Chemistry Reference
In-Depth Information
epoxidized castor oil (ECO) has been shown to have better reactivity than
ELO or ESO when using diaryliodonium salt photo-initiators.
95
However, too
many hydroxyl groups or water can also act as chain-transfer agents thus
retarding the chain growth process and leading to softer polymer
structures.
96,97
Park et al.
98-100
used N-benzylpyrazinium hexafluoroantimonate (BPH)
and N-benzylquinoxalinium hexafluoroantimonate (BQH) as thermally la-
tent catalysts to cure ESO and ECO. BQH showed comparable curing activity
for ECO at slightly lower temperatures than that of BPH. Compared to ESO,
ECO polymerization initiated at lower temperatures when using the BPH
catalyst. The authors also proposed that an observed variation in the thermal
and physical properties of the resulting polymers was due to the different
activities of catalysts.
ENLO showed higher curing rates than ELO during UV-initiated cationic
polymerization, but was still slower than the polymerization of the cyclo-
aliphatic epoxide, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carb-
oxylate. The lower reactivity of ENLO compared to cycloaliphatic epoxide was
attributed to a greater steric hindrance present in the epoxybornyl groups
and to a higher viscosity.
40
During cationic photo-polymerization, the rela-
tive reactivity of the oxiranes was found to be not as important as the vis-
cosity of the reacting system. The polymerization rate was observed to be
diffusion controlled, where adding diluents such as divinyl ethers markedly
accelerated the curing rate and overall conversion rate of epoxies.
41
9.4.4 Miscellaneous Curing Agents
Epoxies can also be polymerized in an anionic fashion for precise control of
molecular weight and polydispersity as well as chain functionality.
101
Ter-
tiary amines, imidazoles, and ammonium salts are commonly used anionic
catalysts for epoxy resin homopolymerization, although their induced curing
mechanisms are very complex and not universally accepted.
102
Boonkerd
et al.
103
successfully synthesized a bio-based elastomer using post-living
anionic polymerization of poly(butadienyl) lithium and ESO; however, the
strongly nucleophilic anions preferentially cleaved ester groups rather than
induced ring-opening of epoxies. Due to a higher oxirane content, ESO is
more reactive than EMO for anionic epoxy ring-opening polymerization.
While a pyridine-initiated epoxy reaction between ESO and 4-methylpyridine
and poly(4-vinylpyridine) has been reported by
¨
zt¨rk and K¨sefo˘lu,
104
no
homopolymerization of epoxy groups, as initiated by pyridine was observed.
Instead, pyridine addition, followed by rearrangement to a pyridone de-
rivative was observed.
Del Rio et al.
105
used coordination catalysts to polymerize EMO. Two main
polymerization mechanisms, cationic and the ionic-coordinative, were ob-
served with the former being predominant. The polymers produced were a
mixture of cyclic and linear structures with different end-groups depending
on the initiator used, but a higher molecular weight was obtained than with
