Matrix trapping radicals

In early studies, flash vacuum pyrolysis, a method that has proven very valuable in preparative studies of closed-shell compounds,was regarded as the method of choice for the production of radicals for matrix isolation studies. " The disadvantage of this method, which is very well suited for preparative studies of closed-shell compounds, is that the reaction occurs on the walls of a hot tube whose surface may trap radicals (this problem may be alleviated by coating the inside of the tube with gold ). Also, unless a very low vacuum can be maintained in the pyrolysis mbe, collisions between radicals may lead to gas-phase dimerization. 

It is interesting to note that the efficiency of radical reactions in solid glassy matrices appears to be uninhibited by the presence of the polymer matrix and in fact in many cases we have observed that the efficiency of radical escape when at least one of the radicals is a small molecule is just as great in polymeric glasses as it is in solution. One can deduce from this that polymeric glasses are not particularly good at trapping radical species unless they are cooled to very low temperatures. 

Once the free radicals have been covered up by the next layer of matrix they are trapped and no longer accessible to other reactants as in most other methods of forming trapped radicals. However, between the point at which they are formed and that at which they are covered up they lie exposed on the surface of the deposit and may undergo reaction with another reactant which can be brought in through a third jet (Fig. 2). For example, oxygen can be introduced and then the following reactions take place in sequence ... 

Information about primary chemical processes and the ensuing secondary processes may be obtained by using suitable radical traps thus isolating molecular primary processes. Direct information about primary processes may be obtained by flash and matrix photolysis. Radical traps may also be used to good advan-... 

In the inhomogeneous coulombic field generated by the trapped cations and anions formed by irradiation, positive holes will migrate in the direction of trapped anions. When, as a result of this process, a matrix (RjH) radical cation becomes adjacent to a solute (RiiH) molecule, positive-hole transfer to the solute may occur. 

One of the drawback of radiation crosslinking of NR latex is the poor ageing property of the film. This poor ageing property may be coming from two causes natural antioxidant degrade under radiation and autoxidation due to the presence of trapped radical in NR matrix. 

Preirradiation grafting (trapped radicals method) involves irradiation of the polymer under inert atmosphere or under vacuiun. As a result, the radicals are formed and remain trapped within the polymer matrix. These radicals subsequently initiate the grafting of a monomer. 

The irradiation of natural rubber in the presence of a vinyl monomer thus leads primarily to a synthesis of graft copolymers, but some block copolymer is certainly always present. Irradiation syntheses may be carried out in solution, either in contact with liquid monomer (with or without a diluent) or in contact with monomer in the vapor phase, or in emulsion or suspension. The rubber may be preirradiated in the absence of air to produce free radicals for later monomer addition, but the life of these radicals is short as a result of mobility within the rubber matrix. Irradiation at very low temperatures makes it possible to use the trapped radicals technique for a variety of natural and synthetic rubbers. Plastics and polymers with a crystalline phase are more readily preirradiated to initiate later grafting by trapped radicals. Irradiation may also be carried out in air to introduce peroxide groupings ... 

The ESR spectra of NF2 radicals trapped in inert matrices at low temperatures show asymmetries which are characteristic of anisotropies in the g and hf coupling tensors. The amount of anisotropy depends on the degree of orientation of the radical, i.e., whether or not the radical is free to rotate, which in turn depends on temperature, matrix material, and matrix-to-radical ratio. Measurements were carried out at 4 to >30 K with noble gas matrices, Ne [8, 9], Ar [5, 6, 9], Kr [6, 7, 9], and also with CCI4 [6] and N2 [9] matrices. 

In the absence of air, these macromolecular radicals remain trapped in the polymer matrix and initiate the grafting in the presence of a monomer (trapped radical method). 

Kashiwabra, H., Shimada, S., Hori, Y. and Sakaguchi, M. ESR Application to Polymer Physics — Molecular Motion in Solid Matrix in which Free Radicals are Trapped. Vol. 82, pp. 141 -207. 

In a KI matrix the electronic absorption maximum of 82 - is observed at 400 nm, and the 88 stretching vibration by a Raman line at 594 cm k 83 shows a Raman line at 546 cm and an infrared absorption at 585 cm which were assigned to the symmetric and antisymmetric stretching vibrations, respectively. The bromides and iodides of Na, K, and Rb have also been used to trap 82 - but the wavenumbers of the 88 stretching vibration differ by as much as 18 cm- from the value in KI. The anion S3- has been trapped in the chlorides, bromides and iodides of Na, K, and Rb [120]. While the disulfide monoanion usually occupies a single anion vacancy [116, 122], the trisulfide radical anion prefers a trivacancy (one cation and two halide anions missing) [119]. 

A number of hydrocarbon radicals having multiple bonds at the radical centre have also been trapped in inert matrices and studied by IR spectroscopy. Thus, ethynyl radical was obtained by vacuum UV photolysis (9) of matrix-isolated acetylene (Shepherd and Graham, 1987) as well as when acetylene and argon atoms excited in a microwave discharge were codeposited at 12 K (Jacox and Olson, 1987). An appearance of diacetylene bands was observed when the matrices were warmed up, while the absorptions of the radical C2H disappeared. Detailed isotopic studies of D-and C-labelled ethynyl radicals showed a surprisingly low frequency of the C=C bond stretching vibration at 1846 cm instead of c.2100cm for a true C=C triple bond (the band at 2104 cm was attributed to the... 

Fourier transform isotopic ( C and D) studies of potential interstellar species - C4H (butadiynyl radical) and QH (hexatriynyl radical) - have also been carried out. The radical C4H was produced (10) by trapping of products from the vacuum UV photolysis of diacetylene (C4H2) or 1,3-butadiene (C4H6) in solid argon at 10 K (Shen et al., 1990). Similarly the radical C6H was obtained (11) by vacuum UV photolysis of matrix-isolated... 

---