Silyl diol ester as a new selectivity control agent in MgCl2-supported Ziegler-Natta systems for propylene polymerization: Catalyst structure and polymer properties

Article abstract of DOI:10.1039/c9ra00715f

In this study, bis(benzoyloxy)dimethylsilane (SDE) was developed as a non-phthalate selectivity control agent (internal donor (ID) and external donor (ED)) in MgCl₂-supported Ziegler-Natta (ZN) systems. The impact of SDE as an ID was investigat

Full text of DOI:10.1039/c9ra00715f

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Silyl diol ester as a new selectivity control agent in
MgCl2-supported Ziegler–Natta systems for
propylene polymerization: catalyst structure and
polymer properties†
Cite this: RSC Adv., 2019, 9, 7420
*
Fatemeh Poorsank, Hassan Arabi
and Nona Ghasemi Hamedani
In this study, bis(benzoyloxy)dimethylsilane (SDE) was developed as a non-phthalate selectivity control
agent (internal donor (ID) and external donor (ED)) in MgCl2-supported Ziegler–Natta (ZN) systems. The
impact of SDE as an ID was investigated in regards to chemical composition, morphology, adsorption
behavior (using FTIR spectroscopy, WAXD and SEM studies) and the propylene polymerization
performance of the catalyst. The results of the adsorption behavior of SDE revealed that SDE, while able
to stabilize the electronically unsaturated surfaces of MgCl2 [(104) and (110)], has a rather high tendency
to be absorbed on strongly acidic Mg ions at the corners of these crystals. The catalyst with optimum
SDE content as an ID afforded a system with reasonable activity and isotacticity (even in the absence of
ED) along with broader molecular weight distribution (PDI: 6.2–10) and greater flexural modulus than
phthalate-based ZN systems. SDE was also used as a new ED in most widely used fourth- and fifth-
generation catalyst systems. The results revealed that in fifth-generation catalyst systems, there was an
increase in hydrogen response (>40%) along with an increase of activity without any change in the
thermal properties of the final polymer in comparison to a conventional ED (alkoxy silane).
Received 26th January 2019
Accepted 19th February 2019
DOI: 10.1039/c9ra00715f
rsc.li/rsc-advances
SCAs.5–12 Therefore, the evolution of a new generation of poly-
propylene catalysts is strongly related to the introduction of new
Introduction
structures and functions that have the potential to act as IDs
and EDs.13 In particular, the fourth generation of catalysts
incorporating phthalate as the most common commercial ID
and alkoxy silane as an ED has been successful in producing
highly isotactic polypropylene, but phthalate is frequently
criticised for its environmental and health risks.14,15 Therefore,
there is an urgent need for efficient organic compounds to
replace it.
Different organic structures have been tested as IDs over the
past decades, including maleates and malonates,16,17 diben-
zoylsulde,18,19 1,3-diethers,20 phosphites,21 succinates,22 salicy-
lates23 and bio-derived diesters.24 These and other electron
donors generally could be separated into two classes.25 The rst
class consists of structures such as 1,3-diethers, which have
a short spacer between their two oxygen atoms and could be
adsorbed only at the MgCl2 (110) surface with the chelating
mode. The second class contains compounds such as phtha-
lates and succinates, which could be adsorbed at both planes of
(100) and (110) MgCl2 with the bridge, chelate or zip coordi-
nation modes due to longer spacers between the coordinating O
atoms.
Magnesium-based Ziegler–Natta (ZN) catalysts hold the greatest
share in the industrial production of isotactic polypropylene.
The basis of these catalysts consists of a highly disordered
MgCl2 support, on which TiCl4 is adsorbed but these formula-
tions have been further complexed by the incorporation of
certain Lewis bases called selectivity control agents (SCA), which
have a crucial role in improving the stereoselectivity of
propylene polymerization. SCA could be added either during
the catalyst synthesis as an internal electron donor (ID) or
externally together with an organoaluminum cocatalyst
(generally triethylaluminium (TEA)) during the polymerization
process as an external electron donor (ED). It has been proposed
that SCAs can bind directly to small MgCl2 crystallites, thus
controlling the amount and distribution of TiCl4 adsorbed on
the support1 or transform atactic centers into isotactic ones.2–4
Consequently, the catalyst activity, microstructure of the
resulting poly(a-olen) including stereoregularity, molecular
weight (Mw), and polydispersity index (PDI), and the hydrogen
response in the polymerization process will be affected by
On the other hand, EDs as a component of ZN catalyst
systems, which can be directly added into the reactor without
affecting the polymerization process, have a signicant effect on
Department of Polymerization Engineering, Iran Polymer and Petrochemical Institute,
P.O. Box: 14965/115, Tehran, Iran. E-mail: h.arabi@ippi.ac.ir
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra00715f
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catalyst performance through its coordination with active replacing conventional alkoxy silane with SDE is to obtain
centers. Therefore, changing the species of ED is a proper polymers with high hydrogen response without any negative
approach to producing polypropylene with the proper molec- effects on catalytic performance. The present report is a part of
ular weight, broad molecular weight distribution and high ongoing investigations from our group to understand the role of
isotacticity, which could be a convenient method to obtain non-phthalate electron donors in ZN catalyst systems.
