How do metallocene catalysts differ from Ziegler-Natta in polyolefin production?

Metallocene catalysts are single-site catalysts with uniform active centers, producing polymers with narrow molecular weight distribution (MWD, PDI ~2) versus Ziegler-Natta's broad MWD (PDI 4-8). Metallocene advantages: precise comonomer distribution (superior copolymer properties), stereochemical control (isotactic, syndiotactic), clearer polymers (fewer catalyst residuals), and ability to make new polymer grades. Challenges: higher catalyst cost, requires methylaluminoxane (MAO) cocatalyst, sensitive to impurities, and different processing behavior (narrow MWD affects melt strength). Applications: LLDPE with unique properties (enhanced toughness, clarity), metallocene PP (better impact strength), specialty grades impossible with Z-N. Commercialized processes: ExxonMobil Exceed, Dow INSITE, Borealis Borstar. Often hybrid plants with both catalyst types for product flexibility.