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The Cossee-Arlman mechanism propagates a polymer through alkene coordination followed by insertion into a metal–carbon bond. Repetition extends the metal-bound chain. Regioselectivity controls monomer connectivity; it does not by itself specify tacticity.

Write $R$ for the current growing chain. Propene first binds at the available site of $[\mathrm{C}{\mathrm{p}}_2\mathrm{ZrR}{]}^{+}$, giving a coordinated alkene adjacent to the metal-bound chain. In the Cossee-Arlman mechanism, the chain migrates to the substituted alkene carbon while the terminal carbon forms the new Zr–C bond:The electron-pair arrows for insertion take the old Zr–R bond toward the substituted alkene carbon and the alkene pi bond toward the new Zr–terminal-carbon bond. A four-center arrangement permits both bonds to change in one migratory insertion; Zr remains Zr(IV). The product again has an accessible site, so coordination and insertion can repeat.

Predominant 1,2 insertion leaves a primary Zr–CH${}_2$ chain end and generates head-to-tail polypropylene. It avoids the more crowded alternative secondary Zr–CH(Me) end and unfavorable methyl contacts with the cyclopentadienyl ligands in the insertion geometry. This explains regioselectivity. An unbridged, unsubstituted zirconocene does not by itself establish a fixed tacticity; the schematic alignment of methyl groups in a polymer drawing is not evidence for an isotactic product.

Addition consumes the carbon–carbon double bond of vinyl chloride and retains chlorine as a side substituent. The interior repeat of A is thereforenamely polyvinyl chloride. Benzoyl peroxide supplies initiating free radicals; head-to-tail addition gives the usual backbone connectivity. End groups and tacticity are not uniquely specified by this repeat formula.

Syndiotactic polypropylene is the expected principal product:The bridged cyclopentadienyl–fluorenyl metallocene has approximately $C_s$ site symmetry. Activation by methylaluminoxane gives a cationic zirconium–alkyl polymerization-active site. Repeated coordination chemistry and migratory insertion of propene extend the chain. The two environments encountered during successive insertions favor alternating stereochemistry, giving syndiotactic tacticity. There is no stoichiometric small molecule eliminated in the propagation step; initiator fragments and chain-transfer products belong to the end groups.

Polypropylene is formed from propene. Head-to-tail insertion gives the repeat connectivity $-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{Me})-$. The stereochemical sequence of its substituted carbons determines its tacticity.

Tacticity describes the stereochemical ordering of configurational units along a polymer chain. It is distinct from the head-to-tail or head-to-head connectivity described by regioselectivity.