Chemistry Reference
In-Depth Information
6.4 Poly(
a
-olefin)s
Many
-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many
such polymers were found to be stereospecific and crystalline. Polymerizations of
a
a
-olefins of the
general structure of CH
2
=CH
is 0-3 and R denotes CH
3
, CH-(CH
3
)
2
,
C(CH
3
)
3
,orC
6
H
5
, can be catalyzed by vanadium trichloride/triethyl aluminum [
80
]. The conversions
are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [
81
].
Addition of Lewis bases, such as (C
4
H
9
)
2
O, (C
4
H
9
)
3
N, or (C
4
H
9
)
3
P, to the catalyst system further
increases crystallinity [
82
].
ð
CH
2
Þ
x
R, where
x
6.4.1 Properties of Poly(
a
-olefin)s
Many poly(
-olefin)s reported in the literature are not used commercially for various reasons.
Table
6.6
lists some of the olefins polymerized by the Ziegler-Natta catalysts [
72
,
83
].
a
6.4.2 Poly(butene-1)
Isotactic poly(butene-1) is produced commercially with three-component coordination-type catalysts.
It is manufactured by a continuous process with simultaneous additions to the reaction vessel of the
monomer solution, a suspension of TiCl
2
-AlCl
3
, and a solution of diethyl aluminum chloride [
84
].
The effluent containing the suspension of the product is continually removed from the reactor.
Molecular weight control is achieved through regulating the reaction temperature. The effluent
contains approximately 5-8% of atactic polybutene that is dissolved in the liquid carrier.
The suspended isotactic fractions (92-98%) are isolated after catalyst decomposition and removal.
The product has a density of 0.92 g/cm
3
and melts at 124-130
C.
Isotactic polybutene crystallizes into three different forms. When it cools from the melt, it
originally crystallizes into a metastable crystalline one. After several days, however, it transforms
into a different form. Noticeable changes in melting point, density, flexural modulus, yield, and
hardness accompany this transformation. The third crystalline form results from crystallization from
solution. The polymer exhibits good impact and tear resistance. It is also resistant to environmental
stress-cracking.
6.4.3 Poly(4-methyl pentene-1)
Another commercially produced polyolefin is isotactic poly(4-methyl pentene-1). The polymer
carries a trade name of TPX. This material is known for high transparency, good electrical properties,
and heat resistance. Poly(4-methyl pentene-1) has a density of 0.83 g/cm
3
. This polyolefin exhibits
poor load-bearing properties and is susceptible to UV degradation. It is also a poor barrier to moisture
and gases and scratches readily. This limits its use in many applications.
Poly(4-methyl pentene) is produced by the same process and equipment as polypropylene. A post
finishing de-ashing step, however, is required. In addition, aseptic conditions are maintained during
manufacture to prevent contamination that may affect clarity.
