Copolymerization of ethylene with 1-hexene over highly active supported Ziegler–Natta catalysts with vanadium active component but different vanadium contents

Abstract

The kinetics of copolymerization of ethylene with 1-hexene over highly active V-Mg Ziegler–Natta catalysts [VMC: VCl4/MgCl2 + Al (i-Bu)3] at different contents of vanadium (4 and 0.1 wt% of V) was studied. Data on copolymerization ability of these catalysts, molecular weight characteristics, and compositional homogeneity of the produced copolymers were attained. It was found that the introduction of 1-hexene led to broadening of the molecular weight distribution of copolymer (an increase in the Mw/Mn value) relative to homopolyethylene due to a decrease in the molecular weight (Mn) in the region of 1–100 kg mol−1 without changes in the molecular weight in the region of Mn > 500 kg mol−1 for two used VMCs catalysts. This result testified to non-uniformity of the active centers of these catalysts in the chain transfer reaction with participation of 1-hexene. Therefore, the active centers of VMCs producing high-molecular polyethylene were virtually not involved in the chain transfer reaction with 1-hexene. At the same time, these centers were more reactive in the incorporation of 1-hexene. This led to unusual distribution of butyl branches in ethylene–1-hexene copolymers produced over VMCs, namely, to an increased content of butyl branches in the high-molecular fraction of copolymers (Mw > 1000 kg mol−1).

Full text

Vol.:(0123456789)
Iranian Polymer Journal
https://doi.org/10.1007/s13726-024-01401-x
ORIGINAL RESEARCH
Copolymerization ofethylene with1‑hexene overhighly active
supported Ziegler–Natta catalysts withvanadium active component
butdifferent vanadium contents
ZenghuiZhao1· TatianaB.Mikenas2 · PengGuan3· MikhailA.Matsko2· VladimirA.Zakharov2· WeiWu4
Received: 15 May 2024 / Accepted: 1 September 2024
© Iran Polymer and Petrochemical Institute 2024
Abstract
The kinetics of copolymerization of ethylene with 1-hexene over highly active V-Mg Ziegler–Natta catalysts [VMC: VCl4/
MgCl2 + Al (i-Bu)3] at different contents of vanadium (4 and 0.1 wt% of V) was studied. Data on copolymerization abil-
ity of these catalysts, molecular weight characteristics, and compositional homogeneity of the produced copolymers were
attained. It was found that the introduction of 1-hexene led to broadening of the molecular weight distribution of copolymer
(an increase in the Mw/Mn value) relative to homopolyethylene due to a decrease in the molecular weight (Mn) in the region
of 1–100kg mol−1 without changes in the molecular weight in the region of Mn > 500kg mol−1 for two used VMCs catalysts.
This result testified to non-uniformity of the active centers of these catalysts in the chain transfer reaction with participation
of 1-hexene. Therefore, the active centers of VMCs producing high-molecular polyethylene were virtually not involved in the
chain transfer reaction with 1-hexene. At the same time, these centers were more reactive in the incorporation of 1-hexene.
This led to unusual distribution of butyl branches in ethylene–1-hexene copolymers produced over VMCs, namely, to an
increased content of butyl branches in the high-molecular fraction of copolymers (Mw > 1000kg mol−1).
Graphical Abstract
Keywords Ziegler–Natta catalysts· Vanadium–magnesium catalysts· Ethylene–1-hexene copolymerization· Copolymer
heterogeneity· Molecular weight distribution
Extended author information available on the last page of the article

Iranian Polymer Journal
Introduction
Polyethylene (PE) is the most widely used polymer in the
world [1]. Its annual production exceeds 100million tons,
which constitute ca. 40% of the total world consumption
of all thermoplastics. The output of polyethylene is ever
growing [2]. At present, a large part of PE in the world is
produced with the use of highly active Ziegler–Natta (ZN)
catalysts, which contain titanium chlorides deposited on
the MgCl2-containing support (supported titanium–mag-
nesium catalysts, TMCs), in a combination with organoa-
luminum co-catalyst (OAC) [3–5].
The majority of produced PE grades, for example, tube
and film grades of high density polyethylene (HDPE) and
also linear low density polyethylene (LLDPE) are copoly-
mers of ethylene with α-olefins. It is very important to
control MWD [6] and short chain branching (SCB) dis-
tribution [7–9] for high density polyethylene intended for
piping applications. A general method to produce poly-
ethylene with broad/bimodal MWD and SCB over tita-
nium–magnesium catalyst is the use of tandem reactors [8]
with different polymerization conditions [10–12].
The composition of active component may exert a con-
siderable effect on the molecular weight characteristics
of PE and ethylene–α-olefin copolymers [6, 11, 13]. Sup-
ported vanadium–magnesium catalysts (VMCs) contain-
ing vanadium chloride as the active component supported
on MgCl2 differ considerably from TMCs regarding the
regulations of molecular structure of PE [14–16] and
copolymers of ethylene with α-olefin [17, 18]. There are
some features of VMCs distinguishing them from TMCs
in homopolymerization of ethylene and copolymerization
of ethylene with α-olefins as follows:
1. Formation of polyethylene with a higher molecular
weight during polymerization in the absence of hydro-
gen and a higher copolymerization ability of VMCs in
the chain transfer reaction with hydrogen, which allows
obtaining polymers with an acceptable molecular weight
[17, 19–21];
2. Broader MWD for the obtained polymers (homopoly-
ethylene and copolymers of ethylene with α-olefins)[20,
21];
3. Formation of polyethylene with a higher molecular
weight during polymerization in the absence of hydro-
gen and a higher copolymerization ability of VMCs in
the chain transfer reaction with hydrogen, which allows
obtaining polymers with an acceptable molecular weight
[17, 19–21];
4. Higher copolymerization ability of VMCs in copolym-
erization of ethylene with α-olefins and the formation of
copolymers with a more uniform distribution of branch-
ing [18, 22, 23].