polypropylene with a high melt ow index (MFI) and good
mechanical properties.26
Benzoic ester,27 organic amine,28 ether,29 alkoxysilane30 and
Experimental
Materials
amino-silanes28,31 have been studied as EDs. Of course, at
present, alkoxy silanes are the most widely used EDs in the
Research-grade propylene (purity >99.9%) was supplied by Arak
Petrochemical Co (Iran). Nitrogen and hydrogen with a purity of
99.99% were supplied by the Arkan Gas Company (Iran). Trie-
thylaluminum (TEA), titanium tetrachloride (TiCl4), bis(chlor-
omethyl) dimethylsilane and potassium benzoate were
purchased from Aldrich Co (Germany). Cyclohexyl-methyl-
dimethoxy-silane (C-Donor) was obtained from Arak Petro-
chemical Co (Iran). N-Heptane and n-hexane as diluents were
purchased from Dijung Co. Dimethylformamide (DMF), sodium
sulfate, sodium chloride, diethyl ether and diisobutyl phthalate
(DIBP) were purchased from the Merck Company. Micro-
spherodial MgCl2$2.1 (C2H5OH), having an average particle size
of 28.72 mm, was prepared in accordance with the method
described in the literature.34
polypropylene industry.
It is important to note that in spite of evaluating different
compounds as SCAs, satisfactory results have only been ob-
tained with a few combinations of them.21
Recently, silyl diol ester compounds (Fig. 1) have been
claimed to be an alternative to the existing internal electron
donors for heterogeneous ZN catalysis.32,33 The molecular
structure of these compounds is different from conventional
4th- and 5th-generation compounds (phthalates and 1,3-
diethers). Thus the assessment of its effect on the properties of
polypropylene helps to shed light on the way silyl diol esters
interact with the catalyst and modify its structure.
A step towards analysing these types of donors has been re-
ported, which showed that the systems containing BenMag
(benzoate-containing magnesium/titanium) procatalyst and
silyl diol ester as the ID, showing reasonable activity and ster-
eoselectivity in comparison to similar systems containing dii-
sobutylphthalate (DIBP).32,33
Synthesis of ID
Synthesis of silyl diol ester. The synthesis method for the
However, to our knowledge, there are no prior reports related silyl diol ester ID was followed according to the literature.33 The
to the study of the role of SDE as an SCA on ZN catalysts based synthetic route of bis(benzoyloxy)dimethylsilane (SDE) is shown
on MgCl2–ethanol adduct support, its possible interactions with in Fi. S1 in the ESI.† The synthesis process of SDE was carried
the support and the verication of catalyst preparation condi- out as follows: a 250 mL round-bottom ask was charged with
tion to obtain more satisfactory polymerization results.
0.01 mol of bis(chloromethyl)dimethylsilane, (0.03 mol) of
In this paper, in order to gain insight into the potential role potassium benzoate, and 100 mL of anhydrous DMF. The
ꢀ
of SDE as SCA in MgCl2/ID/TiCl4/ED-TEA type ZN catalysts, we mixture was heated to 100 C with vigorous stirring. Aer 8 h,
have carried out the synthesis of catalysts, taking into account the mixture was cooled to room temperature and then poured
the effect of the amount of SDE as an ID in the catalyst systems into 100 mL of ice-water. The mixture was next extracted with
and compared the performance of the newly developed systems ether (3 ꢁ 50 mL). The combined ether extract was washed with
with the conventional 4th- and 5th-generation catalysts in terms brine once (12.5 mL) and dried with 12.5 g of sodium sulfate.
of catalytic activity and polymer properties in propylene Aer ltration, the ltrate was concentrated, and the solvent
ꢀ
polymerization.
was removed in a vacuum oven at high temperatures (120 C).
The adsorption behavior of SDE on the MgCl2 support was 1H-NMR and 13C-NMR spectra of all samples were recorded
studied using Fourier transform infrared (FTIR) spectroscopy, using a Bruker NMR instrument at 500 MHz.
X-ray diffraction (XRD) analysis and scanning electron micros-
copy (SEM) techniques. In addition, the effects of SDE as a new
Colorless oil, IR (KBr): 840, 1245, 1602, 1719, 2914.
1H-NMR (500 MHz, CDCl3): d 0.311 (6H, s, CH2), 4.210 (4H, s,
ED on catalytic performance were investigated. Our objective in CH2), 7.404 (4H, t, Ar–H), 7.536 (2H, t, Ar–H), 8.34 (4H, d, Ar–H).
13C-NMR (500 MHz, CDCl3): d 5.692, 56.096, 128.376, 129.530,
130.213, 132.856, 167.246. (Yield: 90%). The results of the GC-
mass analysis showed that the purity of synthesized SDE was
approximately 96%.
Synthesis of 9,9-bis(methoxymethyl)uorine. The synthesis
method for 9,9-bis(methoxymethyl)uorine was performed
according to the literature.35 The synthetic route of this
compound is shown in Fig. S2.†
White solid: mp 92–95 ꢀC. 1H-NMR (300 MHz, CDCl3): d 3.38
(6H, s, CH3), 3.67 (4H, s, CH2), 7.29–7.79 (8H, m, Ar–H). 13C
Fig. 1 Silyl diol ester structure. R1: hydrogen or hydrocarbonyl group;
R2: benzene-containing group.
NMR (75 MHz, CDCl3): d 55.5, 59.6, 74.8, 119.9, 125.3, 127.0,
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Table 1 Composition of catalysts with different internal donors
127.8, 140.6, 146.8. Yield: 60%. Purity: 98% (GC area
percentage).
