Ziegler-Natta polymerization copolymerizations

Ziegler-Natta polymerization leads to linear unbranched polyethylene, the so-called high density polyethylene (HDPE), which is denser, tougher and more crystalline. By copolymerization with other alkenes it is possible to obtain linear low density polyethylene (LEDPE) with better mechanical properties than LDPE. Blends of LLDPE and LDPE are used to combine the good final mechanical properties of LLDPE and the strength of LDPE in the molten state. 

The discovery that group IV metallocenes can be activated by methylaluminox-ane (MAO) for olefin polymerization has stimulated a renaissance in Ziegler-Natta catalysis [63]. The subsequent synthesis of well-defined metallocene catalysts has provided the opportunity to study the mechanism of the initiation, propagation, and termination steps of Ziegler-Natta polymerization reactions. Along with the advent of cationic palladium catalysts for the copolymerization of olefins and carbon monoxide [64, 65], these well-defined systems have provided extraordinary opportunities in the field of enantioselective polymerization. 

An important extension of Ziegler-Natta polymerization is the copolymerization of styrene, butadiene and a third component such as dicyclopentadiene or 1, 4-hexadiene to give synthetic rubbers. Vanadyl halides rather than titanium halides are then used as the metal catalyst. 

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

Only a few studies have been devoted to the Ziegler-Natta polymerization of terpene monomCTs. a- and P-pinene, limonene and camphene were investigated by various authors and the corresponding homopolymers were shown to be structurally analogous to those obtained by cationic mechanisms [89, 90]. More recently, this type of catalysis was successfully applied to copolymerization systems involving a-pinene and MMA or St [56]. 

Ziegler-Natta Polymerizations Quinodimethane Polymers Charge-Transfer Copolymerizations Reactions on Polymers Polymeric Dyes... 

Whereas copolymers of two or more cycloolefins via Ziegler-Natta polymerization are seldom reported [2], an abundant literature covers the copolymerization of cycloolefins with linear olefins using this type of catalyst. This mode of introducing one or more acyclic repeat units into the polymer chain formed from cycloolefins will effect drastic changes in the physico-mechanical properties of these polymers, enabling them to be applied on a large scale in various areas. 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

Ziegler-Natta (or Natta-Ziegler) catalyst (ZNC) Able to produce stereoregular pol5uners. zwitterionic polymerization Copolymerization between nucleophilic and electrophilic comonomers. 

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

The class of monocyclopentadienylamido (CpA) titanium complexes has attracted the interest for the polymerization of a-olefins with bulky side groups. This arises since conventional Ziegler-Natta catalysts are less effective in starting the copolymerization of ethene with 4-methyl-l-pentene. Homogeneous catalysts of the zirconium cyclopentadienyl type (Cp2M) with methylaluminoxane exhibit a low catalytic activity. 

Catalysts of the Ziegler-Natta type are applied widely to the anionic polymerization of olefins and dienes. Polar monomers deactivate the system and cannot be copolymerized with olefins. J. L. Jezl and coworkers discovered that the living chains from an anionic polymerization can be converted to free radicals by the reaction with organic peroxides and thus permit the formation of block copolymers with polar vinyl monomers. In this novel technique of combined anionic-free radical polymerization, they are able to produce block copolymers of most olefins, such as alkylene, propylene, styrene, or butadiene with polar vinyl monomers, such as acrylonitrile or vinyl pyridine. 

The copolymerization theory presented in Chapter is of limited applicability to processes involving heterogeneous Ziegler-Natta catalysis. The simple copolymer 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 compare different catalysts and monomers. 

Medium Density High Density Polyethylene MDPE (or MDHDPE) is produced by copolymerization of ethylene with a-olefins using Ziegler-Natta, supported chromium or single site catalysts. MDPE cannot be produced by free radical polymerization. MDPE has a linear structure similar to LLDPE, but comonomer content is lower. Density is typically 0.93-0.94 g/cm. MDPE is used in geomembrane and pipe applications. 

---