Large-ring acetylenes strained

In general, Rh catalysts are not very effective in stereoseleetively polymerizing sterically crowded monosubstituted acetylenes such as t rt-butylacetylene and ortho-substituted phenylacetylenes. Rh catalysts are also not capable of polymerizing disubstituted acetylenes. One exception is cyclooctyne, whose very large ring strain ( 38 kJ/mol) enables fast polymerization with [(nbd)RhCl]2, giving an insoluble polymer in good yield. ... 

This stereoelectronic requirement would lead to a large distortion of the normal geometry of a five-membered ring and introduce strain. It is this distortion and strain that disfavor the 5-endo-trig cyclization. In contrast, 5-endo-dig cychzation is feasible because the acetylenic system provides an orbital that is available for a nearly planar mode of approach. 

---