We have developed a new modification of highly active
ZN vanadium-magnesium catalyst with the improved
morphology for ethylene slurry polymerization [21, 24].
These catalysts have a high activity and produce PE and
ethylene–α-olefin copolymers with broad bimodal molecular
weight distributions and high bulk density of the polymer
powder with a narrow particle size distribution.
It was found that activity of the ZN titanium–magnesium
catalysts enhanced sharply as Ti content decreased from 5
to 0.07 wt%[25, 26]. The catalyst with a low titanium con-
tent (≤ 0.1 wt% of Ti) produced PE with a narrower MWD
(Mw/Mn = 3.1–3.5) as compared to catalysts with a higher
titanium content (3–5 wt% of Ti; Mw/Mn = 4.8–5.0)[26].
According to ESR data, superactive titanium–magnesium
catalysts with a low titanium content (≤ 0.1wt%) interacted
with organoaluminum co-catalysts to produce isolated
Ti3+compounds, the content of which correlated with the
activity in the ethylene polymerization [27, 28]. Ultrahigh
activity of TMCs with titanium content below 0.1 wt% was
related to a high concentration of active centers (more than
40% of the total titanium) [26]. These centers were repre-
sented by the isolated Ti3+ compounds anchored to fourfold
coordinated magnesium ions of the support (the 110 face)
[26]. According to ESR data, the supported catalyst with the
composition VCl4/MgCl2 and low concentration of vana-
dium (ca. 0.1 wt%) also contained only the isolated surface
compounds VCl4[14].
Recently, we have reported our work on supported VMCs
with different vanadium contents (0.12 and 4.0 wt% of V)
in ethylene homopolymerization[29]. Polyethylene pro-
duced in the presence of hydrogen over these catalysts had
a broad bimodal molecular weight distribution (MWD)
with a pronounced shoulder in the high-molecular region
(Mw ≥ 106g mol−1). Data concerning the effect of hydrogen
on the molecular weight and MWD of the produced poly-
ethylene were obtained in the study. The analysis of these
data revealed that the formation of the high-molecular shoul-
der on the MWD curve was associated with the presence of
VMCs in the active centers having a decreased rate constant
of the chain transfer reaction with hydrogen in comparison
with the active centers on which polyethylene with moderate
and low molecular weights was formed. Thus, the presence
of VMCs in two groups of the active centers with different
reactivity in the chain transfer with hydrogen underlaid the
formation of PE with bimodal MWD on these catalysts.
In the present work, we investigated the copolymerization
kinetic for the reaction of ethylene with 1-hexene over novel
highly active VMCs having different vanadium contents.
Data were obtained on copolymerization ability of these cat-
alysts, molecular weight characteristics and compositional

Iranian Polymer Journal
uniformity of the produced copolymers. The active cent-
ers of VMCs producing the high-molecular polymer were
shown to have a higher copolymerization ability and yield
the copolymer with a higher content of branches in com-
parison with the active centers producing the low-molecular
copolymer.
Experimental
Catalyst preparation
The supported vanadium–magnesium catalysts with low
(VC-1) and high (VC-2) vanadium contents were prepared
by deposition of VCl4 on highly dispersed MgCl2, which was
prepared according to literature [24, 29]. The VC-1 catalyst
contained 0.12 wt% of vanadium, 18.2 wt% of magnesium
and 1.4 wt% of aluminum. The VC-2 catalyst contained 4.0
wt% of vanadium, 18.5 wt% of magnesium and 1.6 wt%
of aluminum. The catalysts have the average particle size
11.7µm and a narrow particle size distribution (the SPAN
value was 0.57).
Polymerization
Polymerization of ethylene or copolymerization of ethylene
with 1-hexene was carried out in a 1L stainless steel reactor.
The reactor was equipped with an external heating jacket
and an internal cooling coil to maintain the polymerization
temperature. It was also equipped with a magnetic coupled
stirrer with variable stirring speed. A pressure sensor was
used to keep the polymerization pressure by controlling eth-
ylene valve; also, a temperature sensor was used to keep
the polymerization temperature by controlling hot and cold
water valves.
The polymerization conditions were as follows: heptane
was used as a solvent (0.25L); polymerization tempera-
ture was 80°C; and ethylene polymerization pressure was
5–10MPa. Hydrogen or 1-hexene was charged into the reac-
tor at the beginning of the polymerization; TIBA (triisobu-
tylaluminum) was used as a co-catalyst (4.8mmol L–1). The
reaction time was 15–120min.
Characterization
The vanadium concentrations in the catalyst were determined
by ES-ICP (an Optima 5300 DV spectrometer). MWD meas-
urements were carried out using a high-temperature gel per-
meation chromatography (GPC) PL 220 system equipped with
RI and DV detectors in 1,2,4-trichlorobenzene at a flow rate
of 1mL·min–1 and 160°C. The polymers were analyzed using
a set of Olexis columns. The instrument was calibrated using
polyethylene and polystyrene standards with a narrow MWD.
Polyethylene and copolymer of ethylene with 1-hexene
separation into fractions with narrow MWD was performed
using a PolymerChar PREP mc2 fractionation station (Spain)
according to literature [22], a sample (0.7–1g) was dissolved
in a definite volume of xylene for 2h. A calculated amount
of 2-(2-butoxyethoxy)-ethanol was then added to the polymer
solution to cause partial precipitation of the polymer. The total
volume of the liquid (xylene and 2-(2-butoxyethoxy)-ethanol)
was always 180mL. A hot polymer solution was filtered into
a collecting flask. The precipitated polymer was dissolved in a
new portion of xylene. The precipitation-dissolution procedure
was repeated to obtain 5–6 fractions with a narrow MWD. The
last fraction was washed with pure xylene. The obtained frac-
tions were precipitated with acetone; the polymer precipitate
was separated from the mother liquor by filtering and then
dried and brought to a constant weight by drying.
