The features of syndiotactic polypropylene

The mechanism of stereoselectivity of propylene insertion in propylene-ethylene copolymerization on a C_S symmetrical zirconium complex i-Pr(Cp) (Flu) ZrCl₂ catalyst is discussed. Calculation results indicate that not only the β-carbon in the growing chain end of the polymer but also the substituent of the β-carbon play an important role in the selectivity of the prochiral face of the next-coming propylene monomer. The stereoregularity of propylene units connected to an ethylene unit (PPE) in propylene-ethylene copolymer was observed to be lower than that in propylene sequences (PPP) in the ¹³C NMR spectrum, which supports the calculation results. Furthermore, the structure and properties of propylene-olefin (ethylene, 1-butene, 1-pentene, 1-hexene, and 4-methyl-1-pentene) copolymers prepared with the i-Pr(Cp) (Flu) ZrCl₂ catalyst system were studied. Propylene-1-butene copolymer exhibits peculiarly lower melting point depression because 1-butene units enter into the unit cell of the crystal structure of syndiotactic polypropylene.