Internal donor
(ID)
Catalyst
Ti (wt%)
Mg (wt%)
Cl (wt%)
Preparation of SDE-MgCl2 and SDE-TiCl4
A3
B
D
SDE
BMF
DIBP
4.6
5.4
4
10.3
13
19
50.4
47
50
The compound SDE-MgCl2 was prepared by dosing silyl diol
ester (SDE/Mg ratio of 0.028) to a suspension of MgCl2 in
heptane (25 mL gꢂ1 of MgCl2) at 20 ꢀC for 0.5 h and was kept at
this temperature for 1 h. The sample was washed once with
heptane and the FTIR spectrum of its solid component was
recorded. The SDE-TiCl4 was prepared by adding 0.1 mol of SDE
The vent valve was opened, and the unreacted monomer was
evaporated quickly. The product was dried overnight at room
temperature.
ꢀ
to 80 mL of heptane at 40 C under nitrogen, followed by the
dropwise addition of 0.10 mol of TiCl4. Aer the reaction at
40 ꢀC for 1 h, a yellowish solid product was separated by
ltration, washed with heptane and dried in a glove box under
nitrogen.
Characterization
The contents of titanium and magnesium were determined by
atomic absorption spectroscopy. The content of chloride in the
catalysts was determined by microwave digestion and potenti-
ometric titration, the UOP 291ctest method. The SEM images
were performed using a TESCAN VEGA scanning electron
microscope. The distribution of elements (Mg, Cl, Si and Ti) in
the catalyst was analyzed using an energy dispersive spec-
trometer (EDS) on an SEM microscope. Fourier-transform
infrared (FTIR) spectra of all samples were recorded on an
ABB Bomem MB-100 spectrometer using dry KBr powders with
15 scans in the range of 400–4000 cmꢂ1. The C]O IR band, in
the case of catalysts, is very complex, and to resolve it, decon-
volution was performed using a Gaussian-line-shape function.
Polypropylene isotacticity was determined using a procedure
based on dissolving a shot of polypropylene in o-xylene at
135 C: cooling the solution to 25 C under controlled condi-
tions, ltering the solid phase, distilling the o-xylene from the
solution, and measuring the mass of the dissolved substances.
The average molecular weights (Mn and Mw) and MWD of
polypropylene were measured using Polymer Laboratory PL 220
high-temperature gel permeation chromatography (PL 220,
Agilent Company) in a nitrogen atmosphere at 160 ꢀC, using
1,2,4-trichlorobenzene as a solvent, within ꢃ0.1 error. The
Preparation of the solid catalyst
Into a 1 L stainless steel Buchi reactor, purged with nitrogen,
175 mL of TiCl4 were introduced at 0 ꢀC. While stirring, 10.0 g of
microspherodial MgCl2$2.1 (C2H5OH) and an amount of ID
were added. The Mg/ID molar ratios were varied in every catalyst
synthesis experiment to acquire varying degrees of donor
incorporation. The temperature was raised to 100 ꢀC within
60 min and kept at this value for 2 h. Then, the stirring was
stopped and the liquid was siphoned off. Aer siphoning,
175 mL of fresh TiCl4 were added, and the temperature was
ꢀ
raised to 120 C within 15 min and kept at this value for 1 h.
Aer sedimentation and siphoning at 120 ꢀC the solid was
ꢀ
washed 8 times with anhydrous hexane at 60 C. Six catalysts
were prepared in the same process: catalysts A1–A4 with bis(-
benzoyloxy)dimethylsilane (SDE) as the ID, Mg/ID molar ratio:
2–10; catalyst B with 9,9-bis(methoxymethyl)-uorene (BMF) as
the ID, Mg/ID molar ratio: 6; and catalyst D with DIBP as the ID,
Mg/ID molar ratio: 6. The list of the prepared catalysts and their
chemical compositions are described in Table 1.
ꢀ
ꢀ
Propylene polymerization
Polymerization was carried out in a 1 L stainless steel Buchi injection volume was 20 mL. The molecular weight distribution
reactor. Before starting the reaction, the temperature of the curve of catalysts was deconvoluted to Flory components by
reactor was increased to 125 ꢀC and was simultaneously purged MATLAB R2014a soware. The melt ow index (MFI) is typically
by nitrogen to remove excess oxygen and humidity. In the start- stated in terms of grams of polymer passing through a stan-
up of the reaction, the reactor was cooled to 0 ꢀC while kept dardized capillary under a standard load over 10 min. The MFI
under purging with nitrogen. Next, the reactor was charged with of polypropylene was measured on an MFI-4 (Gottfert Company)
500 mL of heptane under nitrogen atmosphere, and aerward, at 230 ꢀC and at a constant load of 2.16 kg over 10 min. All
the temperature and pressure of the reactor were increased to differential scanning calorimetry (DSC) measurements were
20 ꢀC and 2 bar, respectively. Triethylaluminium (TEA) and the performed with METTLER TOLEDO differential scanning. The
ꢂ1
ꢀ
ꢀ
catalyst, with and without an ED, were transferred separately to sample was heated to 200 C at a heating rate of 10 C min
the precontacting vial and aer 20 min were injected into the and maintained for 5 min to remove its thermal history. Then, it
reactor under nitrogen atmosphere. As soon as the injection of was cooled to 25 ꢀC at a rate of 10 ꢀC minꢂ1 followed by
the catalyst system was carried out, the propylene valve was reheating at 10 ꢀC minꢂ1. The melting point and heat of fusion
opened, and the prepolymerization intended to reach better were determined in the second scan. Wide angle X-ray diffrac-
morphology was started at 2 bar monomer pressure. At the end tion (XRD) measurements were carried using a SIEMENS, D5000
of the prepolymerization, the polymerization process was (Germany) X-ray diffractometer. The step size in the WAXD
continued at 70 ꢀC and at 6 bar monomer pressure for 2 h. Aer measurements was 0.02, and the time per step was 1 s. The
the illustrated polymerization procedure, the polymerization exural modulus of the prepared polypropylenes was deter-
was terminated.