The total number of methyl groups (CH3
tot) in the ini-
tial homopolyethylenes and ethylene–1-hexene copolymers
as well as in the individual fractions of polymers isolated
by their fractionation according to molecular weights was
determined by FTIR spectroscopy (the absorption band at
1378 cm–1) using a Shimadzu FTIR 8400S spectrometer
and also by 13C nuclear magnetic resonance (13C NMR) of
solutions of the low-molecular fractions of polyethylene and
copolymers of ethylene with 1-hexene in o-dichlorobenzene
using a Bruker Avance-400 spectrometer according to litera-
ture [23, 30–32].
The number of terminal CH3 groups (CH3
termin) in the
polymers was calculated from the Mn values (GPC data)
upon formation of two terminal groups per one polymer
chain or estimated from 13C NMR data (for the low-molec-
ular fractions of polymers). The number of methyl branches
(CH3
br) in homopolyethylene was calculated as follows:
The number of butyl branches (Bu) in ethylene–1-hex-
ene copolymers (the content of 1-hexene) was calculated
as follows:
Results anddiscussion
Estimation ofthecontent ofmethyl branches
inpolyethylene fractions withdifferent molecular
weight
It was known that ethylene homopolymers obtained over
VMCs in the presence of hydrogen had a broad bimodal
(1)
(
CH
br
3)
=
(
CH
tot
3)
−
(
CH
termin
3)
(2)
(
Bu)=
(
CH
tot
3)
−
(
CH
termin
3)
−
(
CH
br
3)

Iranian Polymer Journal
molecular weight distribution and contained not only ter-
minal methyl groups, but also methyl branches[25, 32]. We
determined the distribution of these methyl branches over
PE fractions with different molecular weight. Data on the
molecular weight of the individual fractions and the content
of the methyl branches in these fractions for polyethylene
produced over the VC-2 catalyst are listed in Table1.
As can be seen in Table1 that low-molecular fractions
F1 and F2 virtually did not contain methyl branches. Frac-
tions F3–F6 with molecular weights from 20 to 165kg mol−1
contained 2.0–2.5 methyl branches per 1000 C. Accord-
ingly, the number of branches per chain increased with
the molecular weight of these fractions from 2.9 for frac-
tion F3 with Mn = 20 kg mol−1 to 23.6 for fraction F6 with
Mn = 165kg mol−1 (Table1). Data on the number of CH3
branches in F3–F6 fractions indicated that for these fractions
with Mn from 20 to 165kg mol−1 there was one methyl
branch per each 500 fragments (–CH2-) of the polymer
chain. Data on the content of the methyl branches in dif-
ferent fractions of the polymer were taken into account to
calculate the degree of branching in the individual fractions
of ethylene–1-hexene copolymers obtained over VMCs.
Ethylene–1‑hexene copolymerization overVMCs
withdifferent contents ofvanadium
Table2 lists data concerning the effect of 1-hexene content
on the activity of the catalysts VC-1 and VC-2, molecu-
lar weight characteristics of the produced copolymers and
content of the butyl branches in copolymers. Therewith,
to maintain a close concentration of 1-hexene in the reac-
tion medium (the conversion of 1-hexene not higher than
15–20%) we controlled the polymerization time and pro-
duced no more than 20g of the polymer in each experi-
ment. For this reason, the C2/C6 copolymerization over
the VC-2 catalyst, which has a high activity per gram of
catalyst (Table2, exp. 6) was carried out for a short time
(15–22min) (Table2, exps. 7–10). Over the VC-1 catalyst,
which had a low activity per gram of catalyst, the copo-
lymerization was performed for a longer time (120min)
(Table2, exps. 2–5).
According to the data of Fig.1, the introduction of 1-hex-
ene during polymerization considerably changed the shape
of the kinetic curves, which was most pronounced for the
VC-2 catalyst (Fig.1b). In the case of the ethylene homopo-
lymerization on this catalyst (exp. 6 in Fig.1b), the activ-
ity increased to the maximum value for quite a long initial
period (27min). The introduction of 1-hexene in polym-
erization sharply decreased at this period (exps. 7–10 in
Fig.1b). In the process, the achieved high activity substan-
tially exceeded the maximum activity in ethylene homopo-
lymerization. Activity of the catalyst in the copolymeriza-
tion of ethylene with 1-hexene noticeably decreases with
the reaction time to the values corresponding to ethylene
homopolymerization. These phenomena are observed also
for copolymerization on the VC-1 catalyst, but at a much
lower extent.
According to the data of Table2 and Fig.2, copolym-
erization was accompanied by a decrease in the Mn value
for copolymers obtained on catalysts VC-1 and VC-2 with
an increase in the concentration of 1-hexene in the reac-
tion medium. These data indicated that in the presence of
1-hexene the additional chain transfer reaction with 1-hexene
proceeded, as it was observed also for copolymerization of
ethylene with 1-hexene over supported titanium-magnesium
catalysts[29]. It should be noted that homopolyethylene and
ethylene–1-hexene copolymers produced over the VC-2 cat-
alyst have a lower molecular weight compared to the poly-
mers produced over the VC-1 catalyst (Table2 and Fig.2).
Therewith, the ethylene–1-hexene copolymers obtained
on both catalysts have a higher polydispersity (the Mw/Mn
value) in comparison with homopolyethylene. This was most
Table 1 Data on the molecular weight and MWD of individual fractions and the content of methyl branches in these fractions for polyethylene
produced over VC2 catalysta
a Polymerization conditions: 80°C, PC2H4 = 10bar, PH2 = 2bar, TIBA as a co-catalyst (4.8mmol L–1), for 2h; polymer yield 3.6kg (g cat)−1;
bcalculated from 13C NMR data; cData calculated from the results presented in this table for individual fractions
Polymer code Content of
fraction (%)
Mn (kg (mol)−1)Mw/Mn(CH3
tot)/1000C
(CH3 termin)/1000C (CH3
br)/1000C (CH3
br)per
polymer
chain
Initial – 10.6 10.4 3.9 2.6 1.3 –
F1 21.4 2.1 2.0 – 13.3/ 12.8 b– –
F2 13.9 7.7 1.6 3.3 3.6/ 4.0 b0 –
F3 18.9 20 4.0 3.4 1.4 2.0 2.9
F4 19.1 38 3.7 3.2 0.74 2.5 6.8
F5 13.0 91 3.3 2.8 0.3 2.5 16.3
F6 13.7 165 3.0 2.2 0.17 2.0 23.6
Sum c– 8.5 14.5 – 3.2

Iranian Polymer Journal
pronounced for copolymers obtained on the VC-2 catalyst
(exps. 6–10 in Table2).
