68 Polymers were also synthesised by reacting both the alkynated 3.1 and azidated 3.2
oils directly providing renewable polymer 3.11, where both monomers were derived from renewable sources. In 2012 Shah et al. expanded this chemistry further by synthesising a range of azidated oils derived from different vegetable oils via their epoxides.139 The group used a range of oils with varying amounts of unsaturation, ultimately leading to a varied amount of azide functionality (Fig. 3.4).
Figure 3.4: Common fatty acids in plant oils.
Castor 3.12, canola (rapeseed) 3.13, corn 3.14, soybean 3.2, and linseed 3.15 oils were azidated using the methods previously employed. The azide content was determined by elemental analysis and ranged from 3.2 azide groups per monomer in castor oil 3.12 up to 6.2 in linseed oil 3.15. These monomers were polymerised thermally to produce samples suitable for mechanical testing. Thermal testing of the polymers (derived from: castor 3.16, canola 3.17, corn 3.18, soybean 3.11 and linseed 3.19) showed that the higher the azide functionality the higher the Tg, tensile strength and elongation at break of the polymer produced from it, although the
69 opposite was seen with the degree of swelling (Table 3.2). The material produced from castor oil 3.16 didn’t follow this trend with both the Tg and elongation at break higher than predicted based upon its azide value. This was rationalised to be due to the extra hydroxyl group present in the ricinoleic derived side-chains (1.25) causing extra H-bonding interactions between chains. They concluded that in general an increase in the unsaturation of the starting oil allows an increase in azide functionality of the monomer which in turn causes an increase in mechanical and thermal properties of the polymer due to cross-linking.
Polymer Monomer 1 (alkyne) Monomer 2 (azide) Nf a Tg b (°C) TSc (MPa) Elongation at breakd (%) 3.11 3.1 3.2 (soybean) 4.6 10 1.3 40 (72) 3.16 3.1 3.12 (castor) 2.9 1 0.6 54 (72) 3.17 3.1 3.13 (canola) 4.2 -5 0.9 31 (81) 3.18 3.1 3.14 (corn) 4.5 1 1.1 40 (76) 3.19 3.1 3.15 (linseed) 6.2 16 3.4 61 (57)
a Number of the azide groups (Nf) per triglyceride was calculated using the equation: Nf = (Mn × nitrogen content)/(molecular
mass of the azide group×100). b Scan rate for DSC measured using 10 °C /min. c TS = Tensile strength. d Values in brackets are for degree of swelling.
Table 3.2: Thermal and physical properties of Shah et al. biopolymers.
More recently in 2013, Bakhshi et al. synthesised soybean oil derived 1,2,3-triazole polyurethanes which showed a range of antibacterial activity towards E. coli, S. aureus and C. albicans.195
In summary:
Various azide-click polymers (3.11, 3.16 – 3.19) could be produced from a range of vegetable oils with varying levels of unsaturation.
Polymerisation was successful using heat (100 °C) or copper catalysis (Cu(AAC)) but gave short network chains.
70 Thermal onset of degradation was approximately 300 °C but a wider range of degradation temperatures were found for materials prepared via copper catalysis.
Tensile strengths were relatively low (0.6 – 3.4 MPa) with elongations at break ranging from 30-60%.
Tg’s were found to range from -13 °C to 80 °C with those materials derived using copper catalysis generally showing higher values.
Results indicated that by increasing H-bonding in chains (castor oil) elongation at break may be improved without the need for greater azide functionality.
Materials made from 1,2,3-triazole derived vegetable oils show antibacterial properties.
3.2 Aims and objectives
The materials produced by Shah et al. 3.7-3.19 had relatively low tensile strengths (0.6 – 3.4 MPa) and elasticity’s. Inspired by the increased elongation at break of the castor oil derivatives 3.12, hinting that H-bonding between chains was important, and as part of the Clark group’s interest in renewable elastomers investigations were carried out to establish whether azide-click chemistry could be used to prepare more
elastic materials without a decrease in tensile strength. To this end:
A range of dimeric fatty amides (capable of H-bonding) containing azide functional groups were prepared where the nature of the linker could be varied, (Fig. 3.5). It would be expected that by increasing the linker length
71 from C2-, C4- to C6- the Tg values of the materials derived from them should decrease.
Figure 3.5: Azidated vegetable oil derived diamides with varying linkers.
Six monomers were prepared from oleic 1.22 and linoleic acids 1.23 to determine the effect of the number of azide groups upon polymer properties (2 oils, 3 linkers).
Six monomers were also prepared from the triglycerides, rapeseed and soybean oils (2 oils, 3 linkers). The triglyceride oils are cheaper than purified fatty acids and are thus more likely to be used commercially as renewable feedstocks. Rapeseed oil was chosen as it is mainly made up of oleic acid 1.22 residues (61 %) while soybean oil is mainly linoleic 1.23
residues (56 %). Thus, the use of oleic 1.22 and linoleic 1.23 fatty acids should act as good models for the triglycerides themselves.
Polymerisation reactions of the 12 monomers (4 oils, 3 linkers) were investigated and thermal and mechanical properties measured to determine the effect that increasing the cross-linking (azide value) and monomer length (linker) had on their physical properties.
72
3.3 Synthesis of C2- linked Amide Click Polymers
3.3.1 Synthesis of C2- linked azido dioleamide (3.22)
The simplest azide monomer, potentially containing two azide units on average was prepared from oleic acid 1.22. Oleic acid 1.22 is an 18 carbon chain fatty acid with a single cis unsaturation between C9 and C10, (Fig. 3.4). The initial step was the formation of the diamide 3.20. This was achieved via a condensation reaction between oleic acid 1.22 and ethylenediamine at 120 °C overnight, (Scheme 3.2). Purification afforded C2-linked dioleamide derivative C2-DOA 3.20 in a 35 % yield.