Propene monomers

The absolute-energy minima labeled as a in Figures 1.7a,b correspond to 0i —60° for the re monomer coordination and are sketched in Figures 1.4 and 1.6a, respectively. These models minimize the interactions between the growing chain (at 0i —60°) and the methyl of the propene monomer (re coordinated). Therefore, these are assumed to be preinsertion intermediates suitable for the re monomer primary insertions. 

Hence, this analysis indicates that the stereoselectivity of these models is due, not to direct interactions of the tt-ligands with the monomer, but to interactions of the n-ligands with the growing chain, determining its chiral orientation (0i —60° preferred to 0i +60°), which in turn discriminates between the two prochiral faces of the propene monomer.15,37... 

The name of a polymer is usually written with the prefix poly-(meaning many ) before the name of the monomer. Often the common name of the monomer is used, rather than the lUPAC name. For example, the common name of ethene is ethylene. Polyethene, the polymer that is made from ethene, is often called polyethylene. Similarly, the polymer that is made from chloroethene (common name vinyl chloride) is named polyvinylchloride (PVC). The polymer that is made from propene monomers (common name propylene) is commonly called polypropylene, instead of polypropene. 

A catalyst will generate an isotactic polymer as long as consecutive propene monomers bind to the metal with the same ri-face (Scheme 8.39 path A), as illustrated in the scheme by black and white Ti-orbitals of the alkene. On the other hand, if the second monomer binds with the opposite Ti-face to the first (path B), a syndiotactic polymer will result. 

Copolymers and terpolymers of ethylene and propene, commonly known as EPM and EPDM polymers, respectively, are useful elastomers [Ver Strate, 1986], EPM and EPDM are acronyms for ethylene-propene monomers and ethylene-propene-diene monomers, respectively. The terpolymers contain up to about 4 mol% of a diene such as 5-ethylidene-2-norbomene, dicyclopentadiene, or 1,4-hexadiene. A wide range of products are available, containing 40-90 mol% ethylene. The diene, reacting through one of its double bonds, imparts a pendant double bond to the terpolymer for purposes of subsequent crosslinking (Sec. 9-2b). 

The experimental observation was that C2-symmetric metallocene complexes of zirconium (Fig. 6) produced isotactic polymers, while Cs-symmet-ric metallocene complexes (Fig. 6) produced syndiotactic polymers. Pure MM calculations with frozen core showed that the stereoselectivity is not related to direct interactions of the -ligands of the chiral metallocene with the entering monomer, but to interactions of the -ligands with the growing chain. It is therefore the chirally oriented growing chain which discriminates between the prochiral faces of the propene monomer. For C2-symmetric complexes, identical enantiofacial orientation in all insertion steps results in isotactic polymer formation for Cs-symmetric complexes the enantiofacial orientation alternates between insertion steps and leads to syndiotactic polymers. 

Figure 29. Models of preinsertion intermediates for the primary propene insertion including the unbridged rac-like bis(2-phenyiindenyl) and bridged rac-CzH4(l-lnd)2 ligands. The propene monomer is on the left, while the isobutyl group simulating the growing chain is on the right.

As a reaction initiating species, isoprene radical cation CsHg can initiate co-oligoma zation by direct attack on a neutral propene monomer which may result in a cyclic product as shown below. 

Traditionally, Ziegler-Natta EP(D)M polymerization is performed in a solution process using a volatile hydrocarbon as the solvent. The ethylene and propene monomers are cooled prior to the exothermic polymerization, to avoid heating to temperatures that are so high that the catalyst activity is significantly reduced. All the monomers and the solvent have to be purified polar moieties especially have to be removed, since they kill the catalyst. After polymerization the umeacted monomers are recycled and the polymer is recovered, a process which includes removal... 

Given a monomer that contains the C=C double bond, it will undergo addition polymerisation. Many addition polymers are made using one type of monomer, e.g. poly(propene) from propene monomers. However, co-polymers can also be produced by using more than one type of unsaturated monomer, e.g. H2C... 

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