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ORIGINAL PAPER
Polypropylene synthesis in liquid monomer
with titanium–magnesium catalyst: effect of different alkoxysilanes
as external donors
Ildar I. Salakhov
1
&G. D. Bukatov
2
&A. Z. Batyrshin
1
&M. A. Matsko
2
&A. A. Barabanov
2
&E. V. Temnikova
1
&
N. M. Shaidullin
1
Received: 30 December 2018 /Accepted: 18 April 2019
#The Polymer Society, Taipei 2019
Abstract
The effect of a set of methoxy- and ethoxysilanes as external donors (ED) on propylene polymerization in liquid monomer with
the supported titanium–magnesium catalyst (TMC) and on properties of the produced polypropylene (PP) are studied. Addition
of the studied donors to the polymerization system significantly increases the PP isotacticity compared to polymerization over
TMC without ED (up to 92–98% vs. 66%, respectively). It is found that activity and stereospecificity of the catalytic system, as
well as the molecular weight of the produced PP, decline as the number and size of alkoxy groups increase and the size
(branching) of alkyl substituents decreases. Bulky substituents in alkoxysilanes have a positive effect on activity and stereospec-
ificity of TMC. However, the double bond in the substituent moiety reduces the catalyst activity. Nitrogen atom in the substituent
(with the same alkoxy groups) increases isotacticity and crystallinity of PP, its flexural modulus and strength characteristics.
Varying electron donors with different number and size of alkoxy groups and different substituents (aliphatic, aromatic, alicyclic,
amino, and vinyl) at silicon atom allows one to control the catalyst activity and isotacticity, as well as the molecular and thermal
characteristics and impact strength properties of PP.
Keywords Titanium–magnesium catalyst .Propylene polymerization .External electron donors
Introduction
Being a crystalline thermoplastic polymer, polypropylene (PP)
is characterized by a valuable combination of properties: it is
strong, rigid, thermally and chemically stable, and optically
transparent in some cases. Owing to this fact, PP is widely used
in many industries, such as the automotive industry, medicine,
packaging, construction, etc. The assortment of PP grades is
being constantly expanded due to the development of novel
and modified Ziegler–Natta catalytic systems [1].
Supported titanium–magnesium catalysts (TMCs) domi-
nate and have been successfully used in commercial
propylene polymerization processes for several decades.
Supported TMCs consist of titanium tetrachloride supported
on highly dispersed magnesium dichloride (TiCl
4
/MgCl
2
).
These catalysts are modified with electron-donating organic
compounds to increase catalyst stereospecificity. These
stereoregulating additives are used both during TMC synthe-
sis (internal electron donor D
1
) and during propylene poly-
merization (external donor D
2
)[2–6]. In the latter case, an
external donor D
2
is added together with an organoaluminum
cocatalyst (usually triethylaluminum, TEA). In general, the PP
catalytic system is TiCl
4
/D
1
/MgCl
2
+TEA/D
2
. Donors D
1
and
D
2
play a crucial role as they provide high stereospecificity of
TMC. The mechanism of donor action consists in blocking
non-stereospecific active sites (AS), converting them to ste-
reospecific ones, and increasing the propagation rate constant
for stereospecific AS [4–6].
Although there is a tendency to use non-phthalate TMCs
(1,3-diethers, succinates, etc.), most of the global PP produc-
tion still involves polymerization with TMCs that contain
phthalates (such as dibuthyl phthalate (DBP) or diisobutyl
phthalate (DIBP)) as D
1
and alkoxysilanes as D
2
[6,7].
*Ildar I. Salakhov
i.i.salahov@gmail.com
1
PJSC Nizhnekamskneftekhim, R&D Center, Nizhnekamsk 423574,
Russia
2
Boreskov Institute of Catalysis, Siberian Branch, Russian Academy
of Sciences, Novosibirsk, Russia
Journal of Polymer Research (2019) 26:126
https://doi.org/10.1007/s10965-019-1794-5
The influence of the structure of alkyl alkoxysilanes was
studied previously [8,9]. It was shown that the highest stereo-
specificity of the catalytic system was achieved for silanes
having at least two methoxy groups and bulky alkyl substitu-
ents at silicon atom. Seppala et al. [8] studied the external
donors based on (mono-, di-, and tri-)methoxy-, ethoxy-, and
propoxysilanes in the presence of TMC. They demonstrated
that the effect of the structure of an external donor on polymer
isotacticity and yield depends on the number and size of alk-
oxy groups, as well as on the size of moieties attached to
silicon atom. Proto et al. [9] investigated the effect of different
alkoxy silanes ((di- and tri-) methoxy-, ethoxy-, and
propoxysilanes) and bulky secondary aliphatic amines
(piperidines) compared to ethyl benzoate (EB) on propylene
polymerization over TiCl
4
/DBP/MgCl
2
and TiCl
4
/EB/MgCl
2
catalysts. They showed that alkoxysilanes and secondary ali-
phatic amines behave in a similar manner in the presence of
TiCl
4
/DBP/MgCl
2
and provide high PP isotacticity. In the
presence of TiCl
4
/EB/MgCl
2
catalyst, these compounds are
not efficient for high stereospecificity, unlike ethyl benzoate
used as D
2
.
