Macromol React Eng 2009 3 257262_Controlled Catalyst Dosing An Elegant

Controlled Catalyst Dosing: An Elegant
Approach in Molecular Weight Regulation for
UHMWPEa
Sudhakar Padmanabhan,* Krishna R. Sarma, Shashikant Sharma, Viral Patel
Introduction
Ultrahigh-molecular-weight polyethylene (UHMWPE)
belongs to the specialty grade of polyethylene (PE) having
an average molecular weight greater than
3 Â 106
g Á molÀ1
.[1]
UHMWPE is unique and is distinctly
different from its homologs as can be seen from the
‘‘ultrahigh’’ molecular weight. For a polymer, its molecular
weight is an important characteristic and plays a key role in
the application properties. Because of its ultrahigh mole-
cular weight, UHMWPE has excellent wear resistance,
outstanding impact strength and very good chemical
resistance.[2]
Consequently it finds applications in diversi-
fied areas with unique requirements.[3]
More than two thirds of the commercial processes
employed for manufacturing UHMWPE are based on
continuous stirred tank reactor using conventional Zieg-
ler-Natta catalysts.[4]
Few plants are also using metallo-
cene-based catalyst systems at very limited capacities.[5]
Among the various grades of UHMWPE, the grade with
molecular weight 4–5 Â 106
g Á molÀ1
is unique because of
the optimum abrasion resistance, impact strength, and
chemical resistance.[6]
Hence, the 4 Â 106
g Á molÀ1
mole-
cular weight grade is having maximum business volume.
At higher molecular weights, though the abrasion resis-
tance is slightly better than that of the lower molecular
weight polymers, the impact strength drops down con-
siderably.[6]
Most of the PE produced based on the market needs are
manufactured using traditional Ziegler-Natta catalysts
which typically comprise titanium halides (TiX4 where X
is generally Cl) supported on magnesium chloride (MgCl2) –
through various chemical modifications.[7]
The activity of
these catalysts not only depend on the total titanium
Full Paper
S. Padmanabhan, K. R. Sarma, S. Sharma, V. Patel
Research Centre, Vadodara Manufacturing Division, Reliance
Industries Limited, Vadodara 391 346, India
Fax: þ91 265 669 3934;
E-mail: sudhakar.padmanabhan@zmail.ril.com
a
: Supporting information for this article is available at the bottom
of the article’s abstract page, which can be accessed from the
journal’s homepage at , or from the author.
Ethylene polymerization using Ziegler-Natta catalysts comprising TiCl4 supported on MgCl2
with aluminum alkyls as co-catalyst produce UHMWPE through a mediated proportional
reduction of the Ti oxidation states. We observe the molecular weight regulation to be a
combined function of hydrogen and the co-cata-
lyst through experimental evidences. The role of
unavoidable chain transfer reactions for molecu-
lar weight regulation has been observed during
the production of UHMWPE along with H2
through controlled catalyst dosing. Process optim-
ization studies at 7.5 atm ethylene pressure could
absorb small deviations in the catalyst systems to
maintain the desired molecular weights.
Macromol. React. Eng. 2009, 3, 257–262
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mren.200900001 257

present in the system but also depend on the percentage of
the reduced titanium species.[4]
In general, when alkyl
aluminum is added to the catalyst system containing
titanium in the 4þ oxidation state, it gets transformed to
lower oxidation states depending on the amount of alkyl
aluminum which basically functions as a reducing agent.
The production of UHMWPE using these catalyst systems is
again a big task taking in to account the possible
termination reactions which can kill the propagating active
species. The presence of excess aluminum alkyls brings
about the termination via transfer of polymer chain to
aluminum, reducing the length/molecular weight of the
polymer chain. It also results in broadening the molecular
weight distribution.[8,9]
Apart from the termination reac-
tions described above which brings down the molecular
weight of the growing polymer chain, the presence of
excess aluminum alkyl takes part in eliminating the
impurities which are detrimental to oleﬁn polymerization.
This is especially true when carrying out laboratory
experiments in 5 L scale and above as the catalyst can
easily be killed by the presence of small amounts of
impurities in the solvent. Hence, optimization of Al content
with respect to catalyst quantity is inevitable and is of
primeimportance especiallyinthecaseofUHMWPE, where
the molecular weight is of paramount importance.
