As the film grows thicker, the diffusion of dissolved oxygen from the bulk liquid into the film becomes a controlling factor in the growth process. This gives rise to two distinct regions within the film, as illustrated in Fig. . One is the oxygen-driven fast growth region, located near the tray surface where the concentration of dissolved oxygen is sufficiently high, thereby enabling fast oxygen-involved film growth pathways to prevail. The other region is characterized by slower anaerobic growth similar to the situation studied in the previous work. Due to the low concentration of dissolved oxygen in the hotter section of the distillation column, film growth is initially governed by carbon-center radical pathways. As the film grows thicker, the flow of radicals from the bulk into the film becomes diffusion-limited, causing the accumulation of solid-bound radical reactive sites to decrease. Note that the loss of solid-bound radical reactive sites is not affected by film thickness, because the dominant consumption pathways of solid-bound radical reactive sites are via the hydrogen atom abstraction with a loosely bounded hydrogen on a monomer, while the monomer concentration in the film remains about the bulk liquid and is not limited by diffusion. Consequently, the film growth via the Diels-Alder pathways is not affected by diffusion and thus becomes competitive with the carbon-center radical pathways. In the reboiler, the elevated temperature and the higher concentration of cyclopentadiene cause the Diels-Alder pathways to dominate even at early times.