In the late 1990s, Ikeuchi et al. obtained patents for a set of
nitrogen-containing alkoxysilane external donors [6,10].
They demonstrated that addition of alkoxysilane with the N-
containing polycyclic group allows one to produce highly
isotactic PP with a broad MWD, i.e., to broaden the MWD
of PP by varying the heteroatomic external donor [6].
Noristi et al. [11] studied the mechanism of action of ex-
ternal donors (ED) and showed that interaction between TMC
and TEA results in substitution of internal donors (ethyl ben-
zoate or phthalate) for ED (methyl p-toluylate or alkoxysilane,
respectively). The different effects of various alkoxysilanes
can be explained by differences in selective deactivation
(mostly of non-stereospecific sites), as well as by differences
in complexation affinity and stability of the complexes formed
between the donors and the catalyst surface [12]. In particular,
donors having high electron density (i.e., high complexation
affinity) and sterically bulky substituents hindering the inter-
action with TEA are the most efficient ones for the formation
of isospecific sites. Bulky substituents in alkoxysilane seem to
be needed to prevent ED desorption from the catalyst surface
via complexation with a cocatalyst [12,13].
It was assumed that alkoxysilane acting as ED is adsorbed
on magnesium chloride near the active site and has a steric
influence on stereoregularity of the growing polymer chain
[14]. Chadwick et al. [14] analyzed the PP samples produced
over TMC with different donors by TREF, GPC, and
13
C
NMR. They explained the influence of different alkoxysilanes
on the stereoregularity and MWD of PP based on the lability of
donor coordination near active sites. Compared to stable coor-
dination, the labile donor coordination leads to a high content of
stereodefects in the polymer chain. These sites will also produce
PP with a relatively low molecular weight as chain transfer
increases at sites with reduced regio- and stereoselectivity.
External electron-donating compounds are needed to pro-
duce advanced polypropylene materials with improved prop-
erties providing the excellent processability/strength balance.
Many compounds as ED have already been proposed [8–10].
Nevertheless, the search for novel types of ED to control the
TMC performance does not stop [6,15,16]. Studies involving
modification of TMCs with ED of different nature allows one
to reveal features and advantages of these compounds upon
PP synthesis and to produce polymers with tailored properties.
It is worth mentioning that the effect of ED on propylene
polymerization is typically evaluated according to changes
in such parameters as polymer isotacticity, yield, and molec-
ular weight characteristics. However, when designing novel
plastic materials, it is equally important to study thermal and
physicomechanical properties.
In the present work, we investigated propylene polymeri-
zation in liquid monomer over the TMC-based catalytic sys-
tem in the presence of alkoxysilanes having different substit-
uents as ED. Their effects on activity and stereospecificity of
the catalytic system, as well as on the molecular, thermal,
elastic and strain-strength properties of PP, were studied.
Experimental
Materials
AcommercialcatalystTiCl
4
/D
1
/MgCl
2
(titanium and magne-
sium contents, 2.6 and 16.7 wt%, respectively; the average
size of catalyst particles, 55 μm) containing DIBP as an inter-
nal donor was used in propylene polymerization.
Propylene, hydrogen, and triethylaluminum of polymeriza-
tion purity grade produced at PJSC Nizhnekamskneftekhim
were used. Methoxy- and ethoxysilanes listed below
(Table 1), with the main substance content ≥98%, were used
as external donors D
2
:
dimethoxysilanes –dicyclopentyldimethoxysilane,
diisopropyldimethoxysilane, diisobutyldimethoxysilane and
diphenyldimethoxysilane (USI Chemical);
cyclohexylmethyldimethoxysilane (Wacker ChemieAG);
trimethoxysilane –n-propyltrimethoxysilane (Sigma-Aldrich);
triethoxysilanes –diethylaminotriethoxysilane (Toho Titanium
Company), isobutyltriethoxysilane and vinyltriethoxysilane
(USI Chemical); and tetraethoxysilane (USI Chemical).
Propylene polymerization
Propylene polymerization was carried out in a 5 L steel
autoclave in liquid monomer at 70 °C (T
pm
) and 30 bars
126 Page 2 of 11 J Polym Res (2019) 26:126
Table 1 Alkoxysilanes used as external donors (ED)
No. Chemical
designation of ED
Structure Chemical formula Abbreviation Short
name for
ED
1 Dicyclopentyl
dimethoxysilane
(C
5
H
9
)
2
Si(OCH
3
)
2
DCPDMS D
2 Diisopropyl
dimethoxysilane
(iso-C
3
H
7
)
2
Si(OCH
3
)
2
DIPDMS P
3 Cyclohexylmethyl
dimethoxysilane
(C
6
H
11
)CH
3
Si(OCH
3
)
2
CHMDMS C
4 Diisobutyl
dimethoxysilane
(iso-C
4
H
9
)
2
Si(OCH
3
)
2
DIBDMS B
5 Diphenyl
dimethoxysilane
(C
6
H
5
)
2
Si(OCH
3
)
2
DPDMS DS
6n-Propyl
trimethoxysilane
(C
3
H
7
)Si(OCH
3
)
3
n-PTMS NM
7 Diethylamino
triethoxysilane
(C
2
H
5
)
2
NSi(OC
2
H
5
)
3
DEATES U
8Isobutyl
triethoxysilane
(iso-C
4
H
9
)Si(OC
2
H
5
)
3
IBTES IE
9 Vinyl
triethoxysilane
(CH
2
=CH)Si(OC
2
H
5
)
3
VTES VE
10 Tetraethoxysilane Si(OC
2
H
5
)
4
TES T
J Polym Res (2019) 26:126 Page 3 of 11 126
during 120 min (t
pm
). The following standard conditions
were used: propylene weight m
C3H6
= 1300 g, weight of a
catalyst sample m
cat
=0.01 g; the molar ratio AlEt
3
/Ti =
1300; the molar ratio Al/D
2
= 20. The catalytic complex
was prepared by mixing the calculated amounts of TEA,
the external donor, and TMC in n-hexane in a glass flask.