mined in accordance with ASTM D790.
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Table 2 Effect of Mg/SDE ratio on the performance of catalysts in propylene polymerization
Catalysta
Mg/SDE
External donorb (mg)
Activity (kg PP gcat
)
I.Ic (%)
ꢂ1
A1
A1
A2
A2
A3
A3
A4
A4
2
2
3
3
6
6
9
9
—
13.5
—
13.5
—
13.5
—
1.62
0.5
92
91
1.96
0.78
2.47
1.84
2.42
1.79
93.8
93
97.6
97.5
94.9
96
13.5
a
b
Polymerization condition: 20 mg cat, TEA/cat (weight ratio): 10. Cyclohexylmethyl-dimethoxysilane (C-donor). c Xylene solubility.
site I, active site II and SDE electron donor. Nevertheless, Ti
content on the catalyst and the sum of active sites decreased,
whereas that of active site II increased. So, the addition of SDE
Results and discussion
Effect of ID content on the performance of catalyst A
as a Lewis base improved isotacticity and productivity as has
been shown in Table 2. Introducing excessive amounts of SDE
leads to a reduction in the number of isospecic active site II
(path 2 in Fig. 2), resulting in lower isotacticity and productivity.
Kumawat et al.38 have found that the acidic titanium centre in
the active catalytic species and TiCl2Et on the MgCl2 support
can participate in undesired, decomposing side reactions with
donors. This result provides an explanation for the poisoning of
the active site by the addition of excess SDE in the catalyst
systems (Mg/SDE < 6).
The silyl diol ester compound, SDE, as an ID was synthesized
and characterized. The MgCl2$2.1 (C2H5OH)-supported ZN
catalysts containing different amounts of SDE (catalysts A1–A4)
were prepared and used for propylene polymerization. The
content of silyl diol ester, added in the synthetic processes of
the catalysts, was optimized in relation to the activities of the
catalysts with and without ED. The results of polymerization
with catalysts A1–A4 are presented in Table 2. As shown in the
table, the activity of the catalysts increases as Mg/SDE increases
and reaches a maximum at Mg/SDE ¼ 6. A similar trend was
observed for the isotacticity index (I.I), but the decrease is
sharper when Mg/SDE is above 6. These results are in accor-
dance with the data reported in the literature for succinate used
as the ID.36 To explain this trend, we used the model for active
sites proposed by Soga et al.37 According to this model, a non-
isospecic active site (site I in Fig. 2) has two vacancies,
whereas an isospecic site (site II in Fig. 2) possesses only one
vacancy. We predicted that the introduction of SDE excludes
part of the unstable non-isospecic active sites. It can selectively
replace one of the vacancies of the non-specic sites, the other
vacancy reshapes the isospecic sites (as we have proposed in
path 1 in Fig. 2) and a new equilibrium is formed among active
Elemental distribution in catalyst A3 particles
The distribution of four elements (Mg, Cl, Si and Ti) in the
catalyst A3 was analyzed using an energy dispersive spectrom-
eter (EDS) on an SEM microscope (Fig. S3†). The inorganic
components of the catalyst can be considered as microcrystals
of MgCl2 with Ti compounds embedded into their lattices.39
Judging by the Cl distribution data (the maximum component
of the catalyst), the catalyst particle has a uniform apparent
density with no voids in the center or edge of the particle. The
distribution of Mg atoms is similar to that of Cl atoms, as ex-
pected from the MgCl2 crystals. The distribution of Ti atoms is
Fig. 2 A plausible mechanism of the additive effect of SDE for the aspecific active site (site I) and the isotactic active site (site II).
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also similar to the distribution of Cl and Mg atoms. Also, the Ti When the activation of isospecic sites prevails over the
and Si atoms are spread uniformly along the diameter axis, poisoning of non-stereospecic ones, in addition to I.I, the
which is very important for propylene polymerization. When the polymer yield also increases. So for catalyst D, the improved
active centers are distributed uniformly, the polymer chains can catalyst activity is due to more activated isospecic sites
achieve the same polymerization rate. If not, the stresses in induced by the ED (C-donor).
