Decomposition, stages skeletonization

The olefin seems to disappear mainly in the second stage after the decomposition of 125. Probably, a nonclassical type of carbenium ion intermediate (128) is formed with the help of the olefin group located at an intramolecularly favorable position, and this intermediate obviously makes the polymer structure complicated by a possible cyclization-skeleton rearrangement prior to the nucleophilic attack of the monomer. 

---