The reaction of ozone with double bonds is standard learning for undergraduate students of organic chemistry. The reaction leads to cleavage of the double bond and, depending on work up, the products are hydroxyl or carbonyl compounds such as aldehyde, ketones or carboxylic acids. Creigee established the mechanism for this process in the mid twentieth century and it is generally accepted that it progresses by a 1,3 dipolar cycloaddition followed by a concerted scission and then recoupling, as shown in scheme 1. The most important feature of this mechanism is the production of compounds such as 1 and 2; carbonyl oxides.
Scheme 1 Production of ozonides via carbonyl oxides
However, there are alternative reactions that the carbonyl oxide can undergo and in fact the reaction in scheme 1 is only important if the carbonyl compound is very reactive so that the scission and coupling occur in a â€œcageâ€; ie the initial products can not diffuse apart. The importance of these alternative reactions of the carbonyl oxide were recognised by Creigee1 during his seminal works in the area but they are rarely referred to in the modern organic literature or the current undergraduate texts. Several of these reactions lead to polymer peroxides, which are useful initiators of radical polymerization. Scheme 2 shows how carbonyl oxides can polymerise to give a mixture of linear and cyclic polyperoxides.
Scheme 2 Polymerisation of carbonyl oxides giving linear polymers due to termination by water or cyclics due to termination by intramolecular (head-to-tail) combination
Of course these compounds are very reactive and are best prepared in situ. For compounds that are alkyl-substituted around the peroxide group ozonolysis of similar substituted alkenes is probably the easiest and sagest route. The instability of these compounds has lead to a sparcity of confirmatory evidence for their existence until we discovered that we could use electrospray mass spectrometry and GC-MS to provide direct evidence of their structure2-4 and showed that many alkyl substituted alkenes produces these polymers on ozonolysis rather than the conventional 5-membered ozonide structure portrayed in the text books.
The presence of these structures allows us to consider alternative work up strategies that produce functionality other than aldehyde, ketone and carboxylic acid. For example they can be used to initiate radical polymerizations2,5-8 and can be considered as latent initiators that become activated on ozonoloysis. These results lead us to consider that if the double bond was located in the polymer chain we could produce block copolymers by thermal cleavage of in-chain peroxides formed after ozonolysis, as shown in scheme 3.
Scheme 3 Peroxide (P) functional polymers formed by ozonolysis are cleaved to give oligo-radicals that intitiate polymerisation and give block copolmers
We applied this technique to the synthesis of poly(styrene-block-methylmethacrylate) 9 and polyisobutylene block copolymers.10
Another aspect of ozonolysis that is useful in polymer synthesis is the reaction of carbonyl oxides in aqueous media. Carbonyl oxides reacts many times faster with water than it does with ketones and aldehydes. Also, the polar nature of water means that the solvent cages necessary for carbonyl oxide-aldehyde recombination have life timestat re too short to prevent reaction with water. The consequence is that the 5-membered cyclic ozonides are never formed in aqueous media. Instead a mixture of peroxidesare forme of which the major product is the hydroxyl hydroperoxide as shown in scheme 4.
Scheme 4 Ozonolysis produces hydroxyl hydroperoxides in water exemplified by the ozonolysis of tetramethyl ethylene
Of course the hydroxyl hydroperoxides are unstable but they liberate hydrogen peroxide on decomposition and this can be used as an initiator in a unique emulsion polymerization.6
Oligoâ€“methacrylates with defined chain end functionality are useful intermediates for paints, adhesives and biomaterials. They can be prepared by ozonolysis of unsaturated polymers, which can be prepared by monomer starve fed emulsion or solution polymerizations.11,12 In our laboratories we are focused on using theses oligomers as components of amphiphilic networks13-15 and as reactive coatings. The method could easily be scaled up and currently we produce 100-200 g batches on a routine basis. Scheme 5 shows some of the end group functionalities that can be prepared in this way.
Scheme 5 Synthesis of aldehyde and carboxylic acid chain-end functional oligomers
For us the most important applications of these oligomers are in biotechnology and we are actively applying them as components of the next generation of scaffolds for tissue engineering.14,15
Learn more about this work
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