Chemistry Reference
In-Depth Information
Characterization of Polyethylene Nascent Powders
Synthesized with TpTiCl
2
(OR) Catalysts
Emilio Casas,
1
Arquı´medes Karam,
*
1
Antonio Dı´az-Barrios,
1
Carmen Albano,
1
Yanixia Sa´nchez,
1
Bernardo Me´ndez
2
Summary: Different kinds of polyethylene and ethylene-1-hexene copolymers were
synthesized with TpTiCl
2
(OR)
Et, i-Pr, n-Bu)
catalysts with and without H
2
. The polymers were characterized by
13
C NMR, capillary
viscosimetry or GPC, and DSC. The homopolymers showed properties characteristic of
ultra-high molecular weight polyethylenes (UHMWPE) with linear structure and high
density polyethylenes (HDPE) with molecular weights in the range of commercial
grades under hydrogen atmosphere. The copolymers showed a 1-hexene incorpora-
tion up to 6 mol-%. Important differences in the thermal properties were observed
between the first DSC (nascent powders) and the second DSC heatings (melt-
crystallized samples), which evidenced the molecular weights influence on the
melt-crystallized samples.
(Tp
hydrotris(pyrazolyl)borate; R
¼
¼
Keywords: hydrogen transfer; 1-hexene; nascent powders; polyethylene; TpTiCl
2
(OR)
Introduction
systems in the reactor, and its related
physical properties, is an important rese-
arch topic.
[2,5]
The development of the
nascent state morphology of polyolefins in
the reactor is reasonably well understood
on the micrometer level. However, at the
molecular scale, the physical properties
of nascent polyolefins are less clearly
understood.
[6,7]
Although different catalyst
types and synthesis conditions clearly play
a role, there is no consensus on the origin
of the high value for the peak melting
point commonly seen in nascent polyethy-
lene.
[2,8-16]
Interpretations which invoke
chain-extended and/or fibrilar crystals,
[8-11]
strained noncrystalline tie points,
[12,13]
instrumental effects,
[2]
or small crystals
which reorganize on heating,
[14,15]
have
been invoked to describe aspects of the
thermal behavior. Nascent samples of
UHMWPE also exhibit a peak melting
point which approaches the equilibrium
value (
Ethylene can be polymerized under various
conditions to yield polyethylenes having
markedly different chain structures and
physical properties as a function of differ-
ent structural parameters that influence
their ultimate properties, such as type,
amount and distribution of comonomer(s)
and branching, average molecular weight
(MW) and molecular weight distribution
(MWD).
[1]
In the last few years, authors
have pointed out that polyethylene reactor
powders have a unique morphology and
higher crystallinity than melt-crystallized or
solution-crystallized samples.
[2-4]
Better understanding of the nature of
the nascent morphology of polyolefins,
namely the formation and molecular organ-
ization of polymer particles initiated by
heterogeneous or homogeneous catalyst
0
m
) for polyethylene, but is irrever-
sibly lowered following a melt crystal-
lization cycle.
[2,10,14,16]
In this sense, the
nascent morphology of UHMWPE repre-
sents a unique combination of morphology,
crystallinity and melting characteristics
T