Here, in this paper, we have demonstrated the capability
of using aliphatic hydrocarbon as a polymerization
medium for producing UHMWPE having controlled poly-
mer properties. We have also developed laboratory process
and catalyst recipes for making UHMWPE of higher
molecular weights namely 6, 8, and 10 Â 106
g Á molÀ1
.
We have demonstrated the effective utilization of side
reactions such as termination due to alkyl aluminum in
regulating the molecular weight of the growing polymer
chains.
Experimental Part
General Considerations
All glassware was scrupulously cleaned and oven dried prior to use.
All manipulations like handling/transfer of catalysts and pyr-
ophoric aluminum alkyls were carried out in a nitrogen glove bag.
Catalyst designated C-00, was prepared by treating Mg(OEt)2 with
TiCl4 in hexane as the medium at high temperature, washed with
hexane to remove dissolved impurities, dried under N2 and used as
a hexane slurry. The other catalyst batches with different Ti3þ
contents were synthesized from C-00 by adjusting the AlRR0
2
quantity and are designated C-16 (Ti3þ
16%), C-21 (Ti3þ
21%), C-25
(Ti3þ
25%),C-32(Ti3þ
32%),andC-50(Ti3þ
50%).TheTicontentinthe
catalysts was evaluated as per standard procedures using spectro-
scopic techniques. The average particle size (APS) of the catalysts
was in the range of 8–16 mm in the order of C-16>C-21>C-25>
C-32>C-50.
Preparation of Catalyst, C-00
10 g of Mg(OEt)2 was added to 150 mL of Varsol, a high boiling
kerosenefractionunderanatmosphereofnitrogenandmechanical
stirring. The temperature was increased to 85 8C and maintained.
Subsequently 35 g (20 mL) of TiCl4 was added to the Mg(OEt)2
suspension under a gentle atmosphere of nitrogen slowly over a
period of 5–6 h. The molar ratio of Mg/Ti was 1:2. After the TiCl4
addition was complete the temperature was increased to 120 8C
and maintained for 60 h to temper the catalyst. The Varsol
contained the precatalyst as a pale yellow to white suspension. The
titanium in this system is present in the 4þ state. The composition
approximately works out to Mg/Ti/Cl as 1:1.4:3.5.
Preparation Activated Catalysts
The actual catalyst for UHMWPE was prepared from the C-00 by
reducing the same using an equal volume mixture of triisobuty-
laluminum and isoprenylaluminum. The molar ratio employed
between the Ti catalyst and the aluminum alkyl was adjusted as
perthecatalystsynthesized. The aluminumalkyl wasgently added
at 25 8C to the C-00 under a stream of nitrogen and mechanical
agitation over a period of 5 h. The color of the slurry changed to
grayish black and hence the catalyst is also referred to as the ‘‘black
catalyst.’’ Here, the titanium is present as a mixture of 4þ and 3þ
titanium. The trivalent titanium varied from about 16–50% based
on the catalyst system.
General Polymerization Procedure
The polymerization was carried out in a 5 L laboratory Buchi
reactor. Calculated amounts of the catalyst slurry and aluminum
alkyls were transferred under nitrogen to 3 L of dry hexane in a
standard catalyst charging vessel. The same was then transferred
under a gentle stream of nitrogen into the 5 L laboratory Buchi
reactor which was already baked at 80 8C for 4 h and cooled under
drynitrogentoambienttemperature.Agitationwasstartedtokeep
the slurry homogeneous and thus preventing the catalyst particles
from settling down. The required quantity of ethylene was allowed
to enter at 75 8C. The reaction was stopped by cutting off the
ethylene supply after 2 h from the time the reactor temperature
registered 75 8C. The polymer was isolated after cooling down
the reactor, washed with acidic methanol, methanol, and acetone
followed by drying under reduced pressure.
Results and Discussion
Catalyst Systems for UHMWPE
In general, the typical Ziegler catalyst namely titanium
supported on MgCl2 employing a unique combination of
aluminum alkyls, AlRR0
2 (an equal mixture of triisobutyl-
aluminum and isoprenylaluminum) as the activator and
hydrogen as the molecular weight regulator has been used
for the generation of UHMWPE.[10]
Essentially such
258
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mren.200900001

catalysts(C-00)wereusedformakingHDPEuponactivation
with TEAL.[11]
Tailoring the precatalyst to produce
UHMWPE through process optimization in hydrocarbon
media with rigid polymer specifications has been a
challenge in industrial arena. The process overview is
shown in Scheme 1.
