Chemistry Reference
In-Depth Information
The copolymerization theory presented in Chapter 9 is of limited applicability
to processes involving heterogeneous Ziegler
Natta catalysts. The simple copoly-
mer model assumes the existence of only one active site for propagation, whereas
the supported catalysts described above have reaction sites that vary in activity
and stereoregulating ability. In addition, the catalytic properties of the active sites
may vary with polymerization time. The simple copolymer model can be used
with caution, however, by employing average or overall reactivity ratios to com-
pare different catalysts and monomers.
Because of the nonuniformity of active sites on supported Ziegler
Natta cata-
lysts, LLDPEs produced by these processes contain a mixture of olefin copolymers,
ranging from unbranched linear polyethylene to amorphous copolymers high in 1-
olefin content. As a result, the properties of such LLDPEs depend strongly on the
details of the distributions of comonomers and molecular weights. Production quality
of polyolefins is often controlled on melt flow index (a single-point melt viscosity)
and polymer density. These properties are unfortunately not adequate to ensure prod-
uct uniformity since both the above-mentioned distributions can vary while the qual-
ity control parameters remain within control limits. Marked performance differences
have been noted between nominally uniform LLDPE batches even when the mea-
sured molecular weight distributions were invariant
[11]
.Suchobservationshave
been the impetus for the development of TREF analyses.
Impact-modified polypropylenes are produced by combining the homopolymer
with an ethylene-propylene copolymer
Natta processes yield
such products in cascaded reactors. The first reactor in the sequence produces a rigid
polymer with a high propylene content and feeds the second reactor, where the
ethylene-propylene elastomer is polymerized in intimatemixturewith the firstmaterial.
The various regular polymers that can be produced by polymerization of buta-
diene and isoprene are summarized in reactions (1-16) and (1-17). In addition to
the structures shown in these reactions, it should be remembered that 1,4 poly-
merization can incorporate the monomer with
cis
or
trans
geometry at the double
bond and that the carbon atom that carries the vinyl substituent is chiral in 1,2
and 3,4 polymers. It is therefore possible to have isotactic or syndiotactic polybu-
tadiene or polyisoprene in the latter cases. Further, these various monomer resi-
dues can all appear in the same polymer molecule in regular or random sequence.
It is remarkable that all these conceivable polymers can be synthesized with the
use of suitable catalysts comprising transition metal compounds and appropriate
ligands. In diene copolymerization, also, the monomer sequence can be regulated
by the choice of catalyst and polymerization conditions.
rubber. Ziegler
11.5.4
Comparisons of cis-1,4-Polydienes
Cis
-1,4-polyisoprene (coded in the industry as IR) is available as natural rubber
and synthetically by two routes: living anionic polymerization with lithium alkyls
in nonpolar solvents or by Ziegler
Natta polymerization with Ti-Al catalysts.
Both syntheses are indeed scientific triumphs. It is instructive, however, to note
