Radical polymerization reactions spectroscopy

On condensation at low temperatures, on dissolution in inert solvents or on raising its partial pressure substantially above 1 mbar (100 Pa) S2O polymerizes with partial disproportionation. Since sulfur radicals have been detected in such condensates by ESR spectroscopy [10] it has been proposed that a radical-chain reaction takes place according to Scheme 5. 

The polymerization proceeds via a radical chain-reaction mechanism, judging from some features of the polymerization initiation by irradiation or upon heating, no formation of oligomers, and polymer formation irrespective of the medium or atmosphere. The propagating radicals are readily detected by ESR spectroscopy during polymerization in the crystalline state (Fig. 2), because termination between the propagating radicals occurs less frequently in the solid state [50]. 

The deuterium-labelled initiator method was also applied to the polymerization of macromonomer to determine the number of initiator fragments in a polymacromonomer chain (N) and initiator efficiency (/)-77 Isotactic and syndiotactic PMMA macromonomers having a styrene group as polymerizable function were polymerized with AIBN-<7 2 in toluene at 60°C and the resultant polymacromonomers were analysed by 2H NMR spectroscopy (Table 8). The N values were less than unity (0.50 0.72), indicating some kind of chain transfer reaction in the radical polymerization of macromonomer. The / values were 0.18 0.28 and much smaller than those for styrene polymerization (0.5 0.7). The isotactic macromonomer gave larger N and / values than the syndiotactic... 

The role of Acac" in the polymerization reaction has been clarified by ESR spectroscopy [8]. It was demonstrated that the initial Acac" radical formed is consumed in the course of the polymerization reaction and substituted by another, C-centered radical attributed to the growing polyacrylamide chain. Using Acac or dibenzoylmethane as initiators, NMR studies revealed the presence of the initiator in the polymer product [12]. 

A polymeric pinacol,poly[3-methyl-2-(4-vinylphenyl)-2,3-butanediol],has been prepared by radical polymerization of the styrenic diol monomer and shown to be cleanly and quantitatively converted to a non-conjugated ketone in the solid state by reaction with a photochemically-generated acid [151,348, 350]. The rearrangement reaction can be readily monitored by IR spectroscopy as the disappearance of the hydroxyl OH absorption is accompanied by appearance of a new ketone carbonyl absorption (Fig. 116). Since a polar alcohol (isopropanol) dissolves the polar diol polymer in the unexposed regions but cannot dissolve the less polar ketone polymer produced in the exposed regions, the resist functions as a negative system with alcohol as a developer. The diol polymer is stable thermally to 225 °C in the absence of acid. 

ESR spectroscopy was successfully applied to quantify radical concentration in the polymerizations [4, 8-11], However, the direct detection method of ESR did not reveal information on many additional points that ate very significant in radical polymerization chemistry so far. For example, the length of propagating chain is not known, direct observation of the penultimate unit effect is almost impossible, and detailed mechanisms of radical reactions remain extremely difficult to examine. These problems have not yet been fully resolved but the development of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP), enables us to resolve some of these problems. 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

Using multi-wavelength transmission spectroscopy together with labeled compounds additional quantitative information on the reaction can be readily obtained. For example, Figure 5 shows the analysis of end groups (initiation radicals) per molecule as a function of conversion, in a methyl-methacrylate polymerization reaction. The initiator used in this case is benzoyl peroxide, a strong chromophore in the ultraviolet portion of the spectrum. " ... 

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