different parts of a polymer particle in the process of polymer-
In the absence of ED, catalyst D shows a lower I.I in
ization are different, and the particles would break up easily.40,41 comparison to catalyst B, which is in agreement with previous
reports that claim that about 70 wt% of DIBP will be extracted by
TEA, while it is just 2 wt% for diethers (BMF).45,46 For catalyst A3,
the I.I is quite high (97.5%) in the absence of the ED. This
conrms the reported data that silyl esters are likely to be quite
resilient to decomposition by both Al2Et6 and TiCl2Et on the
Effect of ED/cat and TEA/cat ratios on the performance of
catalyst A3
The effect of the different amounts of ED (C-donor) on the
MgCl2 surface.38 Table 3 shows that the catalyst A3 provides
polymers with an MWD ranging from 6.2–7.7, which is broader
in comparison to catalyst B and catalyst D. It is also broader
than polymers prepared with the maleate47 and dibenzoylsul-
de18 internal donor and is similar to that prepared by the
salicylate23 and succinate48 internal donor. These results could
be attributed to the different active sites and/or the numbers of
active sites in silyl diol ester-based catalysts.49,50
The molecular weight distribution curves of the polymers
obtained with catalysts A3, B and D in the absence/presence of
an ED are shown in Fig. 3. It shows that for each of these
catalysts, the presence of an ED could not remarkably change
the MWD, except for a partial shi of a low molecular weight
fraction to a higher region. This change is more obvious for
catalyst D, which could be attributed to the replacement of most
of the phthalate by an ED.
The thermal properties of the polymers show that the Tm is
in the range of 158.02 to 161.27 ꢀC and that the Xc is in the range
of 29.98% to 43.36% (Table 3). The Tm and Xc of the produced
PP are inuenced by the kind of ID in the catalyst. For all
catalysts in Table 3, the Tm and Xc improved with the intro-
duction of an ED. Catalyst B, as we expected,48 showed the
highest Tm and Xc values in the absence of an ED, and catalysts
A3 and D go to the next positions respectively. The Xc of the
catalyst with succinate as the ID,48 both in presence and absence
of an ED, is about 47%, which is higher than for catalyst A3. Of
course, it was done in completely different polymerization
conditions. The addition of ED has no signicant effect on the
thermal properties of the produced PP using catalyst A3.
Conversely, it leads to signicant improvements for catalyst D.
Table 3 shows that catalyst A3 provides the polymer with an
catalyst activity, I.I and molecular weight was also studied using
catalyst A3 (Table S1†). The ED/cat ratio of catalyst A3 has little
effect on the polymerization activity. However, I.I increases to
97.4 (ED/cat ratio: 0.66) rst and then decreases to 96.3 (ED/cat
ratio: 1). This shows that ED, in an optimum amount, has
a positive effect on blocking non-isospecic Ti sites.
It is important to note that in the absence of ED, the activity
and isotacticity value of catalyst A3 are at optimal levels, but the
Mw of the polymer decreases in comparison to polymers
produced in the presence of ED. The PDI value of the polymers
is in the range of 6.2–7.7 (Table S1†). The inuence of the TEA/
cat ratio was also investigated with the C-donor as the ED (Table
S2†). Upon increasing the TEA/cat ratio from 5 to 20, the activity
of the catalyst passes through a maximum at 10. This shows that
excess TEA can lead to an extensive reduction of active centers
(Ti4+ and Ti3+) and turned it to Ti2+ which shows no polymer-
izing effect.42,43 The excess TEA also reduces the Mw of the
polymers obtained with catalyst A3, because it acts as a chain
transfer agent (Table S2†). However, the inuence of the TEA/
cat ratio on the isotacticity is not remarkable.
Effect of ID structure on catalytic performance
The performance of catalysts A3, B and D in the presence and
absence of ED are shown in Table 3. In the absence of an ED, the
activity of the catalyst decreases in the following order: B > A3 >
D. This clearly indicates that the structure of the ID strongly
inuences the nature and structure of the active centres.
Introducing the ED decreases the activity of the A3 and B cata-
lysts, while it enhances the activity of catalyst D. It is well known
that the ED has two effects on the ZN catalyst: (a) it poisons the
non-stereospecic sites and (b) it activates the isospecic sites.44
Table 3 Effect of the different internal donors on the performance of catalysts in propylene polymerization
TEA/cat
(weight ratio)
TEA/ED
(weight ratio)
Activitya
(kg PP gcat )
Flexural
modulus (Mpa)
I.Ib (%)
Mn
Mw
PDI
Xcd (%)
ꢂ1
c
Tm (ꢀC)
Catalyst
A3
A3
A3
B
B
D
54
10
10
54
54
54
54
15
15
0
15
0
0.8
97.1
97.4
97.6
97.5
96
1.54
2.06
2.20
4.40
2.20
1.15
152 000
84 000
101 000
68 500
104 000
54 000
943 000
647 000
502 000
312 000
614 000
383 000
6.22
7.71
4.96
4.54
5.91
7.08
159.95
158.18
160.22
159.64
161.27
158.02
37.69
36.46
43.36
39.59
40.97
29.98
1.57
15
0
97.5
87.7
1.39
D
a
Polymerization condition: 20 mg catalyst, ED ¼ C-donor. b Xylene solubility. c Tm: melting temperature. d Xc: crystallinity degree.
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Fig. 3 Molecular weight distribution curves of the samples obtained with three prepared catalyst in the presence (
) and absence (——) of an
ED.
improved exural modulus (1.57 Mpa) in comparison to catalyst
D (1.39 Mpa), which could be attributed to the broader molec-
ular weight of the polymer produced with catalyst A3.
