Stereo modifiers are used in olefin polymerization to control the stereoregularity of the polymer. Stereoregularity refers to the arrangement of the monomer units in the polymer chain, which can affect the physical and chemical properties of the polymer. There are two main types of stereo modifiers used in olefin polymerization: internal and external. Internal stereo modifiers are incorporated into the polymer chain during polymerization, while external stereo modifiers are added to the reaction mixture to affect the polymerization process. Internal stereo modifiers include metallocene catalysts, that can produce polymers with high stereoregularity. Metallocene catalysts are able to control the orientation of the monomer units in the polymer chain, leading to polymers with a more uniform structure. External stereo modifiers include Lewis acids and chiral compounds, which can affect the polymerization process by altering the reactivity of the monomer and the catalyst. Lewis acids can coordinate with the monomer or the catalyst to change the orientation of the monomer units in the polymer chain, while chiral compounds can induce a preference for one stereoisomer over another.