After adding the reagents into the flask and mixing for
4 min, the catalytic complex was loaded into the reactor
at 20 °C. Propylene and hydrogen (usually 0.15 mol) were
then added. Temperature was increased to 70 °C. Catalyst
activity was evaluated according to polymer yield (kg PP/
g cat). The samples of PP powder produced were stabi-
lized using a 0.2 wt% blend of phenolic and phosphite
thermostabilizers and subjected to granulation using a
Thermo Fisher Scientific twin screw extruder.
Polyropylene characteristics
To determine the isotacticity index (I.I.) of PP, a PP sam-
ple was dissolved in o-xylene, and the solution was slow-
ly cooled down to 25 °C under controlled conditions.
After filtering off the solid phase and evaporating o-xy-
lene from the solution, the content of xylene-soluble atac-
tic fraction of PP (XS, wt%) was determined. The I.I.
value was calculated as I.I. = 100–XS (wt%). The melt
flow index (MFI) of the polymer was measured on a Ray-
Ran extrusion plastometer according to ASTM 1238 at
230 °C under constant load of 2.16 kg. The molecular
characteristics of PP samples were determined on a
Polymerlab 220 high-temperature gel permeation chroma-
tography system at 160 °C in 1,2,4-trichlorobenzene used
as an eluent, at a solvent flow rate of 1 cm
3
/min.
PP samples were analyzed by differential scanning
calorimetry (DSC) on a DSC 204F1 Phoenix instrument
according to ASTM D 3418 recommendations in argon
atmosphere (flow rate, 75 mL/min). The calorimeter was
calibrated using mercury, indium, tin, bismuth, and zinc
reference standards. Samples (10.0 ± 0.5 mg) in a closed
aluminum crucible with perforated lid were placed into a
calorimetric cell. Measurements were performed during
the controlled temperature program (heating–cooling–
heating) in a temperature range between 25 and 210 °C
at a heating rate of 10 °C/min. The melting point and the
enthalpy of fusion were determined from the thermo-
grams recorded during the second heating. Temperature
and enthalpy were measured with an accuracy of ±0.1 °C
and ± 3 rel. %, respectively. The crystallinity was calcu-
lated using the formula: χ=(ΔH
melt
/ΔH
100%
)·100%,
where χis the crystallinity of the analyzed sample, %;
ΔH
100%
is the enthalpy of fusion of a fully crystalline
isotactic PP (χ= 100%), which was assumed to be equal
to 209 J/g [17]; and ΔH
melt
is the enthalpy of fusion of
the analyzed sample, J/g.
Elastic and strain-strength properties of the cast PP samples
were evaluated using the following parameters: the flexural
modulus (E) according to ASTM D 790, tensile at break (σ
b
)
and elongation at break (ε
b
) according to ASTM D 638, and
notched Charpy impact strength at +23 °C (A
23°C
)according
to ASTM D 256.
Before testing for elastic and strength characteristics,
the PP samples were conditioned at a temperature of 23
± 2 °C and relative humidity of 50 ± 5% during at least
40 h after making an incision (in the case of determining
the Charpy impact strength with an incision) and before
testing. The sample for measuring the flexural modulus,
has the following dimensions: length, 80 mm; thickness,
4 mm; width, 10 mm. The sample for measuring the
Charpy impact strength has the following dimensions:
length, 80 mm; thickness, 4 mm; and width, 10 mm.
The notch was made on the milling machine by a
single-tooth cutter. The sample for measuring tensile at
break and elongation at break has the following dimen-
sions: length, 150 mm; thickness, 4 mm; width, 20 mm
andneckwidth,10mm.
Results and discussion
Electron-donating compounds of alkyl(aryl)alkoxysilane
type R
n
Si(OR’)
4-n
(R stands for the alkyl or aryl radical,
n=0–3) are esters of orthosilicic acid and its substituted
derivatives. In this study, we used alkoxysilanes with at
least two alkoxy groups as ED, since compounds having
a single alkoxy group exhibit weak stereoregulating
properties at propylene polymerization [8].
Alkoxysilanes with different types of substituent R (ali-
phatic, aromatic, alicyclic, amino and vinyl groups)
were used. Properties of the catalytic systems with dif-
ferent donors were compared to each other and to prop-
erties of a TMC without ED. The data on the effect of
the studied alkoxysilanes on propylene polymerization
and on the characteristics of the resulting PP are sum-
marized in Tables 2,3,and4,inFig.1(GPC data) and
Fig. 2(DSC data).
