Cationic Oligomerization of Oxiranes Leading to Linear Oligomers Activated Monomer Mechanism

3 Cationic Oligomerization of Oxiranes Leading to Linear Oligomers. Activated Monomer Mechanism 

Scattered information in the literature indicates 17,30,31,32) that in the presence of water or hydroxyl group-containing organic compounds Unear polyoxiranes with hydroxyl end-groups are formed. It has been taken for granted that this phenomenon is caused by chain transfer. 

In the next step, protonation of the monomer produces the secondary oxonium ion which in turn starts the polymerization  

Indeed, as indicated above, polymerization of e.g. epichlorohydrin with BF3 alone yields mostly cyclic tetramer, whereas polymerization in the presence of BF3 and ethylene glycol gives linear hydroxyl-ended oligomers whose Mn is governed by the ratio [epichlorohydrin (ECH /jethylene glycol (EG)]0 and cychc tetramer is absent 30,31 . 

Typically, the reaction was carried out in 1,2-dichloromethane solution ([ECHJq = 6mol -T1 [EG]o = 0.6 mol l x) at 20 °C with 0.1 mol l-1 of BF3 - ether complex. At 81 % conversion oligomer having Mn = 820 was isolated. This value is very close to the one calculated on the assumption that each ethylene glycol molecule participates in the formation of one chain. Mn calcd. = 800. 

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