The copolymer composed of methyl methacrylate and methacrylic acid is, in contrast to the pure PMMAs, able to form a 6-membered ring during thermal loading. In this case, two methacrylic acid groups must be located adjacent to each other in the polymer chain (see structural formula), which statistically occurs with sufficiently high frequency at a mixing ratio of 2: 1 (PMMA: methacrylic acid). The reaction is possible at a temperature of 190-210 ° since the produced water is a very good leaving group. During irradiation with electrons, the formed 6-membered ring is easier to break apart than the aliphatic radical chain. This results in a higher sensitivity of the copolymer. The adjusted sensitivity then remains unchanged, a reverse reaction with ring opening is impossible.
Fig.: Structural formula copolymer