Regioselectivity in ROMP

The above method of tacticity determination depends upon there being no regioselectivity, i.e. no bias towards HT or HH/TT addition in the polymerisation of the racemate, and in fact this is the case with S-substituted norbomene derivatives. 

In this context a particularly interesting and unique example of the alternating copolymerisation of enantiomers was demonstrate in the polymerisation of 1-methylnorbornene with the ReCl j catalyst [63,75]. The analysis relied on the fact that the hydrogenated forms of these polymers (but see more recent work, p. 52), unlike their unsaturated precursors, exhibited fine structure due to the presence of ring dyad units of different tacticities. 

In this reaction it is envisaged that the lone pair of electrons on the 7-oxy substituent interacts with the electrons of the syn double bond, and the normal [2+2] cycloaddition, which occurs on anti attack, becomes a facile pseudo [3-1-2] cycloaddition, overcoming the apparent steric crowding at the syn face [62]. 

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