Unzipping reaction

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

There sire four ways in which these unzipping reactions may be moderated ... 

By copolymerising with a small amount of second monomer which acts as an obstruction to the unzipping reaction, in the event of this being allowed to start. On the industrial scale methyl methacrylate is sometimes copolymerised with a small amount of ethyl acrylate, and formaldehyde copolymerised with ethylene oxide or 1,3-dioxolane for this very reason. 

The films could be doped with I2, FeCl3 or BF3 to conductivities of the order of 10 -3 S cm 1, much lower than polyacetylene. Spectroscopy suggested that the average length of the polyene sequence was around 10C = C units and the authors suggest that this is due to the random initiation of an unzipping reaction at many points on... 

Unzipping reactions of 2,2,4,4-tetramethyl-3-thioxocyclobutanones with morpholine in tetrahydrofuran (THF) at —40°C led to the formation of mixture of two sulfur-rich heterocycles identified as the pentathiepane 90 and the hexathiepane 91. A mixture of analogous products was obtained when 2,2,4,4-tetramethyl-3-sulfenylchloride-cyclobutanone... 

The unzipping reaction, which gives formaldehyde, is expressed by the following curved arrows ... 

Vacuum pyrolysis of polytetrafluoroethene gives tetrafluoroethene as virtually the only product [21] this unzipping reaction is almost unique amongst depolymerisation processes. At higher pressures the pyrolysis product contains other perfluorinated alkenes and perfluorocyclobutane, the proportions depending on the exact reaction conditions [22]. 

Silicone materials are known to be fairly resistant to thermal degradation under relatively harsh conditions (less than a few hundred degrees centigrade). However, they are known to be subject to post-curing reaction, oxidative chain scissioning and crosslinking reactions, hydrolysis and unzipping reactions 23-... 

The breaking of chemical bonds under the influence of heat is the result of overcoming bond dissociation energies. Organic substances such as polymers are highly heat sensitive due to the limited strength of the covalent bonds that make np their structures. Scission can occur either randomly or by a chain-end process, often referred to as an unzipping reaction. 

Examination of cur data for peaks due to monomer which might be formed from unzipping reactions showed no detectable amount being produced. This is not unexpected because the prepolymer composition is approximately 50% acrylate type monomers and, therefore, degradation by unzipping is unlikely. 

B) /i-scission in specific position, including unzipping reactions... 

These reactions have been reported for phosphonium carboxylates (4, 5). This is, in fact, one of the major factors contributing to the high thermal stability of polymers prepared with phosphines as initiators. A competing reaction is depolymerization via an unzipping reaction. 

At 200°C the initial volatilization rate does decrease after 50 hr, but not to the extent observed at lower temperatures. The total weight loss after 764 hr is 40%. This weight loss is attributed to the formation of cyclic oligomers produced by depolymerization of PFAP(II). After 50 hr the chain scission degradation process has lowered the molecular weight to ca. 450 repeat units. Thereafter, the depolymerization mechanism is viewed as an unzipping reaction initiated at the ends of the low molecular weight chains. 

Chain fission may begin at a weak point of the polymer and then run along a part or the whole of the molecule. The radical depolymerization of poly(methyl methacrylate) and dehydrochlorination of PVC are noteworthy examples of this unzipping reaction. 

Two limiting cases of degradation can be distinguished depolymerization and chain scission. In depolymerization, monomer M is split off from an activated chain end. This is the exact reverse of addition polymerization, taking the form of a kind of unzipping reaction ... 

Despite much investigation, the mechanism of thermal hydrogen chloride elimination is still disputed. It was originally assumed that the dehydrochlorination started at the tertiary chlorine atoms. But, according to recent evidence, PVC does not have any tertiary chlorine atoms. However, there are about 0.5-1.5 double bonds per 1 000 carbon atoms. The dehydrochlorination should start at these double bonds and proceed further by an unzipping reaction ... 

The unzipping reaction involves enolization of the reducing end group [ 1 ]... 

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