Polymerization centers deactivation

If diffusion phenomena are not involved, the formation and deactivation of polymerization centers should reflect in rate-time dependences, other conditions being constant. Rate acceleration period of very widely differing lengths is often observed, followed either by a more or less steady rate or by a deceleration (rate decay) period. As for the polymerization center deactivation, it is quite important to know whether a macromolecule or a metal-polymer bond is formed due to this reaction (see Sect. 4). 

Here Ny is the active center deactivated by Y. H20 is likely to be a Y-type inhibitor. To explain the steady-state period of polymerization it may be assumed that some quantities of Y are adsorbed on the support surface. 

Higginson and Wooding 277) also reported a transfer reaction to solvent for the case of the polymerization styrene in ammonia initiated by potassium amide. There was no termination event in their kinetic scheme, i.e., active center deactivation via a spontaneous termination event was not considered to be a significant event. 

Such experimental results have been rationalized by assuming a chemical deactivation of some of the active centers and the presence of at least two types of species on the catalytic surface These two are isospecific polymerization centers which are unstable with time, and only slightly specific polymerization centers which, in turn, are stable with time. The latter appear to be preferentially and reversibly poisoned by the outside donor. 

From the above results, it is clear that the rate decay must be attributed to a chemical deactivation of the polymerization centers with time. Different mathematical expressions have been proposed, for those catalyst systems most widely studied in the literature, in order to express the law of the decay. For propylene polymerization with TiCyMgCl2—AlEt3/EB or with TiCyEB/MgCl2 - Al Et3/EB, Spitz 45-97) proposed an expression of the following type ... 

A growing chain is deactivated when it reacts with another chain to form a dead macromolecule. The recombination of two growing monoradicals is an example of this. Termination reactions destroy active centers both the rate of polymerization and the degree of polymerization are lowered. The deactivation through reaction of two free radicals in one of the reasons why ionic polymerizations are faster than free radical polymerizations. The deactivation reaction between two free radicals has a small activation energy, and therefore occurs very rapidly. Thus, the concentration of growing free radicals is very low in the stationary state... 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

The experimental evidence for the availability of the coordinative insufficiency of the transition metal ion in the propagation centers was obtained (175) in the study of the deactivation of the propagation centers by coordination inhibitors. On the introduction of such inhibitors as phosphine and carbon monoxide into the polymerization medium, the reaction stops, but the metal-polymer bond is retained. It shows that in this case the interaction of the inhibitor with the propagation center follows the scheme ... 

ORl OX w di-Miutyl peroxyoxalalc deactivation by reversible chain transfer and bioinolecular aclivaiion 456 atom transfer radical polymerization 7, 250, 456,457, 458,461.486-98 deactivation by reversible coupling and untmolecular activation 455-6, 457-86 carbon-centered radical-mediated poly nierizaiion 467-70 initiators, inferlers and iriiters 457-8 metal complex-mediated radical polymerization 484... 

Group 4 metal complexes of the dianion [ BuNP( -N Bu)2PN Bu] polymerize ethylene in the presence of a co-catalyst, but they are readily deactivated [10,14]. This behaviour is attributed to coordination of the lone-pair electrons on the phosphorus(III) centers to Lewis acid sites, which initiates ring opening of the ligand [15]. 

Generally, these equilibrium constants are very solvent dependent. In particular, the values of KEA are expected to be relatively high in protic solvents because halide anions can be stabilized through solvation in such media [143], Also, on the other hand, Kx values will be likewise affected with changes in solvent polarity [50], Low values of Kx in polar and protic media have direct implications on the degree of polymerization control because of the decreased amounts of deactivator (Mt"+1X/L), as a result of halide anion dissociation from the metal center. 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

Figure 7 shows time dependences of polymer yield, Mn of the polymer, and the number of polymer chains produced per vanadium atom [N] in the polymerization of propylene with a toluene solution of V(acac)3/A1(C2H5)2C1 catalyst at —78 °C. The yield of polymer, Y, is proportional to the polymerization time, which indicates that all propagating centers are formed instantaneously and remain active during polymerization without any deactivation. The JVIn of the polymer is also proportional to the time. As a result, the number of polymer chains [Nj calculated as Y/lVIn remains... 

Temperature effects on the polymerization activity and MWD of polypropylene have been examined in the range of —78 °C to 3 °C 82 The MWD of polypropylene obtained at temperatures below —65 °C was close to a Poisson distribution, while the MWD at higher temperatures above—48 °C became broader (Slw/IWIii = 1.5-2.3). At higher temperatures the polymerization rate gradually decreased during the polymerization, indicating the existence of a termination reaction with deactivation of active centers. It has been concluded that a living polymerization of propylene takes place only at temperatures below —65 °C. 

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