Slow fragmentation

O-Alkyl xanthates and A -ary l-A -alkyl dithiocarbamates are effective with vinyl acetate. 97 Dithioesters and trithiocarbonates give severe retardation or even inhibition which is attributed to slow fragmentation of the adduct radical. 

Figure 3. Slow dissociation of naphthalene-dg ions resolved by TRPD. Naphthalene ions were generated by multiphoton ionization of naphthalene vapor at 193 nm. After thermalization for about 5 s at 3 x 10 torr, they were photoexcited by two photons at 355 nm, using an 8-ns pulse from the Nd YAG laser. The plots show the extent of fragment ion signal observed at several delay times following the laser pulse and the competitive slow fragmentation to give the C,oD7, CioDj, and CgDj products.

Techniques based on cyclotron resonance are also interesting because they allow the observation of ions over long time spans. This allows the study of slow fragmentations that are not observable in classical mass spectrometry, and also equilibria between ionic species and ion-molecule reactions. 

Influence of the Polymer Concentration C /d The temporal variations of S(t) and N(t) display two domains for all systems. Results are presented in Table 7. In the first regime of relatively slow fragmentation of aggregates of initial masses S(t = 0) close to 10 the slopes of the temporal variations of the average masses S(t) (A 3) and N(t) (p,... 

On the other hand, with substrates which are relatively good donors, and thus have a more positive BDE(RX ), C-Si bond fragmentation may be slow in MeCN. As an example, a-trial-kylsilylamines undergo electron transfer induced C-Si cleavage faster in the more nucleophilic MeOH than in MeCN. Steric effects may also operate. Thus with triaIkyl(p-methoxyben-zyl)silanes where the alkyl group is bulky the relatively slow fragmentation of the cation radical in MeCN is accelerated by better nucleophiles like alcohols. ... 

In the case of AB diblock copolymers prepared by the RAFT technique, the order of monomer addition must be taken into account. A characteristic example of such a block copolymer synthesis is demonstrated in Scheme 19. Initially, a poly(N, N-dimethylacrylamide) (PDMA) macro-CTA was prepared, followed by the use of PDMA-CTA as an initiator to polymerize successfully the second monomer N,N-dimethyl vinyl benzy-lamide (DMVBA). The final diblock copolymer is not contaminated with homopolymer. It has been discovered that the reverse approach is impossible, probably due to the slow fragmentation of the intermediate radical or due to the slow initiation efficiency of the intermediate radical (styrenic macroradical). 

Even more complex models, which include reversible termination into the slow fragmentation scheme,or consider both irreversible termination and slow fragmentation of RAFT intermediate radicals, or assume different kinetics under RAFT pre-equilibrium and main-... 

Implementing the individual steps presented in Eqs. (1) and (2) into the kinetic treatment is not sufficient for adequately describing all essential features of dithio-benzoate-mediated RAFT polymerization of acrylates. Even including reversibility of the combination step in Eq. (2) or taking both irreversible termination of 1 and 3 and slow fragmentation of 3 into account, does not provide a fully consistent kinetic descrip-... 

Constructed so as to undergo gradual biodegradation, permitting slow fragmentation in the spent state, specifically, after drug release, for efficacious excretion. 

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