Ring Opening Polymerization of Olefin Oxides

The first catalytic polymerization process is ROP of an olefin oxide to produce polyether (reaction 7.1) which Lopitaux and co-workers performed it with (4-methoxy) phenoxy tetraphenylporphynatoaluminum, 1 [3]. They have prepared high-molecular-weight poly propylene oxide with low-molecular-weight distribution by using this catalyst. 

Zevaco and co-workers reported the use of aluminum isopropoxide in RO copolymerization of cyclohexene oxide (CHO) with carbon dioxide [4]. Their results showed that the optimum condition was at 70°C and CO pressures of 114 bar with a substrate to catalyst molar ratio of 300 1 and a CO to substrate molar ratio of 2 1. Zevaco et at claimed that their catalytic 

But Cheng et al. produced some bulky p-diiminate zinc alkoxide complexes, 2, for the use in this reaction [5]. Among different complexes they checked in this process only methoxy and isopropoxy of some P-diimine counterparts, showed acceptable catalytic activity (Table 7.1, entry 1-7). These active catalysts produced atactic polymer with over 94% carbonate linkage and narrow molecular weight distribution. The lower activity of others was attributed to the lower steric hindrance and, therefore, higher stability of dimer complexes that prevented equilibration to the monomeric structures which was necessary to initiation of the polymerization. 

Darensbourg et al. earlier reported the preparation of a zinc alkoxide dimer complex, 3, which had moderate activity in copolymerization of CHO and CO with very broad PDI (Table 7.1, entry 8) [6]. 

Comparison of 2 and 3 in Table 7.1 clearly shows the effect of electron withdrawing group which lowers the activity of dimer zinc complex 3. 

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Coordination polymerization of olefins was first proposed in 1956 for the unusual, at that time, low-pressure polymerization of ethylene and polymerization of propylene with the transition metal catalysts discovered by Ziegler in 1953, and for the ferric chloride catalyzed ring-opening polymerization of propylene oxide to crystalline polymer reported by Pruitt et al. in a Dow patent. Polymerization carried out in the presence of a coordination catalyst is referred to as coordination polymerization . This term is used when each polymerization step involves the complexation of the monomer before its enchainment at the active site of the catalyst [9]. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

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