Hydrogenated cyclic PDMS

In this chapter we show how hydrogenated cyclic PDMS is prepared, characterised and investigated. These studies form the basis of our preparation and characterisation of the first deuterated cyclic polymers [12] [13] [14]. These are per-deuterated PDMS [(CD3)2SiO], and they were obtained by a modification of the method described by Beltzimg and coworkers [15]. Preparative GPC was used to prepare sharp fractions in a similar way to the hydrogenated materials. 

Figure 6 GLC trace of a hydrogenated cyclic PDMS fraction containing rings with 10 to 42 skeletal bonds.

Table I Peak assignments for the EIMS spectrum of a hydrogenated cyclic PDMS fraction. ...

The H spectrum of the deuterated product shows the almost total deuteration due to the absence of peaks at s proximately 0 ppm. All of the phenyl groups have been completely removed from the silane and the hydrolysis has produced deuterated PDMS of high purity. The minute peaks observed at 7.27 and 7.37 ppm are due to residual solvents (CHCI3 and CgH respectively). The large peak at 5.31 ppm is due to the dichloromethane used as a reference for calculating the percentage deuteration in comparison with a similar spectrum recorded with the hydrogenated cyclic tetramer (D4). 

The work summarized in Table 26.1 has been conducted for isolated rings in dilute solution, either at theta conditions or in good solvents. Recent small-angle neutron-scattering studies on bulk rings have shown that they adopt conformations more compact than those in solution (Arrighi et al, 2004). The polymers examined were hydrogenous and deuterated linear and cyclic PDMS. For linear PDMS, Rg oc For cyclic PDMS, Rg oc thus, R c) ... 

To try and confirm foe presence of any linear hydroxyl- terminated deuterated siloxanes, which are not easily resolved from the cyclic siloxanes by GLC, foe technique of GC/CI-MS was applied to the sample. This technique has been reported to clearly show foe presence of hydroxyl end-groups in the analogous hydrogenated PDMS [70]. 

Gas Chromatography/Ammonia Chemical Ionisation Mass Spectroscopic Analysis Of The Small Per-Deuterated Dimethylsiioxane Cyclics. The Cl spectra were also recorded of the small perdeuterated dimethylsiioxane cyclics using foe same instrument as previously with ammonia Three competing processes are known to occur with ammonia in foe case of PDMS [70]. Quasimolecular ions are formed either by hydrogen or ammonium ion addition as well as subsequent trideuteriomefoyl loss. Ions can be observed at M +18, M + 2(18 - 19), M + 1, and M -18. Each of these processes are observed for foe cyclics upto foe decamer and spectra were obtained for foe pentamer up to foe decamer inclusive (Figures 33 and 34 show those of the hexamer and foe nonamer respectively). 

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