Active centers based on polypropylene MWD
Fig. S4† shows the molecular weight distribution curves of
catalysts B and A3, which are deconvoluted to four and ve Flory
components respectively. Each Flory component can be
ascribed to a certain type of active centre that has a character-
istic molar mass (Mn and Mw), PDI equal to 2 and produces
chains, which constitute a certain mass fraction of the polymer
Hydrogen response performance of the catalysts
As can be seen in Fig. 4 and Table S3,† the hydrogen response of
the catalysts are in the following order: catalysts A3 < D < B. It is
in accordance with literature that among catalysts with diether,
succinate and phthalate as the IDs, diether-based catalysts have
the highest hydrogen response.13 The trend in Fig. 4 depicts that
catalyst A3 has the highest similarity to the succinate-based
catalyst. The lower hydrogen response shown by catalysts D
and A3 could be attributed to the presence of active species
having high stereospecicity and high regiospecicity with the
relatively low content of 2,1-inserted species in the isotactic
fraction.51,52
As could be seen in Tables 3 and S3,† aer the addition of
hydrogen, I.I increases and reaches a maximum. The reason is
that chain transfer to hydrogen in 2,1-inserted chains leads to
the formation of a more isotactic polymer than an atactic one
but aer reaching a maximum value; the addition of more
hydrogen causes an increase in solubility of the very low
molecular weight isotactic polymer in xylene and a decrease of
I.I.52
Fig. 4 Hydrogen response of the as-prepared catalysts.
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(Fr, %).53 Thus, there are four and ve types of active centres in 1850–1550 cmꢂ1 spectral range, the spectra recorded upon SDE
catalysts B and A3, respectively, site 1 to site 5 according to the adsorption on activated MgCl2 (Fig. 5c) and solid catalyst A3
sequence of Mw from low to high. The weight average molecular (MgCl2/TiCl4/SDE) (Fig. 5d). The characteristic FTIR absorption
weight (Mw) and the weight percentage of the fraction produced band of SDE is located at 1719 cmꢂ1, which can be assigned to
by a certain active centre (Fr) of catalysts A3 and B are shown in (C]O) as shown in Fig. 5a.
Table S4.† The Mw of polypropylene prepared by each kind of
Fig. 5c shows the absorption spectrum at room temperature
active centre of catalyst A3 is different from B, which indicates of SDE on activated MgCl2. It is worth noticing that the SDE
that the type of active centres in these catalysts are different dosage was limited in order to avoid the presence of loosely
from each other. The highest Mw produced with catalyst B (Mw: coordinated SDE. The (C]O) absorption peak of SDE in this
46.29 ꢁ 104 g molꢂ1) belongs to site 4 of this catalyst, which is sample shis to lower the frequency region and is more broad
much lower than the highest Mw produced with site 5 of catalyst as compared to the free SDE sample.
A3 (Mw: 130.98 ꢁ 104 g molꢂ1).
It is known that the C]O absorption band is very sensitive to
coordination to Lewis acid sites.54 The broad asymmetric shape
of the C]O band in Fig. 5c shows the presence of several low-
coordinated Mg sites on the MgCl2 surface with different
Lewis acidic properties. The Lewis acidity of these sites
increases from 5-coordinated Mg ions of the (104) face (Q5) to 4-
coordinated Mg ions of the (110) face (Q4) which contains the
most three-coordinated Mg ions at the edges and corners of the
MgCl2 crystals (Q3).55,56 The deconvolution of the C]O
absorption band of the MgCl2/SDE system shows three bands
with distinct maxima at ca. 1621, 1633, 1655 cmꢂ1, which are
most likely from the complexes of SDE with the surface acidic
sites of MgCl2. The FTIR spectrum of catalyst A3 is very
complicated with several pronounced maxima (Fig. 5d). These
spectra can be best represented as a superposition of at least 5
separate bands at ca. 1578, 1616, 1630, 1656 and 1691 cmꢂ1. The
integrated band intensity as determined by the deconvolution
of the FTIR spectra for MgCl2/SDE and MgCl2/TiCl4/SDE is
shown in Table 4. The most important observation in Table 4 is
Effects of ED/catalyst A3 ratio on the active centers of the
catalysts
The effect of ED/cat ratio on Mw and Fr of different sites of
catalyst A3 is shown in Table S5 and Fig. S5.† By introducing ED
to catalyst A3, the Mw of the produced polypropylene in all sites
increases, but the fraction value Fr of the active centres changes
slightly. It seems that ED interacts with the empty active sites of
the catalyst and reduces the number of active centres in all sites
of catalyst A3, which increases the Mw of the prepared poly-
propylene and decreases the activity of the catalyst as can be
seen in Table S1.†
FTIR study of silyl diol ester donor in the supported ZN
catalyst
Fig. 5 depicts the FTIR spectra of liquid SDE (Fig. 5a) in
comparison with SDE adsorption on TiCl4 (Fig. 5b) within the
Fig. 5 FTIR spectra, in the (C]O) vibrational region, liquid SDE (a), SDE/TiCl4 (b), SDE/MgCl2(c), and MgCl2/TiCl4/SDE (catalyst A3) (d).
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Table 4 Integrated band intensity as determined by the deconvolution of the FTIR spectra, for MgCl2/SDE and MgCl2/TiCl4/SDE
Integrated band intensity (a.u.)
