meanwhile, the distribution of
lamella thickness I of polypropylene prepared by SID internal
donors ranged from 1.056 to 1.074, which was broader than that
prepared by DIBP internal donor. It was suggested that SID internal
donor in catalyst was more conducive to produce polypropylene
with broad distribution of lamella thickness than DIBP. In addition,
comparing SID-4 and DIBP, it was found that the average lamella
thickness of polypropylene prepared by SID-4 was similar with than
produced by DIBP, but the thickness and thickness distribution of
each lamella in polypropylene for SID-4 and DIBP internal donor
were different.
The thermal properties of polypropylene
Table 7 Meso sequence length calculated from 13C-NMR and the
results of 13C-NMR
Donor
SID-1
SID-2
SID-3
SID-4
SID-5
DIBP
mm
mr
rr
MSL
15.56
22.97
19.16
28.58
22.76
30.66
85.46
88.08
89.61
90.98
88.88
91.67
7.81
6.11
5.25
3.74
5.50
3.82
6.66
5.81
5.15
4.47
5.63
4.51
Nuclear magnetic resonance spectroscopy ( 13C-NMR ) was always
used to study macrostructure of polypropylene39. The methyl
regions results from the 300 MHz 13C-NMR spectrum of the
polypropylene were listed in Table 7. It was found that the content
of the regular triads ‘‘mm’’ of polypropylene prepared by SID-
1~SID-4 internal donor increased gradually from 85.46% to 90.98%,
while the amount of ‘‘mr’’ decreased from 7.81% to 3.74% and the
amount of ‘‘rr’’ also decreased from 6.66% to 4.47%. In addition,
comparing polypropylene prepared by SID-4 and DIBP internal
donor, the content of the regular triads ‘‘mm’’ of polypropylene
prepared by SID-4 internal donor were slightly lower than that
prepared by DIBP internal donor. It suggested that both type and
volume of internal donors could impact on the regular triads “mm”
of polypropylene. 13C-NMR results could also be used to analyze the
average meso sequence length of polypropylene. Average isotactic
sequence length calculated from 13C-NMR result was defined as
Fig. 3 DSC curves of polypropylene prepared by different internal
donor
Fig. 2 showed that the SSA curves of polypropylene prepared by SID
internal donor were all broader and had more peaks than that
prepared by DIBP internal donor, which represented more kinds
and broader distribution of lamellar thickness of polypropylene
prepared by SID internal donor. Therefore, it was assumed that SID
internal donor might produce polypropylene with broad DSC curves.
Fig. 3 displayed that the DSC curves prepared by SID internal donor
were indeed broader than DIBP. Many researchers found that the
broad melting curve of polymer was beneficial to expand the
crystallization temperature range for expanded polypropylene,
which was conducive to the control of foaming of polypropylene41
.
Hence, that polypropylene produced by SID internal donor might be
possibly used as the material for expanded polypropylene.
MSL, which could be calculated by the following equation40
:
6 | J. Name., 2012, 00, 1-3
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Active center with different stereoselectivity
DSC curve model and had high stereoselectivity in catalyst.