MWD curves of copolymers in comparison with
homopolyethylenes are displayed in Fig.3. These data
show that broadening of MWD of the copolymers obtained
on both catalysts occurs due to a decrease in the molecular
weight and appearance of the low-molecular component
in the region of 105–103kg mol−1 without changes in the
molecular weight of the high-molecular shoulder in the
region of 106kg mol−1. According to these data, active
centers of the vanadium–magnesium catalyst producing
high-molecular homopolyethylene virtually did not partici-
pate in the chain transfer reaction with 1-hexene. These data
are radically different from the results obtained during the
copolymerization of ethylene with 1-hexene over titanium-
magnesium catalysts [33]. In the case of copolymerization of
ethylene with hexene-1 over TMCs, the active sites produc-
ing a high-molecular copolymer have an increased reactivity
in the chain transfer reaction involving 1-hexene-1. This led
to a narrowing of the MWD of the copolymer compared to
the homopolyethylene.
Earlier, we have proposed a mechanism of the chain trans-
fer reaction with participation of α-olefin, which proceeded
during copolymerization of ethylene with 1-hexene over tita-
nium-magnesium catalysts [34]. Presumably, the first step of
this reaction was the 2,1-addition of 1-hexene to the grow-
ing polymer chain with subsequent transfer of the hydrogen
atom from the alkyl branch formed after the 2,1-addition of
1-hexene to coordinated ethylene. According to this scheme,
the chain transfer reaction involving 1-hexene proceeded on
the active centers with decreased regiospecificity.
Data on the chain transfer reaction with 1-hexene,
which were obtained for catalysts VC-1 and VC-2, can be
explained by high regiospecificity of the active centers of
these catalysts producing the high-molecular polymer; the
chain transfer reaction with participation of 1-hexene vir-
tually did not proceed on these centers. The reported data
on non-uniformity of the VMC active centers in the chain
transfer reaction involving 1-hexene and also on the effect
of this non-uniformity on polydispersity (the Mw/Mn value)
of the produced copolymers supplemented our earlier data
[22] concerning non-uniformity of the active centers of these
catalysts in the chain transfer reaction with hydrogen dur-
ing ethylene homopolymerization over catalysts VC-1 and
VC-2. Therewith, it was found that the active centers produc-
ing high-molecular polyethylene have a decreased reactivity
in the chain transfer reaction with hydrogen, and this was
one of the reasons for the formation of polyethylene with
bimodal MWD on these catalysts during ethylene polym-
erization in the presence of hydrogen.
Figure4 displays MWD curves for copolymers with a
close content of branches in the polymer chain, which were
obtained over catalysts VC-1 and VC-2 (exps. 5 and 9 in
Table 2 Data on the ethylene–1-hexene copolymerization over VMCs with different vanadium contents
a Polymerization conditions: 80°C, PC2H4 = 10bar, PH2 = 0.5bar, TIBA as a co-catalyst; bthe polymerization time; cthe average polymerization rate Rp during polymerization time, which was
calculated using the polymer yield (kg PE/g cat) and polymerization time values; dthe maximum polymerization rate estimated from the data of Fig.1; efrom the analysis of ethylene homopoly-
mer
Catalyst Exp noa[C6H12]
[M]
τp (min)bYield (g (gcat)−1)Rp
av [kg
(gcat·h)−1)c
Rp
max [kg
(gcat·h)−1)d
Mn (kg mol−1)Mw/Mn(CH3
tot)
/1000 C
(CH3
termin)/
1000 C
(CH3
br)/
1000 Ce
(Bu)/1000 C mol.% 1-hexene
VC-1 (0.12 wt% of V) 1 – 120 555 232 292 50 8.4 1.3 0.56 0.74 – –
2 0.1 120 548 225 354 41 10.0 3.1 0.68 0.74 1.7 0.34
3 0.16 120 603 250 475 41 8.8 4.4 0.68 0.74 3.0 0.60
4 0.24 120 630 267 475 37 9.7 4.6 0.76 0.74 3.1 0.62
5 0.32 120 630 258 583 30 12.0 6.0 0.93 0.74 4.3 0.87
VC-2 (4.0 wt% of V) 6 - 120 13,393 168 248 36 10.1 1.4 0.78 0.62 – –
7 0.1 20 3893 298 375 33 10.6 3.4 0.85 0.62 1,9 0.34
8 0.16 22 3839 260 394 22 13.2 4.5 1.27 0.62 2.6 0.52
9 0.24 15 4107 420 500 16 15.6 6.6 1.75 0.62 4.2 0.85
10 0.32 16 3893 360 438 17 14.4 7.1 1.65 0.62 4.8 0.97

Iranian Polymer Journal
Table2). Copolymers produced on both catalysts had a
broad bimodal MWD; however, the copolymer obtained
on the VC-2 catalyst with a high vanadium content has a
broader MWD (Mw/Mn = 15.6) compared to Mw/Mn = 12.0
for the copolymer produced on the VC-1 catalyst. One can
see in the MWD curves (Fig.4) that the copolymer obtained
on the VC-1 catalyst had a higher fraction of high-molecular
polymer, while the copolymer obtained on the VC-2 cata-
lyst had a higher fraction of low-molecular copolymer. The
decrease in molecular weight and the appearance of a low-
molecular component in the region of 103kg mol−1 led to
broadening of MWD for the copolymer produced over VC-2
catalyst.
