Radicals polymethylmethacrylate

The next simplest group of linear polymers is the vinylidcnc group. Now two of the hydrogens of ethylene are replaced by radicals. Polymethylmethacrylate (alias PMMA,... 

End-chain scission the polymer is broken up from the end groups successively yielding the corresponding monomers. When this polymer degrades by depolymerization, the molecules undergo scission to produce unsaturated small molecules (monomers) and another terminal free radicals. (Polymethylmethacrylate, polytetrafluorethylene, polymethacrylonitrile, polyethylstyrene, polystyrene, polyisobutene)... 

Commercially, polyMethylmethacrylate is made by either suspension or bulk polymerisation using a peroxide free-radical initiator ... 

A radical anion is formed at the cathode and the radical and anion portions polymerize independently, producing polymethylmethacrylate block on the anion and a copolymer on the free-radical portion. 

The formation of syndiotactic polymethylmethacrylate is the result of the prefered chain configuration in the free anionic polymerization, similar to the formation of syndiotactic structure by the free radical polymerization of methylmethacrylate. Bawn, James and North (77)... 

Yet, again, averaging due to internal rotations may not be complete, in which case some but not all components are broadened, as in the study of the radical CH3. CH. C02H (Horsfield et al., 1962) and probably the radical RCH 2.O(Me). CO 2Me derived from polymethylmethacrylate (Section VI, D). 

Chain Free radical Polybutadiene Polyethylene (branched) Polyisoprene Polymethylmethacrylate Polyvinyl acetate Polystyrene... 

Polymethylmethacrylate is one of the heterogeneous polymer and ESR studies on this polymer have been done from the early stage of ESR spectroscopy ( 5). Russian researchers reported ESR ctra observed from the PMMA mechano-radicals (28,46-48). No ir formation of the n hano radicals was mentioned by these authors, although PMMA is one of the suitable polymers on wludi Ihe ptdr formation of mechano-radicals is s rched. An ESR spectrum (38, 49) observed at 77 K from the PMMA milled for 24 hrs. is shown as a in Fig. 8. It is well known (50,51) that PMMA radical produced by the main chain scission shows the characteristic quintet-quartet spectrum when PMMA is irradiated by y-rays. It is reasonable to assume that one partner of the pair formation of the PMMA mei no-radical is the species. 

Photopolymerizable compositions have found their use in the production of thick (5-100 pm) relief images, especially in the field of printed plates and micromachinery. The compositions contain a monomer, matrix polymer (binder), and photoinitiator. The branched monomers are used most frequently. Many photosensitive compounds generating radical and ion centers upon UV irradiation are used as a part of the compositions. Polymethylmethacrylate and its derivatives are very often used as polymer binders. 

A polymethylmethacrylate film is transparent and does not absorb solar radiation. Therefore photodegradation of this polymer is relatively small. ESR studies on photo-induced radical formation have been reported by a few workers (90-94). 

Kato et al. (92) irradiated polymethylmethacrylate at -196° C under vacuum, and the spectrum shown in Fig. 16 was obtained. This spectrum was identified as due to the free radicals, COOCH3, CHO, and -CH3, which show the singlet, doublet and quartet, respectively. The half-life of methyl radicals at — 196° C was about 5 hr. It is likely that the methyl radicals are produced by the photolysis of ester side groups, just as ethyl radicals are produced after irradiation of polyethyl-methacrylate at —196° C. 

The sample irradiated at — 196° C in the presence of air gave the same spectrum as that under vacuum. This sample, however, gave the spectrum characteristic of peroxy radicals at about — 68° C. An effect of oxygen on radical formation on irradiation with light of 300 nm was also reported (67). Protection against radiation damage in polymethylmethacrylate by ultraviolet light was also reported (98). 

In this polymer, every alternate carbon of the chain is quaternary and no reactive hydrogen atoms are present. Formation of radicals by scission of the main chain results in monomer formation by a chain depolymerization [51, 52, 54, 55], but the polymer is not quantitatively converted into monomer as happens with polymethylmethacrylate. A range of products from C4 to C2 0 is also evolved [52, 53]. 

