Chemistry Reference
In-Depth Information
tapping living pine trees. Pine trees are a widespread species in the world and it is
estimated that they account for about 25% of all tree species.
It has long been considered that rosin is a feedstock for the manufacturing of green
polymers [19, 20]. More than one million metric tons of rosin are produced per year
[21, 22]. It consists primarily of abietic and pimaric type rosin acids with characteristic
hydrophenanthrene structures. Other acidic constituents of rosin differ mainly from
abietic acid (AbA) in that they are isomers of AbA which have double bonds at different
positions in the hydrophenanthrene rings, and these are often further hydrogenated or
dehydrogenated in industry. Therefore, the hydrophenanthrene rings are considered
to have cycloaliphatic and aromatic structures. The intrinsic acidity and rigidity,
coupled with other chemical properties, enable rosin acids to be converted to a large
number of downstream derivatives. These include salts, esters and maleic anhydride
(MA) adducts, and hydrogenated disproportionated rosins which are used in a wide
range of applications such as in the manufacture of adhesives, paper sizing agents,
printing inks, solders and fluxes, surface coatings, insulating materials and chewing
gums. It should be noted that rosin acids are a class of stereoisomers with 3 or 4
chirality centres, depending on the rosin acids. Although it is possible to separate
these stereoisomers through costly and tedious procedures [23-28], this chapter will
not cover stereoselective synthesis of chiral resin acids.
Gum rosin has all the elements, such as abundance, low cost, and functionality,
for it to be a renewable natural feedstock for polymeric materials. A great deal of
effort has been devoted to the preparation of renewable polymers by condensation
polymerisation [17, 29-47]. However, condensation polymerisation generally produces
polymers with low molecular weights due to its sensitivity to impurities and variations
in stoichiometry in step-growth polymerisation. There are only a few reports on
the free radical polymerisation of rosin-derived vinyl monomers. However, radical
polymerisation probably presents the most promising technique to revolutionise
the use of gum rosin, as advanced controlled radical polymerisations can prepare a
variety of polymeric architectures, ranging from block copolymers to bioconjugates
to polymer composites [48-59]. Moreover, the hydrophenanthrene ring system of
the rosin moiety at the side group of polymers offers similar thermal and oxidative
stability, as well as structural similarity, to the commonly used petroleum-derived
benzene, naphthalene, and cyclohexane-based monomers.
To date, there has been no published work other than our own on controlled
polymerisation of rosin acid-derived monomers and their elaboration into more
complex rosin-containing polymeric materials (e.g., block copolymers) [4, 5]. To
utilise rosin acids as a feedstock for functional polymers and composites, there are
four scientific challenges: (a) how to make monomers with purity to the level for
controlled radical polymerisation and to achieve controlled polymerisation; (b) how