We used the known simplified copolymerization Eq.1
[35, 36], which interrelated the concentration of comono-
mer (α-olefin) in the reaction medium and its content in
the copolymer, to estimate the comonomer reactivity ratio
(r1) at copolymerization of 1-hexene with ethylene over
VMCs with different vanadium contents.
Data about the effect of [C2H6]/[C2H4] ratio at copoly-
merization on the content of butyl branches in copolymers
obtained with catalysts VC-1 and VC-2 are presented in
Fig.5.
The calculated r1 value (30 ± 0.5) was close for both
catalysts. This value is close to the data reported in [35]
for copolymerization of ethylene with 1-hexene over previ-
ous modification of VMC with a lower activity.
Branching distribution data in ethylene–1-hexene
copolymers produced over vanadium-magnesium catalysts
with different vanadium contents.
We selected two samples of copolymers with a close
content of butyl branches, which were obtained over VC-1
and VC-2 catalysts (exps. 5 and 9 in Table2) to determine
the branching distribution in these copolymers for indi-
vidual fractions with different molecular weight. These
copolymer samples were fractionated into six fractions
with different molecular weights and narrow molecular
weight distributions.
When estimating the content of butyl branches (the
content of 1-hexene) in the individual fractions of ethyl-
ene–1-hexene copolymers produced over VMCs, we took
into account data on the content of methyl branches in
these fractions (Tables3 and 4 and Fig.6).
(3)
[
C𝛼
]
(polymer)=1∕r
1
([𝛼- olefin]∕
[
C
2
H
4]
(heptane
)
Fig. 1 Kinetic curves of ethylene homopolymerization and copolymerization of ethylene with 1-hexene at different concentrations of 1-hexene
over catalysts: a VC1 and b VC2 for the experiments listed in Table2
Fig. 2 The number-average molecular weight (Mn) of copolymers
obtained over VMCs with different vanadium contents versus the
concentration of 1-hexene in the reaction medium (according to the
data of Table2)

Iranian Polymer Journal
Data of Table1 concerning the molecular weight of the
individual fractions and the content of methyl branches in
these fractions for polyethylene obtained over VMCs indi-
cated that methyl branches are present only in the polymer
fractions with Mn ≥ 20kg mol−1 (one methyl branch per each
500 (–CH2-) fragments of the polymer chain). Data of Table1
were used to derive the scaling factor (K) for the content of
methyl branches in these fractions from the degree of methyl
branching of the initial polymer:
(4)
K=(CHbr
3
∕1000 C)(a fraction with M
n
≥20 kg mol−1)
(2.25 −the average number of methyl
groups per 1000 C in these fractions
)
This factor was used to calculate the number of methyl
branches in different fractions of polyethylenes and
ethylene–1-hexene copolymers with different molecu-
lar weights, which were obtained at a different content
of hydrogen in the polymerization medium. The initial
homopolyethylenes produced over catalysts VC-1 and
VC-2 had the degree of methyl branching (CH3
br/1000 C)
0.74 and 0.62, respectively (Table3, exps. 1 and 6). Thus,
fractions of these polymers (PE and ethylene–1-hexene
copolymers) with the molecular weight ≥ 20 kg mol−1
(5)
K=(CHbr
3
∕1000 C)(the initial polymer)
(1.3 −the number of methyl groups
per 1000 C in the initial polyethylene
)
=1.73
Fig. 3 MWD curves for copolymers in comparison with homopolyethylenes produced over catalysts: a VC-1 and b VC-2 (Experiments numbers
corresponded to those presented in Table2)
Fig. 4 MWD curves for copolymers with a close content of branches
in the polymer chain, which were obtained over catalysts VC-1 (exp.
5) and VC-2 (exp. 9) (Experiments numbers corresponded to those
presented in Table2)
Fig. 5 Effect of [C2H6]/[C2H4] ratio in heptanes at copolymerization
on the content of butyl branches in copolymers produced with cata-
lysts VC-1 and VC-2

Iranian Polymer Journal
(F2–F6) will contain 1.3 and 1.1 CH3
br/1000 C,
respectively.
Data on the content of butyl branches in the individual
fractions with different molecular weights for copolymers
produced over VC-1 and VC-2 catalysts are listed in Tables3
and 4. Data on the branches in separate distribution in the
individual fractions with different molecular weight are
shown also in Fig.6.
As can be seen in Fig.6, the distribution of butyl branch-
ing in the individual fractions with different molecular
Table 3 Data on the content of butyl branches in the individual fractions with different molecular weight for copolymer of ethylene with 1-hex-
ene produced over VC-1 catalyst (exp. 5 in Table2)
Polymer code Content
of fraction
(%)
Mp (kg mol−1)Mn (kg mol−1)Mw (kg mol−1)Mw/Mn(CH3
tot)/
1000 C
(CH3
termin)/
1000 C
(CH3
br) + (Bu)/
1000 C
(Bu)/ 1000 C
Total sample 62 36 300 8.3 6.0 0.8 5.2 4.3
F1 11.9 8 4.1 7 1.7 – 7.0 10.4 3.4
F2 18.1 29 21 31 1.5 6.9 1.3 5.6 4.3
F3 31.9 72 46 79 1.7 6.1 0.6 5.5 4.2
F4 14.4 200 150 490 3.3 6,1 0.2 5.9 4.6
F5 13.5 280 290 640 2.2 5.3 0.1 5.2 3.9
F6 10.2 1280 640 1020 1.6 6.7 0.02 6.7 5.4
Sum 58 22 291 13.1 5.5 4.3
Table 4 Data on the content of butyl branches in the individual fractions with different molecular weight for copolymer of ethylene with 1-hex-
ene produced over VC-2 catalyst (exp. 9 in Table2)
Polymer
code
Content
of fraction
(%)
Mp (kg
mol−1)
Mn (kg
mol−1)
Mw (kg
mol−1)
Mw/ Mn(CH3
tot)/1000
C
(CH3
termin)/
1000 C
(CH3
br) + (Bu)/
1000 C
(Bu)/ 1000 C
Total sample 43 18 210 11.7 6.6 1.6 5.0 4.4
F1 18.7 9 4.3 8.6 2.0 – 6.5 10.5 4.0
F2 22.9 27 21 29 1.4 6.8 1.3 5.5 4.4
F3 28.8 72 57 92 1.6 5.0 0.5 4.5 3.4
F4 10.0 210 140 270 1.9 5.2 0.2 5.0 3.9
F5 11.7 480 440 810 1.8 5.1 0.06 5.0 3.9
F6 7.8 1290 690 1100 1.6 6.9 0.04 6.9 5.8
Sum 41 17 240 14.0 5.1 4.2
Fig. 6 Variations of butyl branching distribution for the molecular weight of ethylene–1-hexene copolymer produced over a VC-1 catalyst
(according to the data of Table3) and b VC-2 catalyst (according to the data of Table4)

Iranian Polymer Journal
weights for copolymers produced with catalysts VC-1 and
VC-2 is similar: fractions F1–F5 with Mw values within
the range of 7–810kg mol−1 contained close amounts
of branches (3.4–4.4 Bu/1000 C), but fractions F6 with
the highest Mw values (1020–1110kg mol−1) contained
higher amounts of butyl branches (5.4–5.8 Bu/1000 C).
