Dark polymerization

Tetraethyldibismuthine [81956-27-6], CgH2QBi2, undergoes a similar reaction at 40°C (67). At 0°C this dibismuthine as well as tetra-4-tolyldibismuthine [114245-28-2], C2gH2gBi2, (105) decompose to form dark polymeric soHds ... 

Photolytic. Photoproducts reported from the sunlight irradiation of propanil (200 mg/L) in distilled water were 3 -hydroxy-4 -chloropropionanilide, 3 -chloro-4 -hydroxypropionanilide, 3, 4 -di-hydroxypropionanilide, 3 -chloropropionanilide, 4 -chloropropionanilide, propionanilide, 3,4-di-chloroaniline, 3-chloroaniline, propionic acid, propionamide, 3,3, 4,4 -tetrachloroazobenzene, and a dark polymeric humic substance. These products formed by the reductive dechlorination, replacement of chlorine substituents by hydroxyl groups, formation of propionamide, hydrolysis... 

C28H28Bi2, (105) decompose to form dark polymeric solids ... 

Preirradiation of the solution of gold salt prior to the addition of monomer enhances the rate of dark polymerization, although the rate is much slower in comparison to polymerization under continuous irradiation. Formation of some long-living active species by preirradiation is obvious. It is not possible, however, to say whether the photoirradiation is effective in producing active species alone or whether it sensitizes further reactions of active species. Certainly, further investigation is required to derive conclusions on mechanism. 

About a year prior to the discovery of the cathode glow, it was recognized that the deposition kinetics of DC cathodic polymerization (deposition on the cathode) was completely different from that of deposition on the substrate floating in glow discharge. In order to describe the difference, it was explained by designating the cathodic polymerization as dark polymerization assuming that the polymerization took place in the cathode dark space. This was obviously a mistake, because it was intuitively assumed that the cathode was in the dark space. It has never occurred to... 

A small amount of dark polymeric material, which may form, is separated at this point. 

We note in Table 1 that although methyl methacrylate polymerizes readily upon plasma initiation, ethyl methacrylate (EMA) and n-butyl methacrylate (BMP) gave only low yields (1- ) of polymer regardless of the length of post-polymerization period. Apparently a small amount of polymer was formed during the plasma initiation period (up to 60 seconds), and no further polymerization took place when plasma was turned off. Parallel experiments of dark polymerization (no plasma initiation) under the same conditions also resulted in no polymer formation. 

Dark polymerization was carried out at 80 C for 2-3 hr. without plasma-exposure. 

Only a few experiments have been reported on j8-elimination of unsaturated hexopyranuronates. Methyl 4-0-acetyl-2,6-anhydro-5-deoxy-3-0-(methylsulfonyl)-D-Zt/xo-hex-5-enonate (178) was treated with alkali-metal hydroxide in methanolic solution, as in the experiments on saturated hexopyranuronates already described. The unsaturated pyranuro-nate 179, bearing a parallel-oriented, enol ether double bond, proved, however, to be unstable. Only dark, polymeric products were obtained. The dienol ether (179) (methyl 4-0-acetyl-2,6-anhydro-3,5-dideoxy-D-gZycero-hexo-2,5-dienonate) is, like the 4H-pyran, highly unstable at room temperature. 

The kinetics of ultrafast polymerization of acrylic monomers exposed to UV radiation or laser beams has been investigated by IR spectroscopy. An 8 fold increase of the cure speed was observed by using diphenoxybenzophenone as photoinitiator instead of benzophenone. llie reactivity of polyurethane-acrylate or epoxyacrylate systems was markedly improved by adding acrylic monomers that contain carbamate or oxazolidone groups and which impart both hardness and flexibility to the cured polymer. Time-resolved infirared spectroscopy was used to directly record the actual polymerization profile for reactions taking place within a fraction of a second upon UV or laser exposure. Comparison with other techniques of real-time analysis show the distinct advantages of this method for an accurate evaluation of the important kinetic parameters and of the dark polymerization which develops just after the irradiation. 