Stereospecificity of the catalytic system
ThedatasummarizedinTable2demonstrate that addi-
tion of the studied donors into the polymerization sys-
tem significantly increases the isotacticity index of PP
(I.I. =92–98%) compared to that of the polymer pro-
duced over TMC in the absence of ED (I.I. = 66.3%,
126 Page 4 of 11 J Polym Res (2019) 26:126
which is close to the results obtained in [18]).
Stereospecificity of the catalytic system upon propylene
polymerization in the order of EDs shown in Table 1
decreases in the following way:
It can be seen from these data that donors containing two
methoxy groups allow one to get highly isotactic PP. The
maximum isotacticity index (98.1%) of PP produced is
achieved when DCPDMS (D) is used. High and close I.I.
values (~97%) were also obtained for DIPDMS (P) and
CHMDMS (C) with alkyl substituents branched at the first
carbon atom from silicon atom. PP with a slightly lower
isotacticity index was produced for the donor B having alkyl
groups branching at the second carbon atom (I.I. = 96.2%)and
the donor DS having aryl groups (I.I. = 96.1%). The donor
with three methoxy groups, n-PTMS, shows PP isotacticity
(96.0%) lower than that achieved when using
dimethoxysilanes. During the interaction with TEA, the third
methoxy group, perhaps, increases the silane lability on the
surface of magnesium chloride near the active site, thus reduc-
ing its average stereospecificity and, respectively, PP
isotacticity.
Among the donors with three ethoxy groups (donors U, IE,
and VE), the highest isotacticity of PP is observed for the
donor U, which has an amino group branched at the nitrogen
atom bound to the silicon atom (I.I. = 95.3%). The lower
isotacticity of PP is observed for the donor IE, which has
isobutyl group branching at the second carbon atom from
silicon atom (I.I. = 94.6%), and for the donor VE having a
vinyl group instead of isobutyl group (I.I. = 92.5%). In gen-
eral, triethoxysilanes seem to form more labile complexes on
the catalyst surface compared to methoxysilanes as alkoxy
groups are more bulky. As a result, the average stereospecific-
ity of active sites and isotacticity of the produced PP decrease.
Among all the studied donors, the lowest isotacticity of PP
(I.I. = 91.8%) is observed for the donor T containing four
ethoxy groups.
It can be seen from these data that the stereoregulating
ability of the investigated donors decreases for the following
order: dimethoxy (D, P, C, B, DS) →trimethoxy (NM) →
triethoxy (U, IE, VE) →tetraethoxy (T). In other words, the
greater the number and size of alkoxy groups in ED, the lower
PP isotacticity is. An analysis of the effect of the structure of
alkyl groups showed that the stereoregulating ability of ED
decreases in the order of substituents: branched substituents at
the first carbon (or nitrogen) atom from silicon atom (D, P, C,
DS, and U) →branched substituents at the second carbon
atoms (B vs P, IE vs U) →linear substituents (C vs D, VE
vs IE or U). In general, our findings are consistent with the
data obtained by Seppala et al. [8] on the influence of the
alkylalkoxysilane structure on PP isotacticity.
for methoxysilanes, DCPDMS DIPDMS CHMDMS DIBDMS DPDMS n-PTMS
I.I., % 98.1 97.0 96.8 96.2 96.1 96.0
and for ethoxysilanes, DEATES IBTES VTES TES no ED.
I.I., % 95.3 94.6 92.5 91.8 66.3
Table 2 Data on bulk propylene polymerization over TMC with different alkoxysilanes as ED
a
No. ED type I.I., % Activity, kg PP/g cat MFI, g/10 min M
n
,kg/mol M
w
,kg/mol M
z
,kg/mol M
w
/
M
n
T
melt,
°C χ,%
1 D DCPDMS 98.1 56 2.0 103 441 1300 4.3 170.5 47.0
2 P DIPDMS 97.0 53 3.5 86 385 1200 4.5 169.8 46.9
3 C CHMDMS 96.8 50 7.0 74 300 800 4.1 169.1 46.6
4 B DIBDMS 96.2 51 12 66 270 730 4.1 167.8 46.3
5 DS DPDMS 96.1 44 14.8 50 250 710 5.0 167.2 45.1
6 NM n-PTMS 96.0 39 14.2 47 220 640 4.7 164.7 44.5
7 U DEATES 95.3 40 30 44 200 540 4.5 167.0 49.4
8 IE IBTES 94.6 42 29 46 200 620 4.3 164.5 43.4
9 VE VTES 92.5 20 16 62 260 640 4.2 163.2 41.0
10 T TES 91.8 31 86 30 140 410 4.7 163.7
146.2
42.9
11 no ED 66.3 34 50 43 180 560 4.2 161.9
156.0
27.5
a
Polymerization conditions: m
C3H6
=1300 g; m
cat
= 0.01 g; AlEt
3
/Ti = 1300 mol; Al/Si= 20 mol; 3.5 L (0.15 mol) of hydrogen; t
pm
= 120 min; T
pm
=
70 °C
J Polym Res (2019) 26:126 Page 5 of 11 126
Activity of the catalytic system
Addition of an external electron donor activates the polymer-
ization system. Thus, while the polymer yield in the absence
of an external donor is 34 kg PP/g cat, addition of CHMDMS
(C) increases this value to 50 kg PP/g cat (i.e., by almost
50%). However, activity increases not in all cases. Thus, poly-
mer yield does not rise for the donor TES (T) and decreases for
the donor VTES (VE).
