An interpretation of the effect of resin molecular weight on the dissolution of Novolak is offered. It is based on Eyring's transition state theory and on the percolation model of Novolak dissolution. The rate-determining step of Novolak dissolution is the deprotonation of phenol by base at the front edge of the penetration zone. In order for this reaction to occur at a particular site, an ion pair of base must appear there, and to make this possible, all base ions of the corresponding percolation channel have to move forward in synchronism. That requires the simultaneous thermal activation of all the sites of the channel. At this point the mechanism of energy transport intervenes: In a system of polymer chains, thermal (vibrational) energy propagates much faster along the chains than between them, and the critical energy fluctuations needed for the activation of a site will reach this site almost exclusively via the chain to which the site belongs. It can be shown that the probability that a particular site will receive an activating quantum is inversely proportional to chain length. The probability that all sites of a percolation channel will be activated simultaneously is inversely proportional to chain length to a power that is the number of sites involved in the move. The probability of this event decreases steeply with chain length, as is observed. These principles lead to a quantitative description of the dissolution of Novolak films as a function of the molecular weight of the resin. [References: 18]
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