Double bonds, conversion

Figure 1. Reaction rate of the decrease of double bonds, conversion and swelling degree as a function of time. Insert Kinetic plot of swelling degree versus reaction time (silicone acrylate lb values from [3,4])...

Table I. Double bond conversion as measured by solid state NMR spectroscopy for pTrMPTrA samples with various storage and treatment conditions...

Polymer Thermal History Percent Double Bond Conversion for Various Polymer Storage/Treatment Conditions ... 

The double bond conversion as measured by NMR immediately following the reaction was 73%. The conversion is significantly higher than that measured in the DPC experiments described above due to the more substantial rise in temperature accompanying polymerization on a larger scale. 

Polymerization Behavior. Both Fourier-transform infrared spectroscopy (FTIR) and differential scanning photocalorimetry (DPC) were used to characterize the polymerization behavior, curing time, and maximum double bond conversion in these systems. 

The polymerization rate was essentially zero in each of the systems (even with unreacted double bonds present and continued initiation) after 9 minutes of exposure to UV light. The maximum functional group conversion reached in each system was 96% (1 wt% 1651), 87% (0.5 wt% 1651), and 83% (0.1 wt% 1651). If equal reactivity of the double bonds is assumed, only between 0.16 to 2.89% of unreacted monomer will be present at these total double bond conversions. Unreacted monomer can affectively plasticize the polymer network rendering it more pliable and decreasing its mechanical properties, and unreacted monomer may compromise the biocompatible nature of the system if the monomer leaches to a toxic level. Therefore, it is desirable to identify polymerization conditions which maximize the conversion of monomer. 

Attainment of a maximum double bond conversion is typical in multifunctional monomer polymerizations and results from the severe restriction on bulk mobility of reacting species in highly crosslinked networks [26]. In particular, radicals become trapped or shielded within densely crosslinked regions known as microgels, and the rate of polymerization becomes diffusion limited. Further double bond conversion is almost impossible at this point, and the polymerization stops prior to 100% functional group conversion. In polymeric dental composites, which use multifunctional methacrylate monomers, final double bond conversions have been reported ranging anywhere from 55-75% [22,27-29]. 

DMTA of partially polymerized samples of TEGDA reveals an increase of Young s modulus due to thermal aftercure near 120°C. Parallel DSC-extraction experiments show that this aftercure requires the presence of free monomer. Near the end of the polymerization the free monomer is exhausted and only crosslinks are formed. T(tan then increases markedly with double bond conversion. 

Fig. 3 shows the maximum extents of double bond conversion x, obtained at various light intensities for polymerizations of TEGDA at 20 and 80"C, respectively. The increase of ultimate conversion with light intensity is observed at both temperatures. This effect is not caused by self-heating of the polymerizing samples (9). 

We now turn to a correlation of the DMTA results with DSC measurements. In Figure 10 the extent of C=C double bond conversion, as measured with DSC, is plotted versus exposure time. Also plotted is the amount of monomer extracted afterwards from the DSC samples. It can be seen that for exposure times longer than 6 s the rate of polymerization decreases suddenly to a much lower value, but not to zero. At the same time the free monomer is exhausted so further reaction necessarily means further crosslinking by reaction of pendent double bonds. According to our mechanical measurements thermal aftercuring also ceases to have an observable effect on E (Figure 9a and b). 

The maximum extent of double bond conversion in TEGDA as measured with DSC increases not only with temperature but also with light intensity. Mechanical measurements show, however, that the intensity dependence vanishes when equal doses are applied. This means that at low intensities the polymerization continues for a considerable time at a rate which is imperceptible with DSC. 

The setting reaction of the composite system still needs major improvement due to the incomplete double bond conversion that occurs during these polymerization reactions. The final double bond conversion ranges anywhere from 55-75% [22, 81-87] which implies that a minimum of 6.25% of the monomer is... 

Fig. 6. The characteristic behavior of the propagation kinetic constant, kp, and the termination kinetic constant, k as a function of double bond conversion for a multifunctional monomer polymerization...

Fig. 10. Kinetic gelation model prediction of the relative fraction of crosslinks, primary cycles, and secondary cycles as a function of double bond conversion for the polymerization of a multifunctional monomer...

The curing reaction of acrylates is typical of vinyl monomers. Therefore, the degree of double-bond conversion is the measure of the degree of cure. The best results are obtained when using oligomers as binders and monomers as reactive thinners. Examples of difunctional and polyfunctional acrylates are in Table 4.4. A partial list of the most common acrylate oligomers is below. - ... 

Figure 23 Normalized emission of pyridine 1 in EGDMA before irradiation (right spectrum), after 2 min irradiation (middle spectrum), and after 20 min irradiation (left spectrum). Double-bond conversions, determined by FTIR spectroscopy, are 0, 38, and 76%, respectively. (From Ref. 112.)...

Figure 24 A plot of emission maximum of pyridine 1 in EGDMA as a function of the double-bond conversion. (From Ref. 112.)...

Performing the reaction with uniformly labeled hexadecanoic acid and separating the isolated tetradecanoic acids also demonstrated the introduction of the 11-12 double bond. Conversion of the unsaturated products into epoxides followed by GLC separation showed radioactivity in both the saturated and unsaturated acids. Similar to the results discussed above with RBLR, a closely related insect, the Z E ratio of unsaturated acyl compounds was not the same as that in the pheromone. With the orange tortrix moth, the Z E ratio of unsaturated acids was ca. 2 1, whereas the pheromone was found to be all Z. 

Figure 37. Rate of vinyl double bond conversion of TMPTA-MP, measured using RTIR methodology [179-181]. Initiator ChAD electron donor A-phenylglycine substituted as indicated in the key.

The results obtained are reported in Table 8 and show that the methyl affinities (k4/k2) of MB, NB and TDE are comparable with those known for acyclic and cydic olefins, respectively, having approximately the same degree of substitution at the double bond. Conversely, the k4/k2 ratio for (III,a) is 5 times greater than that expected for two separated double bonds with comparable degree of substitution, but 1—2 orders of magnitude sillier than those observed for other dienic systems, e. g. butadiene, cydopoitadiene, etc. This result illustrates the opposite effects due to resonance stabilization and steric hindrance. 

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