Polymerization, degree Ziegler-Natta

Polymerization by Ziegler-Natta Catalysts. Polymerization by transition-metal complex catalysts gives oligomers of the same structures as those obtained cationically, with distinctly higher conversion degrees (Equation 29). 

The summations extend from n = 2 to n. = oo.) Keii [Kinetics of Ziegler-Natta Polymerization, Kodansha, Tokyo, 1972] has noted that under steady-state reaction conditions, the number of polymer molecules with degree of polymerization n desorbing per unit catalyst surface area in unit time may be written as... 

The degree of polymerization is given by the same expression that described traditional Ziegler-Natta polymerizations (Eq. 8-50). 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Polyethylenes obtained by free-radical polymerization have highly branched structures as a consequence of chain-transfer reactions (see eq. 3.42 and the structure below it). Ziegler-Natta polyethylene is mainly linear (CH2CH2)/7- It has a higher degree of crystallinity and a higher density than the polyethylene obtained by the free-radical process. 

With conventional polymerization processes, atactic chains are predominantly formed for the formation of isotactic and syndiotactic chains a special catalyst system is required, e.g. Ziegler-Natta catalysts. Such a process is called stereospecific polymerization. It enables the manufacture of, i.a., technically usable PP and also unbranched PE (see 4.1). The newest development is the metallocene katalyst it enables the building-up of chains-to-measure with very high degrees of chain regularity also the manufacture of syndiotactic polystyrene is technically possible in this way (see Qu. 2.47). 

Piers investigated ferrocenylboranes containing pentafluo-rophenyl substituents as potential cocatalysts in Ziegler-Natta polymerization. The bifimctional borane (83) (M = Fe, X = CeFs) was found to effect methide abstraction from Cp2ZrMe2 under mild conditions. However, significantly reduced polymerization activity was observed with the monoborylated species (81) (M = Fe, X = CeFs). This effect was rationalized by a stronger degree of interaction with the electron-rich iron center, which lowers the Lewis acidity of the boron center. 

The classic Ziegler-Natta heterogeneous catalysts initiate polymerization at diverse, somewhat randomly scattered active sites on modified TiCls crystals, with the result that polymer chain length is variable even though essentially complete isotacticity can be achieved. Furthermore, these catalysts offer little opportunity to manipulate the degree of stereoregularity and hence the mechanical properties of the product—the Ziegler-Natta polypropylenes are exclusively isotactic. 

The polydispersity with heterogeneous Ziegler-Natta polymerization may range from 5 to 30. High-density polyethylene (HDPE) produced by co-ordination polymerization has a degree of branching of about 1-5 per 1000 monomer units, compared with 25-50 for... 

In polyethylene, the tertiary carbon atom, which dominated the chemistry of the oxidative degradation of PP, is present only at branch points. This suggests that there may be a difference among LDPE, LLDPE and HDPE in terms of the expected rates of oxidation. This is complicated further by the presence of catalyst residues from the Ziegler-Natta polymerization of HDPE that may be potential free-radical initiators. The polymers also have differences in degree of crystallinity, but these should not impinge on the melt properties at other than low temperatures at which residual structure may prevail in the melt. Also of significance is residual unsaturation such as in-chain tra s-vinylene and vinylidene as well as terminal vinyl, which are defects in the idealized PE strucmre. 

Ziegler-Natta catalyst makes it possible to polymerize cr-olefins into elastomers with controlled degree of crystallinity and crosslikability. The first EPR s... 

On the other hand, Usmanov et al. [17] and Sianesi and Caporiccio [20] imply that Ziegler-Natta catalysts of the trialkylaluminum-titanium (or vanadium) tetrachloride type are ineffective because the rate of polymerization is slow and the degree of polymerization of the product is also low [19,20]. 

---