Sample
Q3 (1620 cmꢂ1
a
)
Q4 (1633 cmꢂ1
b
)
Q5 (1655 cmꢂ1
c
)
MgCl2/SDE
MgCl2/TiCl4/SDE (catalyst A3)
6.2
4.8
7.4
3.5
11.3
1.7
a
Q3: defect Mg2+ at corners. b Q4: strong acid Mg2+ (110 surface). c Q5: weak acid Mg2+ (100 surface).
that in the presence of TiCl4, the IR absorption band related to Fig. 5b is probably an overlap of two closely positioned bands of
CO adsorbed on strongly acidic Mg ions (Q3, Q4) moderately different monodentate SDE complexes with TiCl4. These data
loses its intensity, whereas the band assigned to CO adsorbed provide evidence of the co-presence of SDE adsorbed on Mg
on weakly acidic Mg ions (Q5) is almost eliminated. In accor- ions and of TiCl4-SDE complexes. The TiCl4-ID complexes
dance with other reports, the lower content of the ID in catalyst loosely attach to the MgCl2 surface through a few bridging
A3 is most likely the result of the competition of TiCl4 and SDE chlorine atoms.60 The band at 1714 cmꢂ1 is also related to
for the same surface adsorption sites.57 The data in Table 4 loosely coordinated SDE (Fig. 5b).
provides evidence that SDE is preferentially adsorbed on the
strongly acidic Mg ions (Q3, Q4) and almost all fractions of the
SDE adsorbed on the Q5 is substituted by TiCl4. These data are
Effect of ID on the crystalline structure of the catalysts
in accordance with previous reports of the rather high mobility
of the electron donors on the Q4 surface and of the relative high
stiffness of the same donors on the Q5 surface.4,58–60 The band at
1691 cmꢂ1 regarding its low Dv value, indicates the presence of
the free or weakly bound carbonyl groups of the SDE molecules.
There is also a band at 1578 cmꢂ1, which could be attributed
to another type of carbonyl species in the catalyst that is the
complexes between TiCl4 and SDE. As a comparison, the FTIR
spectrum of the (TiCl4-SDE) complex is depicted in Fig. 5b. This
spectrum shows a broad asymmetric feature with bands at 1569
and 1584 cmꢂ1. The complexes of TiCl4 and esters of aromatic
acids were thoroughly described in the literature,60–64 and it has
been reported that the esters of benzoic acid form strong
monodentate complexes with TiCl4, the Dv value for which
Powder XRD pattern of all catalysts is shown and compared with
that of the MgCl2$2.1 (C2H5OH) adduct in Fig. 6. The crystalline
structure of the adduct (Fig. 6) is different from what has been
reported in the literature for anhydrous MgCl2,67–70 which shows
that the transformation of anhydrous MgCl2 to an active form
leads to an irregular structure suitable for use as a support for
the catalyst. The general comparison of the XRD pattern of
catalysts B, D and A3 shows the disordered d-MgCl2, namely
a complex of the a-form (cubic close packing (ccp)) and (b-form
hexagonal close packing (hcp)) of MgCl2. These two crystalline
forms can be identied from the XRD pattern in the range of
30–35ꢀ (Fig. 6). In this range, two peaks near to 34ꢀ and 32.0ꢀ
where considered, which were attributed to the (104) plane of a-
MgCl2 and the (011) plane of b-MgCl2, respectively.
range from 135 to 145 cmꢂ1 63,65,66 Therefore, the broad bands in
.
Fig. 6 XRD patterns of adduct and catalysts: B, D and A3.
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approximation of the MgCl2 crystalline size, for peaks near 34ꢀ
and 50ꢀ was shown in Table 5. The similarity of the FWHM data
for peaks near 34ꢀ and 50ꢀ for catalyst D indicates that DIBP
stabilize both the (104) and (110) faces of MgCl2, while in
catalyst B, a higher FWHM at 50ꢀ shows the ability of BMF to
stabilize the (110) face of MgCl2 more than (104). Catalyst A3
demonstrates behaviour similar to catalyst B, which indicates
SDE prefers to sit at the (110) face of MgCl2.
Table 5 Structural features of MgCl2 in adduct and catalysts
(a/b)
Sample
ratio
FWHMa at 2q z 34ꢀ
FWHMa at 2q z 50ꢀ
Adduct
1.43
0.592
0.77
1.07
0.422
0.573
0.672
0.649
0.196
0.956
0.89
Catalyst A3
Catalyst B
Catalyst D
0.639
a
FWHM: full width at half maximum.
In catalysts A3 and B, the similarity of the a-MgCl2 and b-
MgCl2 phase concentrations as well as that of the FWHM at 50ꢀ
may suggest that the coordination behaviour of the silyl diol
ester (SDE) is similar to the diether (BMF), which could stabilize
the (110) face of MgCl2 more than (104). This is in agreement
with our data reported in the IR section. On the other hand,
there is a keynote question that if the diether (BMF) and silyl
diol ester (SDE) have the same interact with support, why are
their polymer properties, especially their MWD (Table 3), so
different? We predict that SDE could stabilize the (110) face of
MgCl2 through both the chelate and bridge coordination mode,
as has been shown in Fig. 7(a) and (b), which could result in
a broader variety of active centers (Fig. S4†), while the structural
restrictions of diether imposes just the (110)-chelate coordina-
tion mode, creating more uniform active centers.45
The ratio of the intensities of these peaks was shown in Table
5. There is a signicant difference among a/b ratios of catalyst D
and catalysts B and A3. Catalyst A3 and catalyst B showed
a higher b-MgCl2 phase or hcp packing of MgCl2 triple layers in
comparison to catalyst D. This is in agreement with the litera-
ture stating that during catalyst preparation, the DIBP electron
donor could provide stability to the a-MgCl2 through the zip
coordination mode.50,71 This observation indicates that,
although all the catalyst were prepared using the same MgCl2-
adduct, that the structural differences of the electron donors
lead to a noticeable difference in the phase distribution of a-
MgCl2 and b-MgCl2 in the catalyst.