The catalyst batches with different Ti3þ
contents were
synthesized from C-00 (Ti3þ
0%) by adjusting the AlRR0
2
quantity and are C-16 (Ti3þ
16%), C-21 (21%), C-25 (25%),
C-32 (32%), and C-50 (50%).
For a particular ethylene pressure (PC2) and catalyst
system (C-21) ethylene polymerization were carried out at
different Al/Ti ratios for its optimization and the results are
given in Figure 1.
It was observed that when ethylene pressure was 2 atm,
the optimum value of Al/Ti was %7–8 under the specified
operating conditions. This exercise needed optimization
when ethylene pressure was changed during polymeriza-
tion. At an ethylene pressure of about 5 atm we found that
the Al/Tiratio of 4–8showed an increase in catalyst activity
butfurtherincreaseinAl/Tiratiofrom8to16didnotshowa
significant difference in the yield of the polymer obtained.
At different Al/Ti ratios, besides yield the other polymer
properties like bulk density (BD) and average molecular
weight changed, thus providing a lever to alter the polymer
characteristics at the cost of yield. The
efficacies of catalyst and process condi-
tions are shown in Table 1. We have
exploited a commercial Ziegler-Natta
catalyst which is routinely used for
making poly(propylene) (PP) to give
UHMWPE of very high molecular weights and different
polymer characteristics. As seen from Table 1, changing the
Al concentration resulted in tailored polymers with diverse
properties.
Besides AlRR0
2 we have also evaluated triethylaluminum
(TEAL) as the activator for selected catalyst batches. Under
the experimental conditions employed, namely with
0.24 mmol Ti catalyst and Al/Ti ratio of 4, we found that
TEAL activated catalyst showed better productivity com-
pared to that of AlRR0
2 but the molecular weight of the
UHMWPE obtained was comparatively on the lower side to
that of the counter part as shown in Table 2. These results
Controlled Catalyst Dosing: An Elegant Approach in Molecular . . .
Scheme 1. UHMWPE process overview.
Figure 1. Optimization of Al/Ti under specified process conditions.
Table 1. Effect of Al/Ti over polymer properties. General conditions: PC2 7.5 atm, Cocatalyst used AlRR0
2 (an equal mixture of triisobuty-
laluminum and isoprenylaluminum), Al/Ti ¼ 4, 75 8C, 500 rpm and different catalyst concentrations with no H2.
Runa)
Catalyst (Ti) Al/Tib)
PC2 Yield (Ti) BD APSc)
% < 63c)
% > 250c)
Mv
d)
Type Amount atm g Á mLÀ1
g Á mLÀ1
mm mm mm 106
g Á molÀ1
mmol
1 C-21 3.64 5 2.0 19 0.22 100 12 2 2.7
2 C-21 3.64 10 2.0 92 0.26 80 52.3 0.1 3.8
3 C-21 3.64 15 2.0 66 0.25 83 52.7 0.1 5.7
4 C-00 1.20 4 7.5 617 0.4 125 11 3.1 15.0
5 C-00 1.20 8 7.5 767 0.39 110 9.7 1 10.1
6e)
CPP 0.13 40 7.5 4850 0.37 100 8.6 1.4 13.3
7e)
CPP 0.13 60 7.5 6100 0.35 113 6.2 2.1 13.6
8e)
CPP 0.13 100 7.5 6560 0.40 105 10.9 2.5 13.1
9e)
CPP 0.13 20 7.5 2190 0.26 185 38.4 14 13.6
a)
Extracted from extensive studies carried out in the laboratory for making UHMWPE; b)
Fixed based on catalyst performance; c)
Analyzed
by both Malvern PSA and traditional sieve shaker methods; d)
Viscosity-average molecular weight calculated using Margolie’s equation
[(5.37 Â 104
Â RSV1.49
)/106
]; e)
CPP, commercially available PP catalyst.
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mre-journal.de 259

indicate that TEAL acts as a better chain transfer agent than
that of the other aluminum alkyls. The use of TEAL as the
activator for UHMWPE formation is very limited because in
conjunction with hydrogen weend up with high-density PE
(HDPE).
Under similar operating conditions (PC2-5 atm; PH2-NIL;
75 8C) catalyst systems C-16, C-21, C-25, C-32, and C-50 were
screened for polymerization. The results indicated that
polymer obtained from C-25 catalyst system gave the
maximum reduced specific viscosity (RSV) as shown in
Figure 2a. At subsequent stages we realized that we could
get UHMWPE with desired molecular weight with other
catalyst systems as well, by suitable optimization on the
catalyst, activator and hydrogen concentrations.