Tab le 2shows that activity of the catalytic system at pro-
pylene polymerization varies in the aforementioned order of
EDs as follows:
The highest yield of PP (56 kg PP/g cat) is achieved for
DCPDMS, which contains two cyclopentyl groups and two
methoxy groups. The high polymerization activities
(>50 kg PP/g cat) are obtained for donors DIPDMS (P),
CHMDMS (C), and DIBDMS (B), which also contain two
methoxy groups each but differ in terms of the substituent
structure. Although DPDMS (DS) also contains two
methoxy groups, activity of the catalytic system in the
presence of this donor was lower by ~20% than that in
the presence of DCPDMS (D). The possible reasons for
that are as follows. On the one hand, benzyl groups are
less bulky (Bplanar^) than cyclopentyl ones [19]. On the
other hand, the interaction between benzene rings and
TEA can increase silane lability, thus making the active
site temporarily lose its stereospecificity and reducing its
activity due to non-isospecific coordination and insertion
of propylene.
For trimethoxysilane n-PTMS, activity of the catalytic sys-
tem is somewhat lower than that for DPDMS (39 vs. 44 kg PP/
g cat, respectively). As mentioned above, the third methoxy
group interacts with TEA and enhances lability of silane on
the catalyst surface near an active site, thus contributing to
reduction of its stereospecificity and activity, like it was ob-
served for DPDMS.
When donors D
2
containing three ethoxy groups as
DEATES (U) and IBTES (IE) are used, activity of TMC is
almost identical to that observed for trimethoxysilane n-
PTMS. For the donor VTES (VE), which contains a vinyl
substituent in addition to three ethoxy groups, activity of
TMC is even lower than that of TMC in the absence of donor.
The polymer yield is minimal (20 kg PP/g cat). Partial deac-
tivation of active sites may take place in this case due to
addition of the vinyl group of silane instead of propylene to
the active Ti–Cbond.
Addition of TES containing four ethoxy groups as an ex-
ternal donor has a minor effect on polymerization activity. The
yield of PP is 31 kg PP/g cat, which is comparable to the PP
yield obtained without D
2
(34 kg PP/kg cat).
Table 3 Effect of ED on thermal properties of PP.
No. ED type T
melt
a
,
°C ΔH
f
b
,J/g T
cr
c
,
°C ΔT
d
,°C χ
e
,%
1 D DCPDMS 170.5 98.3 116.9 53.6 47.0
2 P DIPDMS 169.8 98.1 117.6 52.2 46.9
3 C CHMDMS 169.1 97.3 114.8 54.3 46.6
4 B DIBDMS 167.8 96.8 116.3 51.5 46.3
5 DS DPDMS 167.2 94.2 116.1 51.1 45.1
6 NM PTMS 164.7 93.1 114.4 50.3 44.5
7 U DEATES 167.0 103.2 117.6 50.0 49.4
8 IE IBTES 164.5 90.7 117.1 47.4 43.4
9 VE VTES 163.2 85.6 118.1 45.1 41.0
10 T TES 163.7
146.2
89.6 115.8 47.9 42.9
11 no ED 161.9
156.0
57.4 109.9 52.0 27.5
a
Temperature of the melting peak maximum
b
Enthalpy of fusion
c
Temperature of the crystallization peak maximum
d
ΔT=T
melt
–T
cr
(see explanation in the text)
e
Crystallinity
Table 4 Physicomechanical properties of PP samples produced with
different alkoxysilanes as ED
No. External donor E, MPa A
23°C
,kJ/m
2
σ
b
,MPa ε
b
,%
1 D 1150 8.7 10.5 63
2 P 1145 7.9 12.2 74
3 C 1140 7.9 17.9 76
4 B 1080 6.8 18.1 320
5 DS 1100 6.8 15.8 300
6 NM 960 6.9 17.7 –
7 U 1160 5.3 22.4 270
8 IE 1050 6.0 17.3 510
9 VE 920 6.0 16.5 520
10 T 990 5.2 12.3 310
11 no ED 270 19.0 15.9 670
for methoxysilanes, DCPDMS DIPDMS CHMDMS DIBDMS DPDMS n-PTMS
A, kg PP/g cat 56 53 50 51 44 39
and for ethoxysilanes, DEATES IBTES VTES TES no ED.
A, kg PP/g cat 40 42 20 31 34
126 Page 6 of 11 J Polym Res (2019) 26:126
Hence, the number and size of alkoxy groups, as well as the
size and structure of substituent in the external donor, affect
activity of the catalytic system. The greater the number and
size of alkoxy groups and smaller the size of a substituent in
D
2
, the lower the activity of the catalytic system is. Overall,
activity of the catalytic system decreases almost for the same
order of D
2
as the one observed at decreasing PP isotacticity.
The increase in lability of silanes in that order of D
2
seems to
cause temporary stereospecificity loss by an active site (de-
creasing PP isotacticity) and reduce its activity due to non-
isospecific coordination, insertion of propylene and slow re-
activation of the active site.