Note the opposite data have been obtained in the case of
ethylene–1-hexene copolymerization over supported tita-
nium–magnesium catalysts else where [29, 31].
In this case, the fraction with the lowest molecular
weight contains a much greater (five-fold) number of butyl
branches compared to the fraction with the highest molec-
ular weight. Thus, the active centers of VC-1 and VC-2,
which produce the fraction of copolymers with the highest
molecular weight, have a higher reactivity in the incorpora-
tion of 1-hexene during copolymerization of ethylene with
1-hexene.
According to our data [37] obtained for copolymeriza-
tion of ethylene with 1-hexene over titanium-magnesium
catalysts of different compositions, the catalysts containing
low-valence titanium ions (Ti2+) have enhanced the copo-
lymerization ability. Presumably, in the case of vanadium-
magnesium catalysts, the enhanced copolymerization ability
could be observed also for the low-valence vanadium ions
(V2+), which produced polymer with an increased molecular
weight.
Conclusion
Vanadium–magnesium catalysts VC-1 and VC-2 pro-
duced polyethylene with some amount of methyl branches
(1.3CH3/1000 C) at homopolymerization of ethylene. It
was found that the low-molecular fractions of polyethylene
with molecular weight Mn below 8kg mol−1 virtually did
not contain methyl branches, while the higher molecu-
lar fractions with Mn from 20 to 165kg mol−1 contained
methyl branches in the amount of 2.0–2.5 CH3/1000°C.
The introduction of 1-hexene during copolymerization
over catalysts VC-1 and VC-2 led to changes in the shape
of kinetic curves: a decrease in the initial period when
the polymerization rate grew with time and an increase in
the maximum polymerization rate. This phenomenon was
most pronounced for the VC-2 catalyst having an increased
content of vanadium. In this case, the maximum activity of
the catalyst reached 20kg (g cat·h)−1 (500kg (g V·h)−1).
With increasing the reaction time, the copolymerization
rate decreased to the values close to those observed at eth-
ylene homopolymerization. The introduction of 1-hexene
during copolymerization over catalysts VC-1 and VC-2
led to a considerable decrease in the molecular weight of
copolymer due to the additional chain transfer reaction
with participation of 1-hexene. It was found also that the
introduction of 1-hexene led to broadening of the molecu-
lar weight distribution of the copolymer (an increase in the
Mw/Mn value) relative to homopolyethylene. Therewith,
the broadening of MWD occured due to a decrease in the
molecular weight (Mn) in the region of 1–100kg mol−1
without changes in the molecular weight in the region of
Mn > 500kg mol−1. This result testified to heterogeneity
of the VMC active centers in the chain transfer reaction
with participation of 1-hexene. Therewith, the VMC active
centers producing high-molecular polyethylene were virtu-
ally not involved in the chain transfer reaction with 1-hex-
ene. Data concerning the effect of 1-hexene in the reaction
medium on the content of butyl branches in copolymers
obtained over catalysts VC-1 and VC-2 were used to cal-
culate the copolymerization constant r1, the value of which
is close for both catalysts and equal to 30 ± 0.5. Data on
the content of butyl branches in individual fractions of
copolymers with different molecular weights, which were
produced over catalysts VC-1 and VC-2 were obtained.
Copolymers produced on these catalysts were found to
have close branching distributions: fractions with Mw from
7 to 810kg mol−1 contained lower number of branches
(3.4–4.4 Bu/1000 C) compared to the high-molecular frac-
tion with Mw from 1020 to 1110kg mol−1, which contains
a greater number of branches (5.4–5.8 Bu/1000 C). Thus,
active sites of these catalysts, which produced copolymers
with the highest molecular weight, had a higher reactivity
in the incorporation of 1-hexene at copolymerization of it
with ethylene. In this case, the distribution of branching
in the fractions of copolymers with different molecular
weights, which were produced over VMCs, sharply dif-
ferred from the results obtained with supported titanium-
magnesium catalysts. The copolymerization of ethylene
with α-olefins over TMCs resulted in the formation of
copolymers in which the content of branches in the low-
molecular fractions substantially exceeded the content of
branches in the high-molecular fractions of copolymers.
Acknowledgements This work was financially supported by Petro
China Petrochemical Research Institute (Daqing) and the budget project
No. FWUR-2024-0037 for Boreskov Institute of Catalysis (Novosi-
birsk). The authors thank Nadezhda Mozgunova for catalyst prepara-
tion, Marina Vanina for GPC analysis and fractionation of polymers
produced, and Igor Soshnikov for polymer analysis by means of 13C
NMR spectroscopy.
