“Preparation of flame-retarding poly(propylene carbonate)” Cyriac, A.; Lee, S. H; Varghese, J. K.; Park, J. H.; Jeon, J. Y.; Kim, S. J.; Lee, B. Y. Green Chem. 2011, 13, 3469-3475. DOI: 10.1039/C1GC15722A
During graduate school in California I was very aware of the tremendous amount of household furniture loaded with flame retardant polybrominated diphenyl ether chemicals. Those chemicals do a great job of reducing the flammability of numerous petroleum-based products. Unfortunately their non-covalent incorporation in the polymers speeds their environmental release. Once in the environment, they break down slowly and bioaccumulate. Additionally there are numerous human health and ecological concerns with these chemicals, including their association with decreased fertility in humans (ref). That is why this paper on flame-retarding poly(propylene carbonate) (PPC) caught my eye. Another reason could have been the flames in their graphical abstract!
As luck would have it the chemistry is neat too, flames aside. The researchers begin by highlighting (bragging about?) the fact that they have an actual pilot plant in which they use their cobalt-salen-based catalyst to polymerize CO2 and propylene oxide to PPC (Note: experiments from this study were not performed in the pilot plant). The catalyst displays high turnover frequency (15,000/h) and produces high molecular weight polymers (Mn = 300,000).
The fact that PPC is made from the greenhouse gas CO2 is its most obvious awesome feature compared to other polymers. The authors highlight another feature introduced by all the oxygen atoms in PPC, namely the polymer’s relatively low heat of combustion (18 GJ/ton) compared to petroleum (55 GJ/ton). Just FYI, this value for most non-oxygenated polymers is around 40 GJ/ton (ref). The authors also state that PPC burns “gently in air without emitting any toxic materials and without producing ash residue”. No reference is provided, which is quite unfortunate, but they do provide this photo that clearly shows PPC producing less polycyclic aromatic hydrocarbon-containing soot.
Various groups have shown that protic chemicals such as alcohols, carboxylic acids, or water can incorporate themselves into growing PPC polymer chains through chain transfer, in what has been dubbed “immortal polymerization.” This report is unique because they use various phosphorus-containing species as chain transfer agents, and show decreased flammability of the resulting PPC. Organophosphorus compounds are currently one of the most commonly-employed flame retardants in consumer products. With respect to industrial scale-up issues, the authors do own up to the scarcity of phosphorus, citing that it makes up only 1000 ppm of the earth’s crust.
They begin with the monoprotic chain transfer agent diphenyl phosphinic acid (Ph2P(O)(OH)) and show that increasing the amount of phosphinic acid relative to the catalyst results in lower molecular weight polymers.
At ratios of chain transfer agent to catalyst of 300:1 or above, the resulting PPC is “non-flammable.” They attribute this non-flammability to the “flame-retarding property of the organo-phosphorus end group.” Their flammability assay involved “igniting [the polymer] for minutes using a gas-lighter,” after which “a flame occurred but was quenched within a second.” This assay is unfortunately crude and would be difficult to reproduce precisely in other labs, but I don’t have any problem labeling a polymer that can withstand direct flame for “minutes” as “non-flammable”.
They go on to perform similar polymerizations with the diprotic chain transfer agent phenylphosphonic acid PhP(O)(OH)2 and triprotic chain transfer agent phosphoric acid. This creates PPC-diols and triols as shown below.
As they saw with the monoprotic example above, an increase in the ratio of chain transfer agent to catalyst causes an increase in the amount of phosphorus incorporated into the polymer chain. The reaction produces non-flammable PPC at ratios of 200:1 and 100:1 or above for the diprotic and triprotic chain transfer agents, respectively.
These PPC-diols and triols open up the possibility for creating polyurethanes by reacting them with a diisocyanate. They did just this by injecting toluene 2,4-diisocyanate into the PPC-diol-forming reaction.
Again, crude flammability tests revealed that this polyurethane was non-flammable. They attribute this flame resistance to the formation of phosphoric acid, which then dehydrates the polymer and aids in the formation of a protective black carbonaceous layer.
Finally, they compared their phosphorus-containing polyurethane to non-phosphorus-containing PPC as well as polystyrene in cone calorimetry studies, which revealed smaller total smoke release, smaller total heat release, shorter time to flameout, and smaller average mass loss for their flame-retarding polyurethane.
I think this is a great example of creative and simple green design. Building the flame-retardant right into the polymer chain will likely minimize issues associated with the release of non-covalently attached small molecule flame retardants from products. Let’s hope it doesn’t introduce any new issues! What do you think?