Unimolecular initiator

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

Tetramethylpiperidine-l-oxy (TEMPO)-containing alkoxyamine derivatives are widely used as unimolecular initiators for living radical polymerization [5], The key step of the presently accepted mechanism of polymerization is the reversible capping of the polymer chain by the nitroxide radical. In 2002, Otsuka and Takahara applied the reversible... 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

In reaction 9.132, molecules A and B form the excited (energized) reactive intermediate species C. Translational energy of the reactant molecules from their relative motion before collision is converted to internal (vibrational, rotational) energy of C. Reaction 9.132 provides a chemical activation (excitation) of the unstable C, with rate constant ka. Note that 9.132 does not involve a third body M for creation of the excited intermediate species, which differs from the unimolecular initiation event in Eq. 9.100. 

Reaction 10.178 is a chemical activation process. Note that this reaction does not involve a third body M for creation of the excited intermediate species, which differs from the unimolecular initiation event in Eq. 10.99. 

Chain Oxidation in the Case of 02 Excess (Unimolecular Initiation). The oxidation rate varies with time, according to a relationship of the type (Audouin et al., 1995) ... 

Standard mechanisms for chain reactions generally miss out the surface termination steps, but these should be included. Such terminations are written as first order in radical since diffusion to the surface or adsorption on the surface are rate determining, rather than the second order bimolecular step of recombination of the two radicals adsorbed on the surface. A complete mechanism will also include the need for a third body in any unimolecular initiation or propagation steps, and in any gas phase termination steps. 

An important special case of the general bond-weighted model is when all bonds are equally likely to break and consequently all bond weights are equal, i.e., wjJ) = IKj- X) for all i-mers. We shall look at this case in some detail for the simplified (but still relevant) case of a unimolecular initial distribution, where v = 1, v, = 0, i= 1- 1. Let P,(x) = N(x)d lnF(x). Then for largen, Fsatisfies the equation... 

The following elementary processes are included unimolecular initiation, radical decomposition, radical addition to unsaturated hydrocarbons, radical isomerization, hydrogen abstraction, radical combination,... 

Bradley considers that the following elementary processes occur in the high-temperature pyrolysis of hydrocarbons unimolecular initiation by carbon—carbon bond rupture, radical dissociation by /3-bond breaking plus simultaneous radical isomerization (via 1—5, 1—4 and forbidden 1—3 and 1—2 hydrogen shifts), H abstraction by hydrogen, methyl and alkenyl radicals, addition of hydrogen, methyl and alkenyl radicals to unsaturated molecules, combination two by two of methyl and alkenyl radicals. 

The most commonly studied unimolecular initiator for NMRP has been (see Scheme 8.2) I. This initiator was first synthesized by Priddy et al. by abstracting an H-atom from ethylbenzene in the presence of TEMPO [3]. They then studied I as a model of the chain-end in polystyrene made in the presence of TEMPO. They found that I decomposes upon heating under anaerobic conditions to form products resulting from both radical coupling and disproportionation (Scheme 8.2). Georges et al. also studied the thermolysis of I but instead of the formation of diphenylbutane as the minor product, they observed acetophenone [4], Acetophenone is likely formed because of the presence of dissolved oxygen during their experiment. 

Rate parameters for the homogeneous, unimolecular initiation reaction are given in Table 4. There is also, as in azomethane, a short chain operative involving the resonance stabilized radical CHj-CH—N—N-C2H5, which is also involved in the free radical initiated isomerization to hydrazone. 

The kinetics of the polymerization of tri- and tetraphosphonitrilic chlorides in solution and in bulk have been studied by Patat and Kol-linsky (58) and Patat and Frombling (57). Hydrocarbons are unsuitable solvents, since they react to give hydrogen chloride successful results w ere obtained in carbon tetrachloride. The proposed mechanism involves unimolecular initiation, either by oxygen (in solution) or another phosphonitrilic molecule (in bulk). A bimolecular propagation step is followed by unimolecular termination. Traces of water were foimd by Renaud to have a significant effect on the polymerization process (62, 63). 

M is Br2 or any other gas that is present. By the principle of microscopic reversibility , the reverse processes are also pressure-dependent. A related pressure effect occurs in unimolecular decompositions which are in their pressure-dependent regions (including unimolecular initiation processes in free radical reactions). According to the simple Lindemann theory the mechanism for the unimolecular decomposition of a species A is given by the following scheme (for more detailed theories see ref. 47b, p.283)... 

Scheme 4 Polymerization of styrene using a nitroxide-based unimolecular initiator.

While possessing many of the key advantages of controlled/ living polymerization methods, nitroxide-mediated free-radical polymerizations do exhibit several limitations. The range of monomers that have been polymerized using nitroxide-mediated techniques include styrenics. acrylamides and (meth)acrylates but these have predominantly been reported in bulk polymerizations (i.e. without solvent) and are conducted at elevated temperature for long time periods. In addition, synthesis of the unimolecular initiator can prove troublesome (dependent upon the type required) and often requires extensive purification in order to attain sufficient purity levels to allow molecular weight control. 

This method for the preparation of a TEMPO-based unimolecular initiating system is based upon the procedure described by Hawker and co-workers. ... 

Scheme 5 Synthesis of nitroxide unimolecular initiator using benzoyl peroxide.

Combine the fractions corresponding to the desired nitroxide unimolecular initiator and transfer the solution into a single-necked round-bottomed flask (250 ml). 

To a clean, dry Schlenk flask (10 mL) that is maintained under positive nitrogen pressure via the side-arm (that is equipped with an appropriate rubber septum) (Figure 3.4), add the TEMPO-based unimolecular initiator (0.099 g, 0.26 mmol). 

Fig. 3.4 Experimental apparatus for bulk polymerization of styrene using TEMPO-based unimolecular initiators.

Since TEMPO is only a regulator, not an initiator, radicals must be generated from another source the required amount of TEMPO depends on the initiator efficiency. Application of alkoxyamines (i.e., unimolecular initiators) allows for stoichiometric amounts of the initiating and mediating species to be incorporated and enables the use of multifunctional initiators, growing chains in several directions [61]. Numerous advances have been made in both the synthesis of different types of unimolecular initiators (alkoxyamines) that can be used not only for the polymerization of St-based monomers, but other monomers as well [62-69]. Most recently, the use of more reactive alkoxyamines and less reactive nitroxides has expanded the range of polymerizable monomers to acrylates, dienes, and acrylamides [70-73]. An important issue is the stability of nitroxides and other stable radicals. Apparently, slow self-destruction of the PRE helps control the polymerization [39]. Specific details about use of stable free radicals for the synthesis of copolymers can be found in later sections. 

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