Low temperatures photopolymerization

A reaction scheme showing the individual steps of the low temperature photopolymerization reaction of the DR and AC reaction intermediates and of the SO, which are the final reaction products, are taken from the optical absorption spectra. All molecules shown in Fig. 16 have been observed in either the optical spectra or ESR-spectra or in both with the exception of only the short lived monomer species. The reactive chain ends are best represented by the diradical and carbene structures, shown in the Figure. 

Fig, 24. Energy level diagram of the photophysical and photochemical primary processes of the low temperature photopolymerization reaction in diacetylene crystals. IC Internal Conversion ISC Intersystem Crossing... 

The low activation energy of the thermal addition polymerization reaction confirms the necessity of a (librational) motion of the molecules in the initiation process. The first addition process differs from all the following addition proccesses by the metastable monomer diradical structure, which — in contrast to the DR , AC , and DC structures with n > 2 — has a limited life-time given by the phosphorescence decay of the monomer triplet state. Therefore, the librational excitation must be performed during the life-time of the monomer reaction centre. In the case of the low temperature photopolymerization reaction the librational excitation has to be prepared optically via the decay of the electronic excitation. This is in contrast to the photopolymerization reaction at high temperatures, where numerous molecular motions are thermally and stationary present in the crystals. Due to this difference two photons (2hv) are required in every dimer initiation process at low temperatures and only one photon (hv -i- kT) is required at high temperatures. The two paths of the photoinitiation reaction are illustrated below by the arrows in Fig. 26. The respective pair states are characterized by M M and M M as discussed below. 

In this article it has been shown, that the low temperature photopolymerization reaction of diacetylene crystals is a highly complex reaction with a manifold of different reaction intermediates. Moreover, the diacetylene crystals represent a class of material which play a unique role within the usual polymerization reactions conventionally performed in the fluid phase. The spectroscopic interest of this contribution has been focussed mainly on the electronic properties of the different intermediates, such as butatriene or acetylene chain structure, diradical or carbene electron spin distributions and spin multiplicities. The elementary chemical reactions within all the individual steps of the polymerization reaction have been successfully investigated by the methods of solid state spectroscopy. Moreover we have been able to analyze the physical and chemical primary and secondary processes of the photochemical and thermal polymerization reaction in diacetylene crystals. This success has been largely due to the stability of the intermediates at low temperatures and to the high informational yield of optical and ESR spectroscopy in crystalline systems. 

In a solid state polymerization reaction monomer crystals of diacetylene molecules (R-CiC-C=C-R) are converted to polydiacetylene crystals (1,2). The primary photochemical processes during the low-temperature photopolymerization reaction have been investigated by ESR (3,4) and optical absorption spectroscopy (5,6). A review ofthe spectroscopy of the intermediate states has been given by Sixl (V. A simple reaction scheme is shown in Figure 1. The reaction is characterized by the uv-photolnitiation of dira-dlcal dimer molecules. Chain propagation is performed by thermal addition of monomer molecules. Thus trimer, tetra-mer, pentamer etc. molecules are obtained. 

It has been reported that low temperature photopolymerization of tetraalkylam-monium methacrylate salts in water provides a highly syndiotactic polymer. ... 

In a solid state polymerization reaction monomer diacetylene crystals are transformed to polymer crystals in successive reaction steps. Nearly perfect polymer single crystals are obtained thermally (kT) or by UV- or X-ray irradiation (hv) of the monomer crystals [1-3]. Within the class of diacetylene molecules (R-C=C-C=C-R) which show this unusual chemical reaction, the TSHD (with side groups R = -CH2SO2-0-CH2) is the best known representative, which is characterized by a variety of reaction intermediates [4-19]. The unconventional reactivity and the unusual properties of the polymer crystal have attracted the interest of both, physicists and chemists. The general feature of the low temperature photopolymerization reaction is shown schematically in Fig. 1 by example of the diradical DR-intermediates. 

Photopolymerization of MA onto PEG with molecular weight about 20,000 at low temperature (25°C) was reported. In this case, no significant influence of the template on the rate of polymerization was found. The effect was slightly more pronounced if PEG with incorporated azo groups was used as template. 

In all experiments described in this work only extremely low concentrations of intermediates are considered. This is due to our interest which is primarily focussed on the most important initial steps of the polymerization reaction, which are characteristic of the overall polymerization reaction mechanism. Consequently only low final polymer conversion is exp>ected and, therefore, complications arising from the interaction between the intermediate oligomer states can be neglected. It will be shown that the low temperature conventional optical absorption and ESR spectroscopy are powerful spectroscopic methods which yield a wealth of information concerning structural and dynamical aspects of the intermediate states in the photopolymerization reaction of diacetylene crystals. Therefore, this contribution will center on the photochemical and photophysical primary and secondary processes of this... 

The DR and AC intermediates of the photopolymerization reaction are stable only at low temperatures. At temperatures above about 100 K they react to form long macromolecules by subsequent addition of monomer molecules. The 10 K optical absorption spectra of Fig. 17 show the result of the thermal reaction as a function of the time at 100 K The initial spectrum showing only the dimer A absorption has been prepared at 10 K by only one UV-excimer laser pulse at 308 nm. Only pure thermal addition polymerization reactions are observed within the DR-series A, B, C,. .. No chain termination reactions are detectable in the optical spectra. The final product P is situated in the vicinity of the final polymer absorption. 

