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

• Published in: Macromolecules 2011-05, Vol.44 (9), p.3436-3451
• Main Authors: Ruiz-Orta, Carolina, Fernandez-Blazquez, Juan P, Anderson-Wile, Amelia M, Coates, Geoffrey W, Alamo, Rufina G
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Polymerization; Preparation, kinetics, thermodynamics, mechanism and catalysts
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma200231a

Using two living nickel α-diimine complexes (rac-1 and rac-4) activated with methylaluminoxane (MAO), isotactic polypropylene (iPP) 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 13C 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 iPP with a higher content of bulky defects than rac-1. Melting temperatures and crystallinity levels of rac-4-derived iPP 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), iPP 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, iPP containing (3,1) regiodefects develops significantly higher contents of the γ-polymorph than any other iPP or random copolymer with a matched X B composition. The experimental evidence is consistent with shorter crystallizable sequences for iPP 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) iPP 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.