Lewis acid-free initiating system

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). 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

The addition of donors to the Lewis acidic sp2-hybridized silicon in Si=N systems is a general method to stabilize the multiple bond, even when non-bulky substituents are used. In most cases the donor adduct reacts just like the free Si=N compound, obviously by initial dissociation of the donor molecule. In the absence of trapping agents and when the steric bulk allows it, silanimines dimerize300,308,311, e.g. 672 gives cyclodisilazane 789 (equation 265). 

In eqn. (65), A+B is a contact (externally solvated) ion pair, A+ B" is a solvent-separated ion pair (with solvent molecules between the ions), and A+, B are free (solvated) ions S stands for solvent. The initiating efficiency of a Lewis acid AB generally increases with a shift of the equilibria (65) from left to right. In the system monomer-initiator-(solvent) the concentration of active formations suitable for initiation and propagation is a function of the equilibrium position. In non-polar, poorly solvating media, only weakly... 

The initial decrease of the apparent rate constants is due to the suppression of free ions in the system. Typically a 2- to 5-fold rate reduction is observed which corresponds to 50 to 80% population of free ions in the system (assuming similar reactivities of ions and ion pairs). The addition of larger amounts of salt has two effects. The first one is the entrapment of Lewis acid and reduction of the polymerization rate. The second effect, which increases the concentration of carbocations, is related to the formation of aggregates of anions (e.g., ionic triplets) as found for SnCI4 [39,269]. 

Hindered pyridines can act in three different ways [150,293-295]. The first is to trap protonic impurities and prevent adventitious initiation by water. This improves the control of molecular weights. The second role is similar to that of salts with common anions. The pyridinium salts formed in the system are accompanied by complex anions which may scavenge free ions in a similar manner as tetrabutylammonium salts. Hindered pyridines may also act as nucleophiles (or donors) and interact with some Lewis acids. These interactions will be directed toward the aromatic ring rather than the nitrogen atom which is protected by bulky tert-butyl groups in ortho position [293]. 

The reaction of the Fe nCls/HOOH system with substrate is first order in both FelliCls and HOOH concentrations but independent of substrate concentration.20 Such behavior is consistent with the rate-determining formation of a reactive intermediate from FeUlCb/HOOH. In the absence of direct structural evidence, conclusions as to the form of the intermediate are speculative. However, without electron transfer between the Fe(III) center and HCXDH, the initial interaction between Fe Cla and HOOH is that of a Lewis acid with a base, a process facilitated by the base-free solvent system. The deactivation of the Fennels/HOOH system by donor ligands such as water or excess chloride (Felhci is inert with respect to HOOH disproportionation or activation) also is... 

We recently initiated efforts within our own laboratory to synthesize terpy-based ligands that incorporate appended Lewis acid/base pairs [36]. Motivated by metal-free systems of frustrated Lewis pairs (FLPs) which bind and form adducts with small molecules, we sought to couple the reactivity of FLPs to a transition-metal fragment to afford a metal Lewis acid/base triad (LABT). The combination of both partners with a transition-metal center opens up the possibiUty to synergistically use these systems for further activation and/or redox transformations - currently a hmitation of metal-free FLP systems. 

The ion pairs or free ions can either initiate polymerization as shown in the second set of equilibria, or collapse, with the coimteranion forming covalent species as indicated by the downward arrow in the first equilibrium. This latter reaction could lead to coimteranion exhange as shown, deactivating the initiating system because of the decreased Lewis acidity of the MtX,j lY formed (eg, when halide ligands are replaced with alkoxy, hydroxyl, or carboxyl groups) or the formation of inactive R—X species (eg, formation of R—F in case of R—X/BF3 or SbFs systems). 

A long induction period is involved in the polymerization of pure trioxan, but this may be greatly reduced or even eliminated by the addition of formaldehyde to the reaction system. It is possible, therefore, that in the polymerization of pure trioxan, initiation does not occur by direct reaction of the Lewis acid with the monomer, as shown above, but that formaldehyde is formed during the induction period and this reacts with the Lewis acid to form the actual initiating species. The polymerization is believed to be free of an inherent termination reaction. 

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