Figure 22 includes the temperature dependent polymerization rates (1), (2) and (3). The thermal polymerization kinetics (1), they — (2), and the UV photopolymerization kinetics (3) have been investigated by the method of diffuse reflection spectroscopy and other methods The activation energy of the thermal reactions (2) and (3) following the photoinduced dimerization processes, (150 + 30) meV, is appreciable lower than those of the dimer DR intermediates. However, the processes which dominate the polymerization reaction are determined not by the short diradicals with n 6 but by the long chains with n 7, which all have a carbenoid DC or AC structure. The discrepancy of the activation energies therefore may be due to the different reactivities of the diradical and carbenoid chain ends. The activation energies of the thermal addition reactions of the AC and DC intermediates at low temperatures have not been determined and therefore a direct comparison with those of the diradicals is not possible. 

All distribution curves are bimodal with maxima at P = 60 and 400. At lower temperatures longer chains are formed. Since there is no gradual shift of the maximum with temperature it must be assumed that the chain grows by at least two different active chain ends, the population of which is strongly temperature dependent. The chemical nature of these chain ends cannot be deduced by the kinetic data. However, it seems reasonable to infer that we are dealing with the same carbene and radical intermediates which have been identified in the photopolymerization of diacetylenes at low temperatures by Sixl and coworkers... 

In vinyl compound polymerization of vinyl acetate, alcohol, bromide, chloride, or carbonate, ascorbic acid can be a component of the polymerization mixture (733-749). Activators for the polymerization have been acriflavine (734), other photosensitive dye compounds (737,738), hydrogen peroxides (740,741,742), potassium peroxydisulfate (743), ferrous sulfate, and acyl sulfonyl peroxides (747). Nagabhooshanam and Santappa (748) reported on dye sensitized photopolymerization of vinyl monomers in the presence of ascorbic acid-sodium hydrogen orthophosphate complex. Another combination is vinyl chloride with cyclo-hexanesulfonyl acetyl peroxide with ascorbic acid, iron sulfate, and an alcohol (749). Use of low temperature conditions in emulsion polymerization, with ascorbic acid, is mentioned (750,751). Clarity of color is important and impact-resistant, clear, moldable polyvinyl chloride can be prepared with ascorbic acid as an acid catalyst (752) in the formulation. 

If crosslinkable materials are used, patterned retardation plates can be realized using the temperature dependence of A . Starting from high An values at low temperature the An for higher temperature can be fixed by photopolymerization up to An = 0 above T. ... 

Dolotov et al. have photopolymerized formaldehyde at low temperatures to give polyoxymethylene. 

Photopolymerization. The free radical solution polymerization of NVK in THE at temperatures in the range of —20°C, to 20°C with photoinitiation of ADMVN as the radical initiator showed an overall rate proportional to the square root of the initiator concentration. At low temperatures and small concentrations of the initiator, weight average molecular weights of 510,000 Dalton were obtained. The same is true, when 1,1,2,2-tetra-chloroethane is used as a solvent. ... 

The nature of the chain initiation species was studied in detail by optical and ESR measurements at low temperatures UV irradiation at 4 K causes the formation of stable triplet diradicals. Most likely one of the two unpaired electrons is located at the phenazine molecule, and the other one at the carboxylic acid group or the diyne unit of the fatty acid molecule. Moreover, it was observed that the photopolymerization via phenazine-excitation proceeds under decarboxylation of the acid molecule. 

Interestingly, itaconate esters can be polymerized at a moderate rate to yield high-molecular-weight polymers in spite of their possession of two bulky substituents. Kamachi et al. observed a 5-line ESR spectrum with small shoulders as shown in Fig. 61 when di-butyl itaconate (DBI) was photopolymerized in bulk with BP at low temperature. They computer-simulated a similar spectrum by assuming the hyperfine splitting constant to be 14.1 G for two P-protons and 10.1 G for another two P-protons, and concluded that the observed spectrum can be attributed to... 

To obtain monodomain nematic elastomers with a global orientation, the photopolymerization was performed in a glass cell whose surfaces were coated with uniaxially rubbed polyimide layers. In this glass cell, the nematic mixture in the low-temperature nematic state was allowed to align globally in the rubbing direction. The photopolymerization was cmiducted at a temperature in the nematic state by... 

In general, there is a relatively high level of understoinding of the photopolymerization intermediates short chain biradicals, bicarbenes, etc.. Most of these low temperature isolation experiments have been conducted on PTS extension to other monomers is... 

Further experiments for analyzing the electronic structure of intermediate states were not very successM, until Sixl et al. [23] and Bubeck et al. [24] published their first low-temperature spectroscopic experiments on partially photopolymerized TS crystals. Thereby, the monomer crystal is cooled to 4.2 K in the dark. Then it is irradiated with UV light (1 < 310 nm) for a short period. After this procedure a large number of different species show up in both, the optical absorption and the ESR spectrum. They persist at helium temperature. Subsequent annealing in the dark produces further intermediate reaction products which are also identified by their optical or ESR spectra. Finally, further irradiation with visible light, which is absorbed only by the intermediate reaction products, produces still more and different reaction products. In a comprehensive series of investigations [25-47] most of these intermediate products have been identified and classified. Moreover, the mechanisms of their production and their reaction kinetics have been analyzed. According to Sixl, there exist three different series of intermediate products ... 

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