The trend when ethylene pressure was 7.5 atm can be
seen in Figure 2b. Unlike the ethylene pressure at 5 atm
(Figure2a),ethylenepressureof7.5 atmdoesnotchangethe
polymer characteristics as seen from the steady RSV values
(Figure 2b) over a range of trivalent Ti content in the
catalysts. At 7.5 atm ethylene pressure the process proceeds
smoothly maintaining the same molecular weight even
with marginal variations in trivalent Ti content in the
catalyst. This could be very favorable for polymerization
processes in the plant level.
Molecular Weight Regulation Using Hydrogen for
UHMWPE Polymerization
Hydrogen is a well known molecular weight regulator used
in polyolefin industry during polymerization of ethy-
lene.[12]
We studied the effect of hydrogen on the solubility
of ethylene in different process solvents over a wide range
of temperature and pressure.[13]
It is interesting to note that
the presence of hydrogen reduces the solubility of ethylene
in hexane significantly at 30 8C. The extent of decrease in
the solubility of ethylene with 5 vol.-% hydrogen over no
hydrogen is nearly 50%. It can be further noted that the
extent of decrease in ethylene solubility with increase in
hydrogen is not linear. The extent of drop in solubility of
ethylene in hexane with 5% hydrogen and with 10%
hydrogen is not significantly different, while the amount of
decrease in solubility due to 5% hydrogen against 0%
hydrogen is significant. Similar trends in solubility were
seenat55 8Calso. Experimentswerealsocarriedoutat75 8C
at 6 atm ethylene pressure with 5 vol.-% hydrogen. The
solubility (mole fraction) of ethylene in the presence of
hydrogen was 0.032 while the ethylene solubility (mole
fraction) in the absence of hydrogen was 0.044. This implies
that presence of hydrogen, under polymerization condi-
tions,notonlydeactivatestheactivepolymerizationsitesof
the catalyst[14]
but also effectively brings down the
Table 2. Effect of cocatalyst over polymer properties. General conditions: PC2 7.5 atm, AlRR0
2 is an equal mixture of triisobutylaluminum and
isoprenylaluminum, Al/Ti ¼ 4, 75 8C, 500 rpm and different catalyst concentration with no H2.
Run Catalyst C-25 (Ti) Co-catalyst Al/Tia)
Yield (Ti) BD APSb)
Mv
c)
mmol g Á mmolÀ1
g Á cmÀ3
mm 106
g Á molÀ1
5 0.24 AlRR0
2 4 617 0.41 125 15
4 0.24 AlRR0
2 8 767 0.39 110 10.1
10 0.24 TEAL 4 630 0.39 108 6.4
11 0.24 TEAL 8 670 0.34 100 4.6
a)
Fixed based on catalyst performance; b)
Analyzed by both Malvern PSA and traditional sieve shaker methods; c)
Viscosity-average
molecular weight calculated using Margolie’s equation.
Figure 2. Effect of Ti3þ
oxidation state over polymer molecular
weight at (a) 5 atm and (b) 7.5 atm ethylene pressures.
260

ethylene available for polymerization thus altering the
polymerization rate.
Knowing the fact that hydrogen can act as a chain
terminating agent as well as affect the polymerization rate
by decreasing the solubility of ethylene in hexane. We have
opted to keep the hydrogen concentration constant and
vary the catalyst concentration to get UHMWPE of different
molecular weights.[9]
Molecular Weight Regulation Using Controlled
Catalyst Dosing at Specified Hydrogen Dosage for
UHMWPE Polymerization
As mentioned earlier, we require process conditions better
than that of 5 atm ethylene operating conditions to achieve
better polymer properties like BD, APS with controlled
amount of coarse and fine materials. Experiments at 7.5atm
ethylene pressure gave us most of the desired polymer
properties. We found that it was an excellent recipe for
making the 4–5million molecular weight grade with
enhanced productivity, desired BD and particle size distribu-
tion (PSD)/APS (Table 3). It was then realized the necessity to
tailor this process for producing higher molecular weight
grades namely >6 Â 106
g Á molÀ1
. Limitations using hydro-
genforachievingthisobjectivehasbeenalreadyexplainedin
terms of the different solubility characteristics of ethylene,
hydrogen, and their mixtures in process solvents. It was at
this juncture, we realized that the molecular weight of the
polymer producedalso depended on the catalystto hydrogen
ratio. Thus, reducing the catalyst concentration for the same
hydrogen dosage level and keeping all other parameters
constant resulted in UHMWPE with the desired character-
istics of BD, APS/PSD, and molecular weight – albeit at a
marginal loss of productivity – which is in order. The results
are tabulated in Table 3.