Molecular characteristics of PP
Tab le 2shows the molecular characteristics of PP at propylene
polymerization in the presence of the TiCl
4
/DIBP/MgCl
2
+
TEA/D
2
catalytic system with different alkoxysilanes. It can
be seen that the polymers with M
w
/M
n
= 4.1 ÷ 5.0 are obtained
under the studied conditions. Addition of alkoxysilanes to the
reaction mixture increases the molecular weight of PP and,
therefore, reduces the melt flow index of the polymer (except
for the case with donor TES).
The melt flow index of polyolefins is one of the most fre-
quently used rheological parameters, which is related to the
80 100 120 160 180 200140
1
2
3
4
5
6
Temperature (°C)
endo
Donor D
Donor C
Donor P
Donor B
Donor DS
Donor NM
Heat flow, mW/mg
0,2 mW/mg
80 100 120 160 180 200140
7
8
9
11
10
endo
Temperature (°C)
no ED
Donor U
Donor IE
Donor T
Donor VE
Heat flow,mW/
mg
0,2 mW/mg
ab
Fig. 2 DSC curves of PP samples produced in the presence of different alkoxysilanes and without ED: amethoxysilanes; bethoxysilanes
34567
0
dW
f/ d logM
log M
4
5
2
-
Donor P
2
3
6
1
6 Donor NM
-
5 Donor DS
-
4 Donor B
-
3
-
Donor C
1
-
Donor D
0.2
0.4
0.6
0.8
34567
0
log M
7
10
8Donor IE 9
11
8
0.2
0.4
0.6
0.8
7-Donor U
-
dW
f
/ d logM
9Donor VE-
10 Donor T
-
11 No ED
-
ab
Fig. 1 MWD curves of PP samples produced in the presence of different alkoxysilanes and without ED: amethoxysilanes; bethoxysilanes
J Polym Res (2019) 26:126 Page 7 of 11 126
weight-average molecular weight M
w
. Therefore, compar-
ative evaluation of MFI values was carried out in this
study for the PP samples produced using different donors.
The melt flow index of PP under the investigated condi-
tions (Table 2) changes in the abovementioned order of
ED as follows:
It is clear that, similar to the case of catalyst stereospecific-
ity and activity, the melt flow index significantly depends on
alkoxysilane structure (the number and size of alkoxy groups,
as well as the structure of substituents). The MFI increases
from 2 g/10 min (for DCPDMS) to 86 g/10 min (for TES)
due to the corresponding decrease of in the average chain
length; in particular, M
w
changes from 441 to 140 kg/mol.
Overall, in the mentioned order of alkoxysilanes the average
PP chain length decreases similarly to deacreasing PP
isotacticity (stereospecificity of the catalytic system).
Increased lability of donors seems to lead not only to forming
stereodefects in polymer chain but to increasing the chain
transfer reaction too.
Figure 1shows the MWD curves of PP samples synthesized
in the presence of the donors under study. It can see in Fig. 1a
that the MWD curves shift toward lower molecular weights in
the order of methoxysilanes D, P, C, B, DS, and NM. In the
second group of donors (U, IE, VE, and T) (Fig. 1b), the MWD
curves also shift toward lower molecular weights (except for the
donor VE that has a reduced MFI value).
Thermal properties of PP
It is interesting to evaluate how the phase structure of isotactic
PP produced with various alkoxysilanes changes according to
thermal properties of PP samples. Two melting peaks appear
on the DSC thermogram of the sample in the absence of ex-
ternal stereospecific additives (without D
2
)(Fig.2b): a weak
low-intensity peak at 156.0 °C and a more intense peak at
161.9 °C. The former peak is ascribed to one of β-
modifications of the crystalline structure of isotactic PP and
the latter one is ascribed to melting of the α
2
-modification of
isotactic PP crystallites [19]. The intensity and enthalpy of
fusion endotherms are rather low and correspond to 27.5%
crystallinity of PP. Such a low heat of fusion, the broad fusion
range, and the presence of an additional low-temperature peak
demonstrate that the content of an amorphous structure (in this
case, low stereoregular PP) in the studied sample is high. This
is confirmed by both low isotacticity (66.3%) and the reduced
M
n
value.
The presence of several melting peaks is one of structural
features of isotactic PP characterizing its ability to undergo
polymorphic transformations. Isotactic PP is known to exist
as α
1
-, α
2
-, β-, and γ-modifications [20]. Formation of a
certain crystalline structure depends on crystallization condi-
tions and stereoregularity of the polymer. Thus, isotactic PP
crystallizes as its α-form (α
1
and α
2
) under atmospheric pres-
sure and under usual cooling conditions [19]. Formation of the
less stable β-form of isotactic PP is initiated by orientational
interactions [21,22] or presence of nucleating agents [23].
Crystallites of the γ-modification of PP can emerge in the
samples with low molecular weight and during high-
pressure crystallization of PP [24].
TheDSCdatasummarizedinTable3demonstrate that the
melting point varies in the abovementioned order of ED as
follows:
It can be seen that the melting point depends on
alkoxysilane structure and decreases in the mentioned order
of ED as PP isotacticity does.
PP samples produced in the presence of different EDs can
be conventionally divided into two groups according to their
thermal characteristics. One group of samples produced in the
presence of dimethoxysilane donors (D, P C, B, and DS) (Fig.
