A5.5 Isostatic and Atactic Addition Polymers
Polypropene has a methyl group attached to every other carbon atom in the polymer chain. This makes its physical properties different to those of polyethene
Isotactic: all the
$\textsf{CH}_\textsf3$
groups are on the same side of the chain. Atactic: the
$\textsf{CH}_\textsf3$
groups are in random positions on both sides of the chain.
Figure 5.1 Isotactic and atactic carbon groups
Isotactic polypropene chains can pack more neatly/regularly, leading to stronger intermolecular (London) forces and a stronger polymer. Atactic polypropene, with its side chains, packs less regularly, with more spaces between the chains, making it softer and generally less useful.
Manufacturers mostly produce isotactic polypropene - they control the way that propene polymerises by using suitable catalysts.