Post-polymerization modification represents a powerful tool to generate a library of polymers with various chemical properties from a common, modifiable polymer backbone. Having accessibility to a well-designed modifiable polymer can lead to the generation of emissive materials, redox-active materials and applications in photopatterning with spatial and temporal control purposes. The modification is made possible by click chemistry. In this thesis, a strained alkyne was utilized as the functional group of choice because it satisfies the click chemistry criteria and undergoes a strain-promoted alkyne-azide cycloaddition without the assistance of a transition metal catalyst. The strained alkyne is highly reactive and although desirable, required a masking-unmasking strategy to be employed, and thus a cyclopropenone masking group was used to prevent side reactivity. The advantage of the cyclopropenone masking group was that it can be unmasked via UV irradiation, only CO gas is lost, and no further purification is required which can be quite advantageous for polymers because their unpredictable physical and chemical nature, can complicate their purification.
In chapter 2, I described the synthesis and characterization of the first dibenzocyclooctyne monomer masked via a cyclopropenone monomer (photoDIBO-monomer) 2.13. The monomer 2.13 was synthesized by performing a nucleophilic acyl substitution between n- hydroxysuccinimide activated norbornene derivative 2.11 in CH2Cl2 overnight. The
success of the acyl substitution was confirmed by 1H NMR spectroscopy where the phenyl
protons show up as multiplets from 7.46 ppm and the olefin protons show up as multiplets
Chapter 4
at 6.43-6.25 which is characteristic of alkene norbornene protons. The IR spectrum showed preservation of the cyclopropenone group with the C=O stretch at 1842 cm-1 and UV-Vis spectroscopy showed the characteristic photoDIBO bands at 323-341 nm. The subsequent ring-opening metathesis polymerization of the photoDIBO-monomer 2.13 (Scheme 4.1) was also a success as confirmed by GPC and 1H NMR spectroscopy. The polymer 2.14 was purified by a simple process, first, removal of the Grubbs III catalyst using a neutral alumina plug, then precipitation into rapidly stirred 0 °C pentane and isolation by centrifugation. The GPC data showed that there was indeed a high molecular species generated with an Mn = 41,640 g/mol. The 1H NMR spectrum helped confirm that the
correct polymer was formed with broad mulitplets at 8.01-6.91 ppm indicative of the phenyl DIBO protons and the disappearance of the photoDIBO-monomer 2.13 olefin multiplet peaks at 6.43-6.25 ppm and reappearance of the olefin protons at 5.91-5.35 ppm
as broad multiplets for the polymer 2.14. IR spectroscopy showed the diagnostic 1842 cm-1 stretch of the C=O, which confirmed that the cyclopropenone group remained
intact during the polymerization in the presence of the GIII catalyst. Finally, the polymerization kinetics study revealed that, although, the polymerization is not considered “living”, it is a well-behaved process at lower molecular weights.
Scheme 4.1. General scheme of monomer 2.13 synthesis and ROMP.
Chapter 3 focused on the post-polymerization modification of the photoDIBO-monomer
2.13 and the photoDIBO-polymer 2.14 (Scheme 4.2). The post synthetic modification was
first explored with benzyl azide as a model reaction. The unmasking of the monomer 2.13 proved to be quick, clean and efficient. The unmasking was tracked by UV-Vis spectroscopy which showed disappearance of the characteristic photoDIBO bands at 332 nm and appearance of 311 nm corresponding to the unmasked strained alkyne. The 1H NMR spectrum was also used to track the unmasking, the photoDIBO-monomer 2.13 showed a distinct splitting pattern of the phenyl protons at 8.01-6.76 ppm and upon irradiation and loss of the C=O the DIBO-monomer 3.1 showed a different splitting pattern with multiplets ranging from 7.24-6.76 ppm. The SPAAC reaction with benzyl azide and the DIBO-monomer 3.1 was also accomplished and tracked by UV-Vis and 1H NMR spectroscopy. The UV-Vis spectrum showed disappearance of the characteristic DIBO bands at 311 nm indicating the strained alkyne had been completely consumed and the 1H
NMR spectrum showed the appearance of additional aromatic multiplet peaks at 7.53-7.51 ppm and the benzylic -CH2- proton peaks at 5.53 ppm. The success of the monomer 2.13
2.9
2.13 2.14
unmasking and SPAAC with benzyl azide served as a great model reaction for the photoDIBO-polymer 2.14. Again, the unmasking was tracked by UV-Vis spectroscopy, 1H NMR spectroscopy and GPC to track the molecular weight increase of the clicked polymer. The UV-Vis spectra for the unmasking and SPAAC of the photoDIBO-polymer 2.14 with benzyl azide was virtually identical to that of the photoDIBO-monomer 2.13. Also, the 1H NMR spectra of the unmasking and SPAAC of the photoDIBO-polymer 2.14 with benzyl azide was very similar to the photoDIBO-monomer 2.13 albeit the peaks were broad. Finally, the GPC confirmed an increase of Mn from 41,640 g/mol for the photoDIBO-
polymer 2.14 to Mn = 44,780 g/mol for the benzyltriazole-clicked-polymer 3.6. The
monomer 2.13 and polymer 2.14 were also clicked with azidomethyl pyrene and azidomethyl ferrocene. The success of these click reactions were confirmed with UV-Vis and 1H NMR spectroscopy for the monomer and the same for the polymer but with the addition of GPC. The fluorescence study of the pyrene-clicked-polymer 3.8 showed that there was excimer formation which is expected because of the close proximity of the pyrene repeating units. The cyclic voltammetry study showed that the ferrocene-clicked-polymer
3.10 was indeed a reversible process. The peaks shape observed for the oxidation of the
polymer indicated a loss of diffusion control at the electrode interface.
Overall, this work, examines a way to synthesize a novel method for the synthesis of a strained alkyne as a pendant group polymer with a clean and efficient unmasking strategy, with simple purification steps allowing the highly reactive strained alkyne to remain dormant until required. Once the polymer is ready for modification, it utilizes the fast mechanism of SPAAC with commercially available azide click partners and because of the high atom economy of the polymer, every repeating unit is functionalized. This work not
only represents a way to generate a library of polymers but also a way to expand the scope of strained-alkynes in materials chemistry with applications that require spatial and temporal control of polymers.
2.14
3.5
3.8 3.10
3.6
Scheme 4.2. General scheme for the different click partners with DIBO-polymer 3.5 to