As can be seen in Fig. 6, all the catalysts showed a broad peak
near 50ꢀ which was assigned to the (110) planes of a-MgCl2 and
b-MgCl2 and to the (018) plane of the a-MgCl2 phase.70 The full-
Effect of SDE as an ED on the performance of the ZN catalyst
The performance of the catalysts A3, B and D in the presence of
SDE, as the ED, in comparison to conventional ED (C-donor),
width
at
half-maximum
(FWHM),
an
appropriate
Fig. 7 Possible SDE complexes: on the (110) surface (a, b) and on the (104) surface (c). The Mg atoms are colored in blue, the Cl atoms in green,
the O atoms in red, the C atoms in dark gray and the H atoms in gray.
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Table 6 Effect of SDE as an ED on the performance of catalysts in propylene polymerization
TEA/cat (weight TEA/ED (weight
MFR
H2 (mmol) External donor Activitya (kg PP gcat
)
I.Ib (%) (g/10 min) Tm (ꢀC) Xcd (%)
ꢂ1
c
Catalyst ratio)
ratio)
D
D
A3
A3
B
54
54
10
10
54
54
15
15
15
15
15
15
4.5
4.5
2.25
2.25
4.5
SDE
C-donor
SDE
C-donor
SDE
C-donor
2.33
4.32
2.28
3.48
7.9
98.57
99.3
97.97
96.66
99.17
99.2
48
22.9
14
7.18
49.8
159.63
159.91
161.13
162.06
158.10
157.73
42.31
47.26
42.10
44.14
47.38
47.26
B
4.5
3.75
35.37
a
d
Catalyst: 20 mg. b Xylene solubility. c Tm: melting temperature. Xc: crystallinity degree.
are shown in Table 6. For catalysts D, A3 and B in the presence of behaviour of these systems, using FTIR and WAXD studies,
SDE, there is a remarkable increase in hydrogen response. This shows that the SDE has quite a high tendency to absorb onto
could be explained by the decrease of Mw, as could be seen in strongly acidic Mg ions (Q3, Q4 (110)) at the edges and corners of
Table 6. The activity of catalysts D and A3 in the presence of SDE the MgCl2 crystals, and polymerization data revealed that the
decreases in comparison to the presence of the C-donor. There highest activity was reached at molar ratio of MgCl2 to SDE
is another consideration in regards to the activity, which is equal to 6 (cat A3). This catalyst also showed a broad MWD (PDI:
contrary to what has been reported for diether as the ED.29 It 6.2–10) and an isotacticity above 96.3% even in the absence of
showed that not only is there no noticeable reduction in the ED, meaning that TEA cannot extract SDE from the catalyst
activity of catalysts D and A3 when SDE is added as the ED (Table system. This performance was compared with the most widely
6) in comparison to when it is presented as the ID (Table 3), but used fourth- (ID: phthalate) and h-generation (ID: 1,3-
also there is a two-fold increase of the activity of catalyst B. diether) catalyst systems. Meanwhile, the performance of SDE
There is no prior report on the effect of SDE as the ED, but this as ED in the TiCl4/ID/MgCl2 (ID: SDE, 1,3-diether, phthalate)
behaviour could be explained by the mechanistic model re- systems was studied. The results showed that in all three
ported by Shen et al.72 It includes the competitive and reversible systems, the hydrogen response increased (>40%) in compar-
coordination of an ED molecule on either the central Ti of the ison to conventional compounds (alkoxysilanes) used as ED,
active center or the Mg adjacent to the central Ti. The former and in h-generation catalyst systems, the activity increased
coordination leads to the deactivation of the active centers, signicantly as well. These ndings can thus serve as a guide for
while the latter causes an increase in the propagation rate the direct industrial development of this new class of non-
constant of the active center. It has also been reported that this phthalate donors.
reaction is completely dependent on the chemical composition
and textural parameters of the catalysts.71 Thus, the data in
Table 6 indicates that SDE in the catalyst systems containing
Conflicts of interest
1,3-diether as the ID (catalyst B) has a stronger tendency to
coordinate on the adjacent Mg than the central Ti.
There are no conicts to declare.
The thermal properties of the polymers were also studied
(Table 6). The results revealed that SDE in comparison to the C-
donor leads to the decrease of Tm and Xc, with an obvious
exception for catalyst B, in which no signicant change in the
thermal properties was observed. Thus, it could be concluded
that the catalyst with optimum SDE content as ID afforded
system with a reasonable activity and isotacticity along with
broader MWD (PDI: 6.2–10) and improved exural modulus
than phthalate-based ZN systems. Meanwhile, it showed an
isotacticity higher than 96% even in the absence of an external
donor (ED), which is similar to the most widely used h-
generation catalyst. On the other hand, developing its applica-
tion as ED in systems containing 1,3-diether as ID leads to the
increase of activity as well as hydrogen response.
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