The polymerization of several allylic triphenylarsonium ylides, which contain a pendant R group at the 2-position, in the presence of triethylborane. is described. Three initial molar ratios of ylide to triethylborane (15, 302 60) were used, leading to different degrees of polymerization. The ylide was generated from the corresponding arsonium salt, in THF at -78 degreesC: using either tert-butyllithium (when R = alkyl) or lithium hexamethyldisilazide (when R = tBuMe(2)SiOCH(2)CH(2)(-)) as the haze. After addition of triethylborane at 0 degreesC, the deep-red solution of ylide was readily discolored. Then oxidation of the resulting polymeric borane led to linear skipped polyenes containing a terminal alcohol function. These polymers can be called poly(2-substituted-1-propenylene)s. Molecular weights have been determined from both H-1 NMR analyses and size exclusion chromatography (SEC). In most, cases, the molecular weight of the polymers increases linearly with the initial ylide/triethylborane molar ratio. which gives credit to a controlled polymerization process. Block copolymers were also obtained from triethylborane by, successive additions of two different 2-substituted allylic arsonium ylides, followed by oxidation (yield: 62-85%).