Data availability All data generated or analyzed during this study are
included in this published article.
Declarations
Conflict of interest The authors declare that they have no conflict of
interest.

Iranian Polymer Journal
References
1. Spalding MA, Chatterjee AM (eds) (2017) Handbook of industrial
polyethylene and technology: definitive guide to manufacturing,
properties, processing, applications and markets. Scrivener Pub-
lishing LLC, Beverly, MA. https:// doi. org/ 10. 1002/ 97811 19159
797
2. Paulik C, Spiegel G, Jeremic D (2019) In: Albunia AR, Prades F,
Jeremic D (eds) Multimodal polymers with supported catalysts.
Springer, Cham. https:// doi. org/ 10. 1007/ 978-3- 030- 03476-4
3. Nowlin TE (2014) Business and technology of the global polyeth-
ylene industry: an in-depth look at the history, technology, cata-
lysts, and modern commercial manufacture of polyethylene and
its products. Wiley-Scrivener Publishing, Salem, MA
4. Sauter DW, Taoufik M, Boisson C (2017) Polyolefins, a success
story. Polymers 9:185
5. Plastics Europe: Plastics-The Facts (2020) Available online:
https:// plast icseu rope. org/ de/ wpcon tent/ uploa ds/ sites/3/ 2021/ 11/
Plast ics_ the_ facts- WEB- 2020v ersio nJun21_ final. pdf. Accessed
27 June 2022
6. Zucchini U, Cecchin G (1983) Control of molecular-weight distri-
bution in polyolefins synthesized with Ziegler-Natta catalytic sys-
tems. In: Industrial developments. Advances in polymer science,
vol 51. Springer, Berlin. https:// doi. org/ 10. 1007/ BFb00 17586
7. Shan CLP, Soares JBP, Pendilis A (2002) Mechanical properties
of ethylene/1-hexene copolymers with tailored short chain branch-
ing distributions. Polymer 43:767–773. https:// doi. org/ 10. 1016/
S0032- 3861(01) 00666-8
8. Alt FP, Böhm LL, Enderle H-F, Berthold J (2001) Bimodal
polyethylene. Interplay of catalyst and process. Macromol Symp
163:135–143. https:// doi. org/ 10. 1002/ 1521- 3900(200101) 163:1%
3c135:: AID- MASY1 35% 3e3.0. CO;2-7
9. Aigner P, Averina E, Garoff T, Paulik C (2017) Effects of altera-
tions to Ziegler-Natta catalysts on kinetics and comonomer
(1-butene) incorporation. Macromol React Eng 11:1700009
10. Cӧpperl L, Pernusch D, Schwarz J, Paulik C (2022) Impact of
polymerization process parametrs on improved comonomer incor-
poration behavior in Ziegler-Natta catalysis. Macromol React Eng
16:2100042
11. Cho HS, Chung JS, Han JH, Ko YG, Lee WY (1998) Polym-
erization of ethylene and ethylene/1-hexene over Ziegler–Natta/
metallocene hybrid catalysts supported on MgCl2 prepared by a
recrystallization method. J Appl Polym Sci 70:1707–1715
12. Masoori M, Rashedi R, Sepahi A, Jandaghian MH, Nikzinat E,
Houshmandmoayed S (2022) Structure–performance relationship
(SPR) of Ziegler Natta catalysts (TiCl4/MgCl2 based) in ethyl-
ene/1 butene and ethylene/1 hexene copolymerization. J Polym
Res 29:317
13. Mikenas TB, Zakharov VA (1984) Polymerization of ethylene in
the presence of the supported catalysts containing vanadium and
magnesium chlorides. Vysokomol Soedin Ser B 26:483–485 ((in
Russian))
14. Zakharov VA, Mikenas TB, Makhtarulin SI, Poluboyarov VA,
Pankratʹev YD (1989) Study of supported titanium-magnesium
and vanadium-magnesium catalysts for polymerization of ethyl-
ene containing different amounts of transition-metal. Kinet Catal
29:1103–1106
15. Karol FG, Cann KJ, Wagner BE (1988) In: Kaminsky W, Sinn H
(eds) Transition metals and organometallics as catalysts for olefin
polymerization. Springer, Berlin. https:// doi. org/ 10. 1007/ 978-3-
642- 83276-5_ 16
16. Spitz R, Patin M, Robert P, Masson P, Dupuy Cnrs J (1994) The
control of molecule weight distribution in Ziegler–Natta catalysis.
In: In: Terano M and Soga K (eds) Catalyst design for tailor-made
polyolefins, 1st edn, vol. 89. Elsevier, Kanazawa, Japan
17. Mikenas TB, Zakharov VA, Echevskaya LG, Matsko MA (2001)
Ethylene polymerization with supported vanadium-magnesium
catalyst: hydrogen effect. Macromol Chem Phys 202:475–481.
https:// doi. org/ 10. 1002/ 1521- 3935(20010 201) 202:4% 3c475::
AID- MACP4 75% 3e3.0. CO;2-V
18. Zakharov VA, Echevskaya LG (1997) Kinetics of the copolymeri-
zation of ethylene with α-olefins initiated by supported titanium-
magnesium and vanadium-magnesium catalysts. Polym Sci B
(Eng. Transl.) 39:291–294
19. Zakharov VA, Echevskaya LG, Mikenas TB (1991) Studying of
reaction of transfer of a polymeric chain with hydrogen at polym-
erization of ethylene over titanium-magnesium and vanadium-
magnesium catalysts. Vysokomol Soedin Ser B (in Russian)
33:102–104
20. Echevskaya LG, Matsko MA, Mikenas TB, Zakharov VA (2006)
Molecular mass characteristics of polyethylene produced with
supported vanadium-magnesium catalysts. Polym Int 55:165–170.
