Main chain scission

Reductive treatment of the ozonides formed from copolymers of MMA with butadiene leads to oligomers with -OH end-groups their presence was confirmed by the appearance of NMR signals between 3.9 and 4.1 ppm and also NMR signals at 62.3 and 64.6 ppm from the adjacent methylene groups. Acceptable values of M for the oligomers were obtained from comparison of the areas of the proton peaks of these methylene groups with those of the methoxyl protons in the MMA units. 

NMR was used to characterize fluorophenyl ketone end-groups in oligomers produced by ozonolysis of polymers of MMA and STY, containing units derived from 2,3-di(4-fluorophenyl)buta-1,3-diene [56]. 

In all the cases examined, the diene is very largely incorporated as 1,4-units but the pendant double bonds of the few 1,2-units can cause complications by giving rise to additional end-groups. In some cases at least, the difficulty can be avoided by replacing the diene by a suitable acetylenic compound as the comonomer used to furnish the in-chain double bonds [57]. For example, phenylacetylene has been used instead of 2,3-diphenylbutadiene to produce oligomers of MMA, having phenyl ketone and carboxylic acid end-groups as indicated by the overall equation 

Another promising method for the preparation of oligomers of MMA uses the photolysis of high polymers containing a limited number of units bearing carbonyl groups. These units can be introduced by using methyl vinyl ketone as a comonomer. It is well established that absorption of a UV quantum by 

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Both side-chain and main-chain scission products are observed when polyacrylates are irradiated with gamma radiation (60). The nature of the alkyl side group affects the observed ratio of these two processes (61,62). 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

In transient elongational flow degradation, it was determined in the authors laboratory, by a detailed mass balance, that main chain scission accounted for >95% of the degradation in dilute solution. Any other type of depolymerization, if present, should then be of minor importance. 

The recombination of fragments stemming from one macromolecule, at times shorter than the diffusion time, prevents the linear increase in RD with the absorbed dose per pulse, as not all main-chain scissions result in the formation of fragments. The effect of molecular oxygen on RD in the case of PBS can be interpreted by formation of peroxyl radicals, e.g. 

In the case of radical formation by the main-chain scission of the polymer molecule, a high concentration of the radicals initiates the reactions involving the geminate pair [30]. 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

Materials that exhibit enhanced solubility after exposure to radiation are defined as positive resists. The mechanism of positive resist action in most of these materials involves either main-chain scission or a polarity change. Positive photoresists that operate on the polarity change principle have been widely used for over three decades in the fabrication of VLSI devices and they exhibit high resolution and excellent dry etching resistance. Ordinarily, the chain scission mechanism is only operable at photon wavelengths below 300 nm where the energy is sufficient to break main chain bonds. 

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

We present here the results of a mass spectroscopic study of the vaporized species produced during UV and VUV irradiation of PMMA. A difference was observed between the mass spectra of the vaporized species produced by the two methods of irradiation, indicating direct main-chain scission induced by VUV absorption of PMMA. Further, the effects of heating and introduction of oxygen on UV etching will be described. 

UV Etching. A typical mass spectrum of the vaporized UV etching products is shown in Figure 4, together with a background spectrum obtained without UV irradiation. The comparison clearly shows that UV irradiation causes an increase in intensity for various mass peaks. For example, the intensity of the peaks of m/e=15, 31, 59 and m/e=41, 69 increased drastically by UV irradiation. The former three are due to side-chain scission caused by UV absorption at the C=0 unit, while the latter two are due to main-chain scission initiated by side-chain scission (11). The structure and mass numbers of typical vaporized species are shown in Table I. From here on, we use the spectral intensity after the background is subtracted. 

Methyl end groups resulting from main-chain scission in ethylene-propylene copolymers have observed by their characteristic 13C NMR resonance and determined quantitatively to give values of G(scission). 

The only radical intermediate observed for poly methacrylic acid was the propagating radical formed by main chain scission. This observation is similar to that noted for gamma radiolysis of poly methylmethacrylate, where the propagating radical is also found as the only stable radical intermediate following radiolysis at 303 K. In both cases the propagating radical is formed by -scission following the loss of the side chain, resulting in formation of the unstable tertiary radical. 

Figure 1. Adiabatic potential curves in the main chain scission of a model compound of poly(isobutylene) 2,2-, 4,4-tetramethylpentane (4). AE3l(=0.61eV), aET,(—0.35eV), and AEf (=2.05eV) are the activation energies of the main chain scission in the lowest singlet excited state (S,), the lowest triplet state (T,), and the ground state, respectively.

The absolute sensitivity was defined as the number of main chain scissions occurring in one photon molecule when one photon was irradiated on a unit surface (1 cm2) of the film. If the spectral dependence of the quantum yield has been obtained, the absolute sensitivity is calculated following the formula (4). The results obtained are shown in Fig. 9. The practical sensitivity will be the integral of the product of the absolute intensity of the irradiated light and the absolute sensitivity in Fig. 9 over all the wavelength of the spectra. 

Elegant work by Van der Hoff [60] seems to suggest that it is the latter, or more correctly there is a Gaussian distribution about the midpoint of a chain. However, as to whether the main chains are primarily broken by ultrasonic action is still open to question since it is possible that the main chain scissions are secondary effects due to chemical reactions initiated by unstable intermediates, such as free radicals or ions, produced by sonication. For example McKay [61] has shown that hydroxyl radicals, generated by the oxidation of Fe by H2O2, are the cause of chain scission in polyacrylamide molecules in aqueous solution. 

Experimental determination of the quantum efficiency of photosensitive polymers of the sort that are used in one-component positive resist systems is a more complex experimental undertaking. Here the quantum efficiency is defined as the number of main chain scissions that occurs per photon absorbed. Guillet and coworkers at the University of Toronto have... 

So far we have only considered polymers that undergo main-chain scission upon exposure to radiation. PMMA is an example of such a material. If, on the other hand, one considers polymeric systems in which both scissioning and crosslinking events occur simultaneously upon exposure, the analysis depicted above will allow determination only of the net scission-... 

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

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