Photoinitiated polymerization reaction

Bosch P, Peinado C, Martin V, Catalina F, Corrales T (2006) Fluorescence monitoring of photoinitiated polymerization reactions synthesis, photochemical study and behaviour as fluorescent probes of new derivatives of 4-dimethylaminostyryldiazines. J Photochem Photobiol A Chem 180(1-2) 118-129... 

Kinetic Study of Photoinitiated Polymerization Reactions by Real-Time Infrared Spectroscopy... 

The crosslinking of such types of silicones can be described by means of a polymerization reaction. The reaction rate (rP) of this process is a function of the light intensity, the exposure time, the acrylate content, the molecular weight of the uncrosslinked silicone, the photoinitiator and also of the oxygen content of the system. A typical reaction rate/time profile is shown in the Fig. 1. 

The polymerization reaction was found to develop both faster and more extensively as IQ was increased, up to a certain value above which identical RTIR curves were recorded. Consequently, the (Rp)max value reaches an upper limit, as shown in Figure 5 where (Rp)max was plotted versus Iq on a logarithmic scale. The slope of the straight line obtained at low light intensities, 0.55, is close to the 0.5 value expected for a photoinitiated radical polymerization involving bimolecular termination reactions. 

In the second dual photo/thermal initiation strategy, the approach described above is augmented by the inclusion of a thermal initiator. Upon illumination, active centers produced by fragmentation of the photoinitiator start the polymerization reaction. The heat evolved from the exothermic photopolymerization elevates the temperature of the system and results in the production of additional active sites by the thermal initiator. This dual initiating strategy provides both the cure on demand (temporal control) afforded by photopolymerization, and the completeness of cure provided by the thermal initiator. 

Photochemical or photoinitiated polymerizations occur when radicals are produced by ultraviolet and visible light irradiation of a reaction system [Oster and Yang, 1968 Pappas, 1988]. In general, light absorption results in radical production by either of two pathways ... 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

The infrared absorber, (I), and photoinitiator, (II), which were used in polymerization reactions present are illustrated below. 

If a swelling agent is added to the reaction mixture, a maximum rate is usually noted for some fairly low agent-to-monomer ratio. DMF, which is merely a swelling agent when mixed with monomer, gives a maximum rate at 10 mol-% in photoinitiated polymerization, at about 25 mol-% in polymerization catalyzed by benzoyl peroxide (9), and at about 30 mol-% with gamma rays (114), all near room temperature. On the other hand, as little as 10 mol-% of DMF reduces the rate at 60° by a factor of about 15. It decreases also the ratio of the fast reaction at 60° to the 25° rate. 

In contrast to some other procedures, the UV photoinitiated polymerization does not require elevated temperature for the reaction to be completed. Therefore, the mobile phase used for packing remains in both the outlet frit and the packing during polymerization of the inlet frit. Consequently, the conditioning time for the column prior to its use is shortened significantly. No bubble formation was observed while using packed capillary columns with photopolymer frits. 

The type of crosslinking achieved with electron beam and uv radiation is very similar, but the way curing initiates is different. Electron beams have the higher energy, and the electron itself has sufficient energy to initiate polymerization. In uv curable materials, the polymerization reaction is not directly initiated by uv light, and a photoinitiator is required to interact with the uv radiation and produce the initiating species. 

Pigments generally inhibit uv curing to some degree since the pigments absorb and/or scatter uv radiation. This interferes with the ability of the photoinitiator to absorb the light energy required to initiate the polymerization reactions. Thus, the majority of commercial radiation curable adhesives are clear or contain silica. 

A typical cationic uv adhesive formulation contains an epoxy resin, a cure-accelerating resin, a diluent (which may or may not be reactive), and a photoinitiator. The initiation step results in the formation of a positively charged center through which an addition polymerization reaction occurs. There is no inherent termination, which may allow a significant postcure. Once the reaction is started, it continues until all the epoxy chemistry is consumed and complete cure of the resin has been achieved. Thus, these systems have been termed living polymers. 

The free radical photoinitiator may be any compound that produces a free radical on exposure to radiation, such as ultraviolet or visible radiation, and thereby initiates a polymerization reaction. 

The proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

Most of the known cationic photoinitiators produce acid species in an irreversible reaction, and once formed these species continue to promote the polymerization reaction even after the end of irradiation. This behavior is of living type and is in contrast to the radical photoinitiated... 

Different techniques have been used to study the products of photoreactions of organometallic compounds for example, irradiation of the arene complexes [CpFe() -arene)]+ resulted in the substitution of the arene by solvent or other potential ligands present in solution. In solutions containing an epoxide monomer, this photochemical reaction generated a species that initiated polymerization. Ion cyclotron resonance Fourier transform mass spectrometry and electrospray ionization mass spectrometry were used to elucidate the mechanism of these photoinitiated polymerizations. 

ESR (electron spin resonance) and optical absorption spectroscopy at low temperatures were used to analyse the individual reaction steps of the optical and thermal polymerization reactions and their kinetics. The reaction steps are the photoinitiation, the chain propagation and chain termination reactions. 

The most probable photochemical primary process of the polymerization reaction is shown in the energy level diagramm of Fig. 24. In the photoinitiation reaction an excited adjacent monomer molecule M is added to the reaction center, best represented by the metastable triplet DRi monomer molecule. Owing to the spin conservation rules, we conclude that an excited triplet dimer- diradical DR2 is formed in the chemical reaction. We may, therefore, formulate the reaction as follows ... 

---