Isotactic Polypropylene with (3,1) Chain-Walking Defects: Characterization, Crystallization, and Melting Behaviors

Using two living nickel α-diimine complexes ( rac - 1 and rac - 4 ) activated with methylaluminoxane (MAO), isotactic polypropylene ( i PP) samples containing five types of regiodefects were synthesized. Both isolated and successive groups of (2,1) and (3,1) enchainments were identified by solution-state 13 C NMR spectroscopy. The extended monomer and bulky nature of these defects add some restraints to the crystallinity level that can be achieved in these polymer samples. The complex based on a cumyl-derived ligand, rac - 4, produces higher molecular weight i PP with a higher content of bulky defects than rac - 1 . Melting temperatures and crystallinity levels of rac - 4 -derived i PP are accordingly lower than the polymers obtained from rac - 1 . Although the nature of the chain-walking defect is similar to the addition of the ethylene unit, the difference in polymer properties is profound. At equivalent point defects per total monomers ( X B ), i PP samples with chain-walking defects display lower melting temperatures and much lower degrees of crystallinity than random 1-alkene copolymers, including those with comonomers excluded from the crystal lattice such as the 1-hexene and 1-octene counits. Furthermore, i PP containing (3,1) regiodefects develops significantly higher contents of the γ-polymorph than any other i PP or random copolymer with a matched X B composition. The experimental evidence is consistent with shorter crystallizable sequences for i PP with chain-walking defects based on (3,1) enchainments. Taking crystallizable and noncrystallizable total units as the basis to compute point defects, the properties of (3,1) i PP adhere to the basis of exclusion equilibrium theory, indicating that the defects are random-bulky or generated in a random fashion but of a defined extended/multimonomer nature.