Molecular weight reduction scheme

Poly(dimethyl glutarimide) (PMGI) (structure 3.7) was shown by Hir-aoka (63) to undergo molecular weight reduction upon irradiation with a sensitivity comparable to PMMA. This polymer is sensitive to DUV radiation below 280 nm soluble in aqueous base resistant to common organic solvents and thermally stable to ca. 185 C, which renders the material very attractive as a thick planarizing layer in the exposure-PCM scheme as will be discussed in a later section. This material is being evaluated for commercialization by Shipley Company (64, 65). 

Hydroperoxides decompose relatively slowly at ambient temperatures in the dark, but in light they are readily photolysed to free radicals, (Scheme 1.1, reaction d). Consequently, the rate of photo-oxidation of the hydrocarbon polymers is orders of magnitude higher than thermal oxidation. In addition, small amounts of transition metal compounds, notably iron, cobalt, manganese and copper, have a powerful catalytic effect on radical formation from hydroperoxides [14], leading to rapid molecular weight reduction by breakdown of the intermediate alkoxyl radical and the formation of carboxylic acids and esters as oxidation end products (see Scheme 1.2) [15]. 

Moreover, alcohol functionalities have been introduced into the polynor-bornene (PNB) backbone by copolymerization of norbornene with a few percent of 5-acetate norbornene and subsequent acetate reduction. After transformation of the pendant hydroxyl functions into diethyl aluminum alkoxides, sCL has been ring opening polymerized (Scheme 31). Owing to the controlled/ liv-ing character of both polymerization processes the isolated poly(NB- -CL) graft copolymers were characterized by well-defined composition, controlled molecular weight and branching density, and narrow MWD (PDI=1.2-1.4) [92]. 

In contrast to aromatic aldehydes, aliphatic aldehydes invariably afforded complex mixtures of products when submitted to the above conditions. Based on their molecular weights, a few products could tentatively be identified as dialkylated, as well as dehydrodialkylated, compounds, such as 24. These perhaps were derived from tautomerization of the initially formed imine 19 to the thermodynamically more stable enamine 20 followed by reaction with a second aldehyde molecule and subsequent reduction (Scheme 3). 

Figure 5.6 shows the same data plotted as a function of cM0 6S to test the low concentration reduction scheme based on c rf] with a typical value of the Mark-Houwink exponent for good solvents. The data have been shifted vertically to achieve superposition at high molecular weights. It is clear that the cM variable produces a better superposition of data at all molecular weights and concentrations. The apparent variation in the values of cM at the intersections in Fig. 5.4 (Table 5.1) is largely due to a lack of data to define the limiting behavior at low molecular weights at some concentrations. The intersection on the superposed plot in the composite Fig. 5.5 is cM = 30000, giving Mc = 30600 for undiluted polystyrene (q = 0.98 at T = 217° C, in good agreement with the value 31200 reported by Berry and Fox (16).

The oxidation-reduction route was also used to prepare copolymers 53 of bis[bis(tri-methylsilyl)amido]germanium and acetylene derivatives130 (Scheme 27). Rhodium compounds such as [Rh(norbornadiene)Cl]2 were used as catalysts. In contrast to other polymers prepared from germylenes, the monomer-to-monomer ratio was not regular. Relatively low molecular weight polymers 53 (Mn = 1 x 103-104) were isolated. 

---