Environmental Engineering Reference
In-Depth Information
5.3.2
Second-Generation Derivatives
Furfural and 5-HMF benefit from the ease of adding functional groups to their
structures. As illustrated in Figure 5.2, 2,5-FCDA, 2,5-FDCA, 2,5-FDCC,
and isopropylidene
bis
-(2,5-furandiylmethylene)diisocyanate can be pro-
duced from the first-generation derivative HMF. These monomers are
bi-functional and well suited for the condensation polymerizations to pro-
duce polyesters, polyamides, and polyurethanes. Modifications to furfural
produce mono-functional monomers such as furfuryl acrylates, which
are precursors for chain reaction polymerizations such as free radical and
cationic polymerization [67]. The major exception is furfuryl alcohol (FA),
which is the hydrogenation product of furfural; FA is a commonly used mon-
omer in polycondensation reactions [67]. There are numerous modifications
to furfural and HMF cataloged, many of which follow oxidation or reduction
chemistry route. In-depth information on these modifications can be found
elsewhere [65, 68, 73].
5.3.3
Addition Polymerizations
The resonance effect of the furan heterocycle prevents furan derivatives that are
not sufficiently substituted from participation in free radical polymerizations [65].
In fact, a whole class of free radical inhibitors has been developed on the basis of
the stabilizing effect of some conjugated furan derivatives [65, 67]. Furyl acrylates
and furfuryl methacrylates do display appropriate reactivity to undergo free
radical polymerization, though it was reported that at high conversion rates,
cross-linking and chain branching occurred in modest amounts, suggesting some
furyl radical participation [67, 74].
Furan derivatives bearing 2-alkenyl structures, such as 2-vinylfuran and
2-isoprophenyl-5-methylfuran, can be activated for polymerization under mild
cationic conditions. In the case of 2-vinylfuran, two types of additions have
been reported - one through normal vinyl growth and the other through alkylation
at C5 - which gives rise to an irregular pattern on the polymer backbone [67].
Side reactions can be eliminated by using the more substituted 2-isoprophenyl-
5-methylfuran which only participates through vinyl additions, producing a
traditional thermoplastic homopolymer [67].
The efficiency of anionic polymerization is also limited in furan derivatives.
The epoxy substituted, 2-furyloxirane is susceptible to anionic polymerization,
though care has to be taken in selecting an initiator. Conventional initiators can
force both the α and β opening of the epoxide ring, producing an irregular poly-
mer structure [67]. Regiospecific Al(iPrO)
3
acts on the α position only, producing
a regular polymer [65].