It can be observed from the above results that besides
hydrogen, the concentration of the catalyst also plays a
major role in regulating the molecular weight. Prima facie it
may be misleading that how the molecular weight
decreases with decrease in effective hydrogen concentra-
tion per unit Ti (Table 3).
FromTable3itcanbeseenthattheexperimentshavebeen
performedkeepingthesameratioofAl/Tiandconcentration
ofhydrogen.Ifhydrogenalonewasresponsibleformolecular
weight regulation the trend in molecular weight as depicted
in the Table 3 should have been reversed viz. an increase in
the molecular weight with increase in catalyst to hydrogen
ratio. In other words the effective hydrogen available in the
system reduces as catalyst concentration increases. Since,
this phenomenon was not observed it is clear that the apart
from hydrogen, the other competing termination reaction
namely transfer to aluminum is responsible for regulating
the molecular weight. This is a vivid example of two
competing termination reactions which are prominent in
the area of olefin polymerization.
Using this approach we could get different grades of
UHMWPE with desired molecular weights of 5–
8Â 106
g Á molÀ1
(Table 3). The BD and APS/PSD were also
optimal. During the process of fine tuning, we also realized
that, other catalyst systems under identical optimized
process conditions could give desired polymer characteristics
(Table 4).
Controlled Catalyst Dosing: An Elegant Approach in Molecular . . .
Table 3. Molecular weight regulation using controlled catalyst
dosing at specified hydrogen dosage. General conditions: PC2
7.5 atm, Al/Ti ¼ 4, 75 8C, 500 rpm and different catalyst concen-
trations with PH2 34 mL in 100 mL bomb.
Run Catalyst
C-25
Yield (Ti) BD APSa)
Mv
b)
g g Á mmolÀ1
g Á cmÀ3
mm 106
g Á molÀ1
12 0.17 716 0.40 106 8.0
13 0.31 915 0.41 107 7.3
14 0.34 855 0.41 111 6.9
15 0.36 890 0.41 108 6.1
16 0.41 980 0.40 109 5.1
a)
Analyzed by both Malvern PSA and traditional sieve shaker
methods; b)
Viscosity-based average molecular weight calculated
using Margolie’s equation.
Table 4. Molecular weight regulation using other catalyst systems, C-16, C-32, and C-50 at specified hydrogen dosage. General reaction
conditions: catalyst 1 mmol Ti, PC2 7.5 atm, PH2 34 mL in 100 mL bomb, Al/Ti ¼ 4, 3 L hexane, 75 8C, 500 rpm, 2 h.
Run Catalysta)
PH2 Productivity BD APS RSV, hb)
Mv
c)
atm (g PE) Á (g cat)À1
g Á cmÀ3
mm dL Á gÀ1
106
g Á molÀ1
17 C-16 0.34 680 0.4138 104 26 6.9
18 C-32 0.34 860 0.397 115 21.2 5.1
19 C-50 0.34 660 0.396 113 25.3 6.6
a)
Prepared by activation C-00 with different amounts of AlRR0
2; b)
Analyzed by both Malvern PSA and traditional sieve shaker methods;
c)
Viscosity-average molecular weight calculated using Margolie’s equation.
ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mre-journal.de 261

Conclusion
In this paper, we have demonstrated the regulation of
molecular weight through controlled catalyst dosing which
provided a convenient handle to tune the molecular weight
especially on larger operating scales. We have highlighted
the significance of moderating competing chain termina-
tion reactions mediated through hydrogen and aluminum
alkyls through process optimization studies.
Acknowledgements: We sincerely thank Dr. A.B. Mathur and
Dr. R.V. Jasra for their continuous encouragement to carry out this
work.
Received: January 10, 2009; Revised: March 25, 2009; Accepted:
March 25, 2009; DOI: 10.1002/mren.200900001
Keywords: catalysts; chain termination; molecular weight reg-
ulation; polyethylene; polymerization; titanium; transfer to
aluminum; Ziegler-Natta catalysis
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