The maximum value of Rp (between 10% and 30% conversion) is one of the reliable parameters to measure the reactivity of the formulation. FTIR profiles also provide the amount of unreacted functional groups remaining in the cured system, which is a parameter that strongly affects the final properties of the polymer. Real-time FTIR is also well suited for dark polymerization reactions that occur immediately after light exposure. One of the disadvantages of this technique appears in composite systems, in which the presence of additives may interfere with the transmission of the light by the polymer system, such as in dental composites. ... 

Figure 9 Photoinitiated cationic polymerization of a di vinyl ether of triethyleneglycol and of an aliphatic dicycloepoxide. Importance of the dark polymerization after a 1 s UV exposure (—)...

J. V. Crivello In photoinitiated cationic polymerizations one does observe considerable dark polymerization after irradiation has ceased. This polymerization will basically be limited by diffusion rates of the monomer and the growing chain ends. When multifunctional monomers are used, network polymers are formed. In these systems since the growing chain end is attached to the matrix, it cannot diffuse and the rate of... 

Sometimes it is reflecting the existence of a second maximum on the molecular mass distribution curve. Incorporation of a chainOtransformation agent reduces the polymer molecular mass both in the light and in the dark polymerization regimes. 

The structure of these high-melting, insoluble, unreactive products was deduced by their fragmentation patterns and spectral properties. The dimerization process involves the attack by the 5-anion (116) on the S—N bond of a second molecule of the isothiazolone and may proceed by one of the pathways (a)—(c). The attempted extension of the reaction to iV-acyl-3-isothiazolones was unsuccessful, affording only intractable, dark, polymeric material. ... 

Up until this point, we have determined that the photoinitiated polymerization of the FIO monomer in the smectic phase is slower than in the isotropic phase when the continuoiis ouq>ut of a mercury lamp is us as the initiating light source. The consequences and origin of this rate phenomenon will be further explored with respect to dark polymerization in the smectic phase after the initiating light source is suddenly terminated. The results will provide direct evidence via exotherm and ESR analysis of the long-lived polymer radical chains in the smectic phase, as well as an estimate of the termination/propagation rate constant ratio in the smectic phase. 

A particularly interesting observation is that if the monomer-dye-reducing agent-oxygen system is exposed to the light of a flash bulb, followed by storage in the dark, polymerization starts after an induction period, as a result of a dark reaction between the reduced dye and oxygen [62]. 

Besides acrylonitrile and acrylates other functional olefins such as vinyl or allyl compounds can also be dimerized. An important example for vinyl compounds is styrene, which can be dimerized to 1,3-diphenyH-butene. Owing to its high tendency to poljmerize spontaneously, the reaction conditions must be chosen carefuUy. Thus the dimerization catalyzed by PdCl2 at 100 °C yielded only 33% of a dimer fraction with more than 60% of a dark polymeric residue [49]. Using Ni( 1/ -03115)2 as the catalyst, styrene is converted to l,3-diphenyl-/mn5-l-butene [58, 59]. With [PdCl r] -C,U,)] dimers and trimers are obtained (Equation 43) [60]. 

Further confirmation for the above mechanism was obtained by direct spectroscopic observation of the perylene cation-radical when perylene was employed as a photosensitizer for these photoinitiators as well as detection of perylene end groups bound to the polymers produced by these systems It is interesting to note that while the direct photolysis of dialkylphencylsulfonium salts is a reversible process, the photosensitized photolysis is an irreversible one. Consequently, in contrast to the direct photolysis, photosensitized photolysis in an inert solvent followed by addition of a monomer results in spontaneous dark polymerization. 

Three classes of molecules are found to be valuable for practical use here diazonium salts, onium salts and organometallic complexes, about which detailed discussion have been pubUshed [4, 105]. Compared to radical type reactions, cationic polymerizations feature (i) low curing speed, (ii) lower viscosity, (iii) small shrinkage after polymerization, and (iv) severe post-irradiation dark polymerization. Sometimes extra thermal processing is needed to increase the conversion of monomers [106]. The above general information is instructive for choosing a suitable material system for laser fabrication. 

---