The Propagation Reaction

The propagation takes place via tertiary oxonium ions [37, 38]:

A cyclic oligomer forms in some instances in addition to the polymer [40]. For instance, in polymerizations with BF3 in methylene chloride at low temperatures a cyclic tetramer forms, probably by a backbiting process [40]. The oxonium exchange reactions may occur with the polymer ether linkages as well as with cyclic tetramers that form, as shown above. The concentrations of the oxonium ions of the ether group on the polymer and on the cyclicte tramers, however, are very small[42].PolymerizationswithPF5,ontheother hand, or with(C₂H₅)₃OPF₆ either in bulk or in methylene chloride solutions , yield no significant amounts of cyclic oligomers [43]. The activation energy of polymerizations of oxetane monomers is higher that of tetrahydro furan (see next section). This indicates that the orientation of the cyclic oxonium ion and the monomer is looser in the S_N2 transition state [42]:

In principle, stereospecificity should be possible in substitute polyoxycyclobutanes, such as 2-methyl, 3-methyl, and others. The 2-methyl derivative however, yields amorphous polymers. This is due to the monomer’s unsymmetrical structure [33] NMR studies of the microstructure of polymers from 3,3-dimethyloxetane [44] and 2-methyloxetane [43] led to no conclusions about the manner of ring opening. The predominant head to tail structures may result from attacks at either the methylene or the methine carbons next to the oxonium ions of the propagating species.

Oxetane compounds also polymerize with the aid of aluminum trialkyl-water acetylacetone catalysts [45, 46]. The reactions can take place at 65°C in heptane and yield very high molecular weight polymers. These polymerizations, however, are ten times slower that similar ones carrier out with propylene oxide, using the same catalyst. The reaction conditions and the high molecular weights of the products led to assumption that coordinated mechanisms of polymerizations take place [46]. Also, it was reported [218] that quaternary onium salts coupled wit bulky organoaluminum diphenolates initiate controlled (living) coordinate anion polymerizations of oxetane to give narrow molecular weight distribution polyethers The catalyst system consists of onium salts, such as quaternal ammonium phosphonium halides that are combined with sterically hindered methylaluminum diphenolates [47]

Crivello reported frontal photopolymerization of oxetane [48]. He proposed a mechanism for the polymerization that is shown in the following scheme:

Crivello reported that only diaryliodonium salts were used as cationic photo initiators in this study. Photo activation was carried out by UV irradiation prior to thermal initiation. Initiation was carried out using an electrically heated wire immersed in the monomer. The velocity of the resulting propagating front is quite high with the temperature of the front reaching 110°C.

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مواضيع ذات صلة

Polyethylene and Related Polymers

Thermodynamics of Ring-Opening Polymerization

Ring-Opening Polymerizations by a Free Radical Mechanism

Polymerization of Trioxane

Ring-Opening Polymerizations of Cyclic Acetals

Polymerization of Oxepanes

The Propagation Reaction

The Initiation Reaction

The Initiation Reaction

Polymerization of Oxetanes

Steric Control in Polymerizations of Oxiranes

Polymerization by Coordination Mechanism