The degradation behaviour of polymethylmethacrylate is easily characterized by thermal volatilization analysis [87] (Fig. 29). Monomer is obtained in very high yield in all cases. A polymer sample prepared by a free radical reaction undergoes a rapid depolymerization at about 275°C as indicated by the first peak. The second peak, situated between 350 and 400°C, corresponds to a second mode of initiation of chain depolymerization. For samples prepared by anionic polymerization, the first peak is not observed. Depolymerization of the whole sample occurs above 350°C. 

The thermal degradation of polymethylmethacrylate was investigated many years ago by isothermal methods [88, 89]. The two mechanisms of chain depolymerization were already identified at that time and analysed. At temperatures below 270°C, the reaction is initiated at the double bonds situated at chain ends and formed by radical disproportionation... 

Fig. 29. Comparison of TVA thermograms for high and low molecular weight polymethylmethacrylate samples prepared by an anionic mechanism with the thermogram for a low molecular weight free radical sample [87], Dotted line, polymer D,M = 1,500,000 full line, polymer E, M = 60,000 dashed line, free radical polymer C, M = 20,000.

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds to the elimination of HC1 and the second to that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded styrene evolution, in fact, does not occur below 350°C. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 300° C. According to McNeill et al. [323], structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of Cl to the aromatic ring, yielding a cyclohexadienyl-type radical which is unable to induce depolymerization of the styrene chain. 

Fig. 33. Change in radical concentration as a function of time for polymethylmethacrylate irradiated with 7-rays (4 Mrad) and warmed at 150, 200, 250 and 300°K [220].

Stepwise decay was also observed when PMMA was irradiated in the presence of ethyl mercaptan (EtSH) [245]. The initial decay rate of the radicals measured at 150°K is proportional to the concentration of EtSH, indicating that the decaying pairs are mixed pairs formed by a radical from PMMA and a radical from EtSH. In fact, radiolysis of pure PMMA results in the formation of pairs of macroradicals. Some are due to main-chain scission, others to hydrogen abstraction from the polymer by CH 3 or CH30 radicals produced by side-chain scission. At 150°K, in pure irradiated polymethylmethacrylate, the mobility of the macroradicals is limited and their rate of decay comparatively low. In the presence of ethyl... 

Isotactic polymethylmethacrylate has been shown to isomerize into the atectic form under the effect of electrons [404] or 7-rays [405]. The yield of isomerization is higher with ionizing radiation than with UV irradiation [405], as is the main-chain scission yield the G value for volatile formation, on the other hand, is lower for 7 than for UV irradiation. The isomerization has therefore been attributed to main-chain scission followed by recombination of the radicals in the cage. This isomerization is completely inhibited in the presence of ethyl mercaptan [406]. 

Various mechanisms of chain scission have been proposed for polymethylmethacrylate. The simplest involves direct breaking of the main chain followed by disproportionation of the radicals [392], viz. 

The primary cation formed by ionization of the polymer would be unstable and decompose according to reaction (55). This type of reaction has been observed in the mass spectrometer for low molecular weight esters [420]. Recombination of the cation with an electron yields an excited radical which immediately decomposes according to reaction (57). Radical A has, in fact, never been observed by ESR. Direct breaking of the main chain occurs according to reaction (51) but recombination of the macroradicals in the cage is very probable as indicated by the observed racemization of isotactic polymethylmethacrylate [405]. This racemization would involve the reactions... 

However, there is no chemical evidence for such a process. Main chain scission in irradiated polymethylmethacrylate is probably due to the decomposition of the macro-radical produced in reaction (5) namely,... 

The nine-line ESR spectrum observed after irradiation at room temperature has been attributed to the propagating radical formed in reaction (7) [76]. However, according to other workers [77] this signal could also result from the addition of another radical to residual monomer molecules. It is of interest to note that the quantum yield of main chain scissions is five to ten times smaller than the quantum yield of side group splitting by reaction (5), whereas both processes occur with the same yield in the radiolysis of polymethylmethacrylate [78]. This indicates that only a fraction of the macro-radicals decompose according to reaction (7) at room temperature. 

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