Comparing active center Istereo and IIstereo for Cat-1, Cat-3 and Cat-4,
relative content of active Istereo and IIstereo both increased from Cat-1
to Cat-4, while Cat-5 contained lower content of active center Istereo
and IIstereo than Cat-4. Meanwhile, the trend of stereospecificity
parameter piso1 and piso2 were both similar with the rule of the
relative content of active center Istereo and IIstereo. It was indicated
that the stereoselectivity of catalyst increased as increasing the
volume of SID internal donor in catalyst, but SID internal donor with
excessive large volume could decrease the stereoselectivity of
catalyst. It was reported that Ti active center attached to the
surface of (110) facets of MgCl2 crystallites by the coordination of
Cl- on TiCl4 with Mg on MgCl244. Meanwhile, ester internal donor
could also coordinate with Mg, which would lead to disperse Ti on
surface of MgCl2 crystallites well and decrease the space of Ti active
center27. It was also confirmed that although some of ester internal
donor could be exchanged by external donor during polymerization,
the ester internal donor remained in catalyst could significantly
Scheme 3 Active Centers ( Csi ) and Active Centers ( Cre
)
It was reported that there were multiple active centers with
different stereoselectivity in heterogeneous Ziegler-Natta
catalysts42, which could produce polypropylene chain with varying
distribution of stereo-defects. The results of 13C-NMR and SSA had
confirmed that internal donor could significantly impact on the
relative content and distribution of stereo-defect in molecular chain
of polypropylene. It was supposed that different internal donors in
catalyst might have different influence on stereoselectivity of active
center in Ziegler-Natta catalyst. Combining the results of 13C-NMR
and DSC, a modeling of DSC melting curve23 could be used to
analyze the stereoselectivity of active center in Ziegler-Natta
catalyst. If all of active center in catalyst was perfectly isospecific,
which mean that the probability of meso linking of adjacent
propylene monomer units was highest (stereospecificity parameter
piso was the highest and defined as 1), polypropylene prepared by
the active center would own very narrow DSC curve and the Tm of
the DSC curve would be highest (Tmo); “mmmm” value of polymer
would be close to 1. Therefore, there were certain relationships
impact on the active center of catalyst31
. Therefore, it was
speculated that as increasing the volume of SID internal donor
remained in catalyst, the space of active center around internal
donor became smaller, which was favor of improving the
stereoselectivity of catalyst. However, the excessive large volume of
SID internal donor would result in excessive small space of active
center, which might be harmful to meso-insertion of propylene
units in the polymer chain. Therefore, the stereoselectivity of
catalyst decreased. In addition, the structure of SID and DIBP was
different, which might lead to the different stereoselectivity of SID
and DIBP catalyst.
between Tm, “mmmm” value and stereospecificity parameter piso
,
which had been reported by Kissin42. Fig. 4 was the fitting straight
of Tm from DSC curve and “mmmm” value from 13C-NMR. It was
o
found that the highest melting temperature Tm was 171.2 oC,
which was obtained by extrapolating “mmmm” value to 1 in the
straight. And the highest melting temperature Tmo could be used to
calculate the stereospecificity parameter piso of active center43
.
Fig.5 was the modeling of DSC curve of polypropylene prepared by
SID catalyst. It was showed that there were four fitting curves to fit
the DSC melting curve of polypropylene, which corresponded to
four active centers with different stereospecificity parameter piso
.
Combining melting temperature of the fitting curve from Fig. 5 with
Tmo from Fig. 4, the stereospecificity parameter piso of each active
center could be obtained 43. The stereospecificity parameter piso and
the relative content of active center Istereo and IIstereo for different
catalyst were listed in Table 8, which corresponded Tm1 and Tm2 in
Table 8 Stereospecificity parameter piso and activation energy ΔEact (si/re) of active center calculated from DSC model
Istereo
ΔEact(si/re)1
-3.14
IIstereo
ΔEact2(si/re)2
-2.86
Samples
piso1(oC)
99.09
99.21
99.26
99.27
99.23
99.49
Yield(%)
30.2
piso2(oC)
98.62
98.66
98.77
98.81
98.76
99.06
Yield(%)
56.2
Cat-1
Cat-2
Cat-3
Cat-4
Cat-5
Cat-D
-3.23
31.8
-2.88
58.0
-3.28
32.2
-2.93
58.2
-3.29
33.9
-2.95
60.5
-3.25
32.5
-2.93
59.6
-3.45
25.6
-3.12
64.8
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Fig. 4 The fitting straight of Tm from DSC curve and “mmmm” value stereo-defect in polypropylene chain was explained by the
from 13C-NMR
stereoselectivity of active center in catalyst for different internal
donors, indicating that appropriately increasing the volume of SID
internal donor could benefit to improve the stereoselectivity of
catalyst, but excessive large volume of SID internal donor would
reduce stereoselectivity of catalyst. In addition, it was found that
the catalyst activities of catalyst containing SID internal donors with
proper volume were higher than that catalyst containing DIBP
internal donor. And the isotactic sequence length and isotacticity
index of polypropylene prepared by isobutyl 2-benzyloxy-3,5-
isopropyl benzoate were close to that produced by DIBP internal
donor.
Acknowledgements
The authors express thanks for the supports of the science and
education integration innovation project of Molecular Science of
Chinese Academy of Sciences.
Fig. 5 Resolution and fitting of DSC curves for four components
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Volume of eofriendly salicylate internal donor for propylene polymerization has important impact
on catalyst active center and microstructure of polypropylene.

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39x19mm (300 x 300 DPI)