2a) has a single phase transition, with the high-temperature
melting peak in the region between 171 and 167 °C that char-
acterizes the content of PP with monoclinic lattice of the α
1
-
modification [19,23]. All the PP samples in this group have a
high stereoregularity (their isotacticity is 96–98%), the highest
-formethoxysilanes, DCPDMS DIPDMS CHMDMS DIBDMS DPDMS n-PTMS
MFI, g/10 min 2.0 3.5 7.0 12 14.8 14.2
-forethoxysilanes, DEATES IBTES VTES TES no ED.
MFI, g/10 min 30 29 16 86 50
-formethoxysilanes, DCPDMS DIPDMS CHMDMS DIBDMS DPDMS n-PTMS
T
melt
, °C 170.5 169.8 169.1 167.8 167.2 164.7
-forethoxysilanes, DEATES IBTES VTES TES no ED.
T
melt
, °C 167.0 164.5 163.2 163.7 161.9
126 Page 8 of 11 J Polym Res (2019) 26:126
values of enthalpy of fusion (94–98 J/g) and crystallinity (45–
47%) (Table 3). One of the reasons for the increase in enthalpy
of fusion is that intermolecular interactions between polymer
chains become stronger (energy of cohesion increases). This
variation in thermal properties indirectly demonstrates that
more ideal crystallites are formed. It is noteworthy that our
data are consistent with the literature data on isotaticity and
crystallinity values of similar isotactic PP samples [20].
Another group of PP samples produced in the presence of
trimethoxy-, tri-, and tetraethoxysilanes differs in terms of
their melting point (it lies in the temperature range of 163–
165 °C). This melting point can be ascribed to melting of the
crystalline monoclinic structure of the α
2
-form of PP [19],
which has lower enthalpies of fusion (~80–90 J/g) compared
to those of the α
1
-form of PP. Polypropylene produced in the
presence of donor U and having a higher melting point T
melt
=
167 °C should be classified into the first group of samples.
DSC thermogram of the PP sample produced in the presence
of donor T (Fig. 2b, curve 10) shows an additional low-
intensity peak with T
melt
= 146.2 °C corresponding to melting
of the less stable β-modification before melting of the α
2
-
modification [22,23]. The presence of four ethoxy groups
compared to other ED can be the important factor causing
the decrease in M
n
of PP in this case (Table 2).
Moreover, comparison of the difference between the melting
point and crystallization temperature ΔT=T
melt
–T
cr
,which
characterizes the crystallizability of polymers [23], demon-
strates that samples in the second group have lower ΔTvalues
(<50 °C). This is probably related to the high mobility of PP
chains due to low viscosity of the melt. For the first group, the
ΔT value is >50 °C, which can be caused by higher molecular
weight and higher stereoregularity of the samples. High-
intensity intermolecular interactions in highly stereoregular PP
samples alter molecular mobility. This results in a longer-term
supercooling, which is required for the formation of crystalline
phase under these measurement conditions.
An analysis of melting thermograms demonstrates
(Fig. 2a, b and Table 3) that addition of EDs of different types
increases the melting point of the resulting PP (from 161.9 to
170.5 °C), the intensity of peaks, and the enthalpy of fusion.
Such behavior of PP samples is as a result of increasing PP
isotacticity (from 66.3 to 98.1%), and the PP crystallinity in-
creases too (from 27.5 to 49.4%).
Hence, the DSC data suggest that polymorphism is ob-
served in PP samples with low isotacticity (PP produced with-
out ED). For the PP sample produced in the presence of donor
T, different forms of crystalline structures seem to emerge
because of the low average molecular weights. Addition of
stereoregulating donors (in particular, the amino donor U
and alicyclic donor D) leads to the synthesis of isotactic PP
with high stereoregularity and predominantly monoclinic
crystalline structures (i.e., the more perfect structures). These
results correlate well with the data on physicomechanical
properties shown below.
Physicomechanical properties of PP
Physicomechanical properties of PP depend on many param-
eters (structure, crystallinity, molecular weight characteristics,
and the content of the amorphous phase), which are influenced
by polymerization conditions.
Tab le 4presents the results of studying the elastic-strength
and strain-strength characteristics of homopolypropylene
samples produced in the absence and presence of
alkoxysilanes as ED. The flexural modulus, impact energy,
and elongation at break are the key parameters of PP. These
parameters are used to evaluate the consumer performance of
polypropylene products.
The flexural modulus, the Charpy impact strength, and the
elongation atbreak of PP samples vary in the abovementioned
order of ED depending on crystallinity and the weight-average
molecular weight in the following manner:
for methoxysilanes, DCPDMS DIPDMS CHMDMS DIBDMS DPDMS n-PTMS
E, MPa 1150 1145 1140 1080 1100 960
A
23°C
, kJ/m
2
8.7 7.9 7.9 6.8 6.8 6.9
ε
b
, % 63 74 76 320 300 –
χ, % 47.0 46.9 46.6 46.3 45.1 44.5
M
w
, kg/mol 441 385 300 270 250 220
and for ethoxysilanes, DEATES IBTES VTES TES no ED.
