Vinyl end group

The carboxyl end can also thermally degrade to form vinyl end groups [22-24], which occur along with the formation of acetaldehyde (Figure 11). This is a side reaction that needs to be avoided, as acetaldehyde can taint mineral water and carbonated soft drinks if present at high levels (>20ppm) in the bottle polymer. Hence lower temperatures (around 160°C) are used in the SSP phase to minimise the formation of acetaldehyde. 

Figure 18 hi NMR spectrum of PET showing a vinyl end group resonance. 

Most bimodal networks synthesized to date have been prepared from PDMS [88], One reason for this choice is the fact that the polymer is readily available with either hydroxyl or vinyl end groups, and the reactions these groups participate in are relatively free of complicating side reactions. These ideas can obviously be extended to higher modalities (trimodal, etc., eventually approaching an extremely broad, effectively-unimodal distribution) [102-104],... 

The formation of polyenes from vinyl end groups was first postulated by Zimmermann and Leibnitz [69] and requires two reaction steps, i.e. (1) the polymerization of vinyl end groups to polyvinyl esters, and (2) the elimination of carboxylic acids to form the polyenes (Figure 2.17). 

Figure 2.17 Formation of polyenes from vinyl end groups, as proposed by Zimmermann and Leibnitz [69]...

Besides the main depolymerization reactions, side reactions should also be considered in the kinetic description of a PET recycling process. This is emphasized by the results obtained from a PET extrusion model [85] shown in Figures 2.19-2.23. The complete set of reactions summarized below in Table 2.10 have been used, but shear effects have not been taken into account. Chain degradation, accompanied by a significant reduction of intrinsic viscosity, occurs even within residence times of a few minutes. Carboxyl end groups, vinyl end groups and acetaldehyde are formed in amounts depending on residence time, temperature and initial moisture content of the PET flakes. 

Figure 2.22 Concentration of vinyl end groups as a function of extruder residence time and temperature for initial water contents of 3 and 3000 ppm...

Both P-hydride transfers result in polypropene molecules with one vinylidene and one n-propyl end group. The two transfers are zero- and first-order, respectively, in monomer. P-Hydride transfer yields vinyl end groups in ethylene polymerization. 

P-Alkyl transfer (R = CH3, CH3CH2) produces polypropene and poly(l-butene) with vinyl end groups ... 

Polyolefins with vinyl end groups can be readily transformed into end-functionalized polyolefins by post-polymerization functionalization to yield a wide variety of end-functionalized polyolefins, which include epoxy-, amine-, and hydroxy-terminated polyolefins. Brookhart, Gibson, and co-workers reported on diimine-pyridine-ligated Fe complexes incorporating sterically less hindered alkyl substituents such as a methyl group ortho to the imine-A s, F12-1, that selectively converted ethylene to oligomers, affording linear a-olefin mixtures (>99%) (see also Section... 

The free radical —O—CH—CH2—O— is supposed to undergo 0-bond scission to form a carboxyl radical and a vinyl end group (Reaction 5). The carboxyl radical then abstracts a hydrogen atom from another repeat... 

The effects of irradiation damage on polymers has also been followed using FT-IR 219,235,236), by obtaining difference spectra after specific exposure durations in a variety of environments. For polyethylene 219), the difference spectra revealed an increase in the vinyl end groups and an increase of ketonic carbonyl and trans-vinylidene double bonds. It was found that for every free radical formed approximately 10 carbonyl groups and 2 carbon-carbon double bonds were formed 236 ... 

Hydrogen can also be used to regulate the molecular weight of the polymer. With Ziegler catalysts it shortens the chains by hydrogenation and is the main method of MW control. The result is a saturated polymer containing few, if any, vinyl end groups. 

If the precursor dienes have vinyl end-groups, ethene is eliminated in the metathesis reaction, but it is sometimes advantageous to use a precursor with one or two propenyl end-groups, eliminating propene or but-2-ene in the metathesis reaction. An example is shown in equation 31, the final stage in the synthesis of the protected precursor (42) of Sophora compound I, the antifungal phytoalexine isolated from the aerial part of Sophora tomentosa L228. 

Figure 3a-b illustrate the LCB effect on the melt rheological properties. The response of the rheological behaviour to the copolymerisation ability and vinyl end group selectivity of the siloxy-substituted metallocenes has been investigated from their dynamic modulus curves. The frequency dependency of the dynamic modulus of the polyethenes produced with catalysts 2 is demonstrated in Fig. 3a. For comparison dynamic modulus for a linear polyethene, prepared by the catalyst -BuCp2ZrCl2, is shown in Fig. 3b. 

There is evidence of a parallel mechanism which forms polymers with vinyl end-groups without hydride as an intermediate via /3-hydrogen abstraction. 

Weak resonances corresponding to the expected para-substituted phenylsilyl end groups appeared, but the resonances corresponding to the potential vinyl end groups were not observed in the H NMR spectra of the poly(MMA)s within detectable limits. One may anticipate that the silane reactivity (which could be directly related to the hydrogen... 

Lewis acid-free catalysts for acyclic diene metathesis obviate the formation of carbocations, which in turn completely eliminates competing reactions, mostly involving cationic oligomerisation via 1-alkene bonds. Thus, metathesis polycondensation occurs quantitatively to yield high molecular weight poly(l-alke-nylenejs with vinyl end groups and ethylene as a byproduct. 

---