https:// doi. org/ 10. 1002/ pi. 1933
21. Wang D, Zhao Z, Mikenas TB, Lang X, Echevskaya LG, Zhao
C, Matsko MA, Wu W (2012) A new high-performance Ziegler-
Natta catalyst with vanadium active component supported on
highly-dispersed MgCl2 for producing polyethylene with broad/
bimodal molecular weight distribution. Polym Chem 3:2377–
2382. https:// doi. org/ 10. 1039/ C2PY2 0163A
22. Matsko MA, Echevskaya LG, Mikenas TB, Nikolaeva MI, Vanina
MP, Zakharov VA (2011) Analysis of the molecular structure of
ethylene hexene-1 copolymers produced over highly active sup-
ported Ziegler–Natta catalysts. Catal Ind 3:109–115. https:// doi.
org/ 10. 1134/ S2070 05041 10200 97
23. Matsko MA, Echevskaya LG, Vanina MP, Nikolaeva MI, Mikenas
TB, Zakharov VA (2012) Study of the compositional heterogene-
ity of ethylene/1-hexene copolymers produced over supported cat-
alysts of different composition. J Appl Polym Sci 126:2017–2023
24. Mikenas TB, Zhao Z, Nikitin VE, Fu I, Zakharov VA, Ley S,
Matsko MA, Wu W, Bessudnova EV, Zhen K, Wang D, Van D
(2019) A method for preparing a vanadium magnesium catalyst
for ethylene polymerization and ethylene copolymerization with
alpha-olefins. Russ patent 2682163C1
25. Echevskaya LG, Matsko MA, Mikenas TB, Nikitin VE, Zakharov
VA (2006) Supported titanium–magnesium catalysts with differ-
ent titanium content: kinetic peculiarities at ethylene homopoly-
merization and copolymerization and molecular weight charac-
teristics of polyethylene. J Appl Polym Sci 102:5436–5442
26. Jiang B, Zhang B, Guo Y, Ali A, Guo W, Fu Z, Fan Z (2020)
Effects of titanium dispersion state on distribution and reac-
tivity of active centers in propylene polymerization with
MgCl2-supported Ziegler-Natta catalysts: a kinetic study based
on active center counting. Chem Cat Chem 12:5140–5148. https://
doi. org/ 10. 1002/ cctc. 20200 0778
27. Koshevoy EI, Mikenas TB, Zakharov VA, Volodin AM, Kenzhin
RM (2014) Formation of isolated titanium (III) ions in superac-
tive titanium–magnesium catalysts with a low titanium content as
active sites in ethylene polymerization. Catal Commun 48:38–40
28. Mikenas TB, Koshevoy EI, Zakharov VA, Nikolaeva MI (2014)
Formation of isolated titanium (III) ions as active sites of sup-
ported titanium–magnesium catalysts for polymerization of ole-
fins. Macromol Chem Phys 215(18):1707–1720
29. Mikenas TB, Zhao Z, Guan P, Matsko MA, Zakharov VA, Wu
W (2022) Ethylene polymerization over supported vanadium-
magnesium catalysts with different vanadium content: the effect
of hydrogen on molecular weight characteristics of the produced
bimodal polyethylene. Catalysts 12:985. https:// doi. org/ 10. 3390/
catal 12090 985
30. Randall JC (1978) Methylene sequence distributions and number
average sequence lengths in ethylene-propylene copolymers. Mac-
romolecule 11:33–36

Iranian Polymer Journal
31. Nikolaeva MI, Matsko MA, Mikenas TB, Echevskaya LG,
Zakharov VA (2012) Copolymerization of ethylene with
α-olefins over supported titanium–magnesium catalysts. I. Effect
of polymerization duration on comonomer content and the
molecular weight distribution of copolymers. J Appl Polym Sci
125:2034–2041
32. Echevskaya LG, Zakharov VA, Golovin AV, Mikenas TB
(1999) Molecular structure of polyethylene produced with sup-
ported vanadium-magnesium catalyst. Macromol Chem Phys
200:1434–1438
33. Matsko MA, Zakharov VA (2023) Heterogeneity of active sites in
the polymer chain transfer reactions at olefin polymerization over
multisite supported Ziegler–Natta catalysts. Polymers 15:4316.
https:// doi. org/ 10. 3390/ polym 15214 316
34. Nikolaeva MI, Matsko MA, Mikenas TB, Echevskaya LG,
Zakharov VA (2012) Copolymerization of ethylene with α-olefins
over supported titanium–magnesium catalysts. II. Comonomer as
a chain transfer agent. J Appl Polym Sci 125:2042–2049
35. Echevskaya LG, Zakharov VA, Bukatov GD (1987) Composi-
tion effect of highly active supported Ziegler catalysts for eth-
ylene copolymerization with α-olefins. React Kinet Catal Lett
34:99–104
36. Böhm LL (1981) Zur copolymerisation von ethylen und α-olefinen
mit Ziegler-katalysatoren. Macromol Chem 182:3291–3310.
https:// doi. org/ 10. 1002/ pol. 1950. 12005 0210
37. Mikenas TB, Zakharov VA, Guan P, Matsko MA (2023) Copo-
lymerization of ethylene with alpha-olefins over supported tita-
nium–magnesium catalysts containing titanium compounds in
different oxidation and coordination states. Appl Sci 13:5030.
https:// doi. org/ 10. 3390/ app13 085030
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Authors and Aliations
ZenghuiZhao1· TatianaB.Mikenas2 · PengGuan3· MikhailA.Matsko2· VladimirA.Zakharov2· WeiWu4
* Tatiana B. Mikenas
mikenas@catalysis.ru
1 China National Petroleum Corporation, Daqing Chemical
Research Institute, Daqing, China
2 Boreskov Institute ofCatalysis, Siberian Branch
oftheRussian Academy ofSciences, Prospect Akademika
Lavrentieva, 5, Novosibirsk630090, Russia
3 Novosibirsk State University, St. Pirogova 2,
Novosibirsk630090, Russia
4 Heilongjiang University, Harbin, China