E, MPa 1160 1050 920 990 270
A
23°C
, kJ/m
2
5.3 6.0 6.0 5.2 19.0
ε
b
, % 270 510 520 310 670
χ, % 49.4 43.4 41.0 42.9 27.5
M
w
, kg/mol 200 200 260 140 180
J Polym Res (2019) 26:126 Page 9 of 11 126
These data demonstrate that the flexural modulus of PP
samples increases as polymer crystallinity goes up in the series
of methoxysilanes and ethoxysilanes. The highest flexural
modulus values were achieved for donors U and D; the lowest
ones were obtained for the PP samples produced in the pres-
ence of donor VE and without donor. Crystallinity of PP sam-
ples produced using these donors was 49.4 and 47.0% com-
pared to 41.0 and 27.5%, respectively.
In the series of donors under study, the Charpy impact
strength (A
23°C
) seems to depend on molecular weight rather
than on crystallinity of the polymer. One can see that the
Charpy impact values correlate well with the M
w
range (ex-
cept for the synthesis with VTES).
The highest A
23°C
value was obtained for PP synthesized in
the absence of ED. This change is consistent with the classical
concepts of impact strength of PP, when a significant content
of the amorphous phase reduces crystallinity of PP (down to
27.5%) and the impact strength of PP increases (up to 19 kJ/
m
2
) despite its low molecular weight.
The elongation at break also significantly varies depending
on ED type. Thus, for methoxysilanes the elongation at break
for PP produced in the presence of donors D, P, and C is 63–
76%, while the ε
b
value of the PP produced in the presence of
B and DS is four- to fivefold higher (300–320%). In the series
of triethoxysilanes, the elongation at break presumably de-
pends on the total effect of molecular weight and crystallinity
of PP. In particular, PP synthesized in the presence of donor U
has the lowest ε
b
value (270%); the highest ε
b
value is ob-
served for the PP samples produced in the presence of donors
IE and VE (510–520%). The maximum value of elongation at
break was obtained for PP synthesized without any donor
(670%). So, the elongation at break increases with decreasing
crystallinity and molecular weight of PP.
It is evident that the flexural and impact strength properties
of PP are improved as molecular weight of the polymer rises
and the content of low stereoregular fraction decreases.
Meanwhile, elastic properties of polymer are reduced; i.e.,
polypropylene becomes Brigid^and more fragile. It is most
clearly seen for methoxysilanes. On the other hand, the ele-
vated content of the amorphous phase in the polymer in-
creases the elongation at break.
PP samples with the maximum crystallinity and high ste-
reoregularity, which were produced in the presence of donors
U and D, have the best flexural modulus (Table 4). The poly-
mer produced in the presence of U has higher tensile and
elongation at break, but lower impact strength (A
23°C
value)
compared to PP synthesized with donor D. It is assumed that
such a combination of elastic and flexural properties in PP
produced in the presence of diethylaminotriethoxysilane can
be due to simultaneous presence of more ideal structures
(smaller spherulites) in the homopolymer and the higher con-
tent of the amorphous phase compared to that for donor D.
However, this content of the amorphous phase is still
insufficient to ensure high impact strength. It is well-known
that α-olefin copolymers are typically inserted into the poly-
propylene structure (during polymerization or the
compounding stage) to enhance its impact strength.
Conclusions
It is demonstrated that the structure of alkoxysilanes as ED
substantially determines the behavior of propylene polymeri-
zation and the properties of PP produced. Investigation of the
effect of a series of methoxy- and ethoxysilanes on PP syn-
thesis revealed the dependences in variation of stereospecific-
ity and activity of the catalytic system, the melt flow index,
MWD, as well as thermal and physicomechanical properties
of the polymer. Addition of the studied donors to the polymer-
ization system is shown to significantly increase polymer
isotacticity compared to that of PP produced over TMC with-
out ED (up to 92–98% vs 66%, respectively). It is found that
the greater number and size of alkoxy groups and the smaller
size (branching) of alkyl substituents in alkoxysilane, the low-
er are activity and stereospecificity of the catalytic system and
PP molecular weight. The presence of bulky substituents in
alkoxysilanes has a positive effect on activity and stereospec-
ificity of TMC. However, the double bond in an alkyl substit-
uent reduces catalyst activity. A nitrogen atom in the substit-
uent (with the same alkoxy groups) increases isotacticity and
crystallinity of PP, the flexural modulus and strength proper-
ties of PP. Therefore, it seems promising to use
aminoalkoxysilane as an ED to improve the
physicomechanical properties of PP.
The dependences of the flexural modulus, Charpy impact
strength, and elongation at break for the studied methoxy- and
ethoxysilanes as EDs demonstrate that crystallinity and mo-
lecular weight have a significant effect on physicomechanical
properties of PP.
Hence, activity and isotacticity of the catalyst, the molecu-
lar and thermal characteristics of PP, and impact strength prop-
erties of PP can be regulated by varying electron donors that
have a different number and size of alkoxy groups and differ-
ent substituents (aliphatic, aromatic, alicyclic, amino, and vi-
nyl) at silicon atom.
Acknowledgements The authors are grateful to Marat Khasanov, a staff
member of the R&D Center of the Public Joint Stock Company
Nizhnekamskneftekhim, for processing the DSC thermograms and
discussing the DSC data.
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