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Solid-phase organik syntheses - Burdges K.

Burdges K. Solid-phase organik syntheses - John Wiley & Sons, 2000. - 283 p.
ISBN 0-471-22824-9
Download (direct link): phaseorganicsynthesis2000.pdf
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Pd2(dba)3tPPh3, Cul, piperidine , 25

R %

R = I
Pd2(dba)3, PPh3, Cul* piperidine , , 25
Bu fBu
R =
Scheme 8.
8).26 The polymerization was carried out beginning with triazene-tethered
3,5-diiodobenzene as a focal point of an AB2 polymerization with 3,5-diio-dophenylacetylene. The degree of branching is limited by steric factors due to the size limitations within the bead or adjacent dendrimers. To ensure that the polymer was soluble after being cleaved, the iodo ends were capped with (3,5-di-te?t-butylphenyl)ethyne. Polydispersity in the product of this synthesis was less than for comparable solution-phase routes (1.3 vs. >2.5). Furthermore, size exclusion chromatography traces showed only monomo-dal distributions for support-grown products while a bimodal elution profile was seen for those made in solution. Moreover, the molecular weight of the product could be controlled by the monomer to the focal point ratio used in the supported route. Purification in the solid-phase route was also relatively convenient.
Polymer resins with different loadings and cross-linking were investigated. Lower loadings and cross-linking gave better yields and narrower polydispersities (Table 4.5). Examination of the dry solid support by polar-
TABLE 4.5. Data for Solid-Phase Synthesis of a Hyperbranched Polymer
By Focal Point-Monomer Ratio
Physical Parameter^ 17:5 35:1 70:1 140:1 280:1 560:1
Mass increase of solid
support, %
I 200 250 470 760 1025 1245
II 100 190 550 900 1230 1350
III 26 120 55 190 235 305
Yield of polymer, %
I 53 35 35 19 20 12
II 7 21 18 17 14 6
III 19 9 7 8 4 3
Polydispersity of polymer
I 1.37 1.33 1.28 1.34 1.29 1.47
II 1.46 1.57 1.38 1.42 1.44 1.73
III 1.56 1.41 1.51 1.49 1.89 1.74
aI, II, and III correspond to resin degree of functionality (in mmol/g) 0.7, 1.7, and 1.7, respectively. In addition, they correspond to the degree of cross-linking of the polystyrene beads with divinylbenzene: 1,1, and 2%, respectively.
ized optical microscopy revealed that the beads became birefringent and grew in size as they took up more monomer. The birefringence was most likely due to internal stress within the expanded polymer beads. Furthermore, at high monomer to focal point monomer ratios, the spherical beads shattered into a fine powder.
There are a number of potential advantages to solid-supported hyperbranched polymerizations. First, since the focal point functional group is bound to the solid support, intramolecular cyclization between the focal point and a peripheral group is impossible. This problem has been implicated as limiting solution-grown hyperbranched polymers.38 Second, the terminal ends can easily be modified with different monomers providing different periphery with a common internal structure. This type of modification can be extended to other monomers to make layered hyperbranched copolymers. Third, cleavage from the support after terminal group capping ensures one unique focal point functional group per molecule. This site could be used to construct multidendron architectures or hybrid structures.
4.4.4. Monitoring and Characterization
Monitoring reaction progress throughout a multistep synthesis is a relatively difficult task.22 Typical methods used for solution-phase synthesis, including thin-layer chromatography (TLC), GC, and most types of mass spectrometry (MS), are less informative for solid-phase methods. However, Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) are particularly useful in solid-phase strategies.
Null-to-null IR monitoring has been used to track the reaction sequence of growing oligomers bound to a polymer resin. This method is simple and nondestructive. FTIR spectroscopy has been used for the specific case of phenylacetylene oligomers.14,15 Infrared analyses are conveniently performed by placing approximately 1 mg of the resin between two NaCl plates, swelling the beads with one drop of carbon tetrachloride, and immediately recording an FTIR spectrum. Attempts to record the spectrum without swelling the beads results in a poor signal. The convergent synthesis of the oligomers, detailed previously, involves a trimethylsilyl-protected acetylene group as the exposed reactive site off the polymer bead. TMS-protected oligomers have a characteristic band at 2156 cm-1 (strong) corresponding to a carbon-carbon bond stretch of the TMS-acetylene (Figure 4.7). After deprotection using TBAF in THF for 5 min, the newly
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