## 3 Results and Discussion
### RAFT Agent Removal  
Since the chain transfer agent was inevitably employed in RAFT polymerization to prepare the copolymers, the residual trithiocarbonate terminal groups in the copolymers are present on the copolymer chains after polymerizations. However, the presence of sulfur residue is detrimental to photolithography [38]. On one hand, the trithiocarbonate terminal groups confer color to the polymeric product, which impacts the UV transparency of photoresist. On the other hand, trithiocarbonate terminal groups may thermally decompose during pre-baking and post-baking stages, generating harmful substances that cause irreversible damage to the photolithography equipment. The results of radical-induced-reduction of PTBM-4 in toluene/isopropanol (1:1) as solvent and with AIBN as initiator are illustrated in Figure 3. As shown in Figure 3B,C, the photographs of PTBM-4 before and after the removal of RAFT terminal groups illustrated that the copolymer changed from a pale-yellow color to colorless, indicating the effective removal of trithiocarbonate end groups. In addition, the results of the elemental analyses of PTBM-4 and the product after removal of RAFT terminal group were displayed in Table 2. After the radical-induced-reduction of PTBM-4, the S content significantly decreased compared to that of PTBM-4. According to the calculation of the decreased S content and the average S content of TPS-SS repeating units in PTBM-4, most of the RAFT terminal group was removed, which more clearly verified the success of the removal of the RAFT terminal group. As shown in Figure 3A, it is observed that there is no bimodal molecular weight distribution nor formation of high molecular weight shoulders. It suggested that the toluene/isopropanol mixed solvent was suitable as a hydrogen donor for PTBM-4 propagating radical. Furthermore, a high radical flux also increases the likelihood that termination will involve reactions with a small radical rather than another propagating radical. Even so, there is a small increase in the molecular weight of the polymer, but no significant change in the molecular weight distribution. Polymers with RAFT terminal groups, when exposed to initiators with a high concentration and being heated, the polymers generated propagation radicals (P\({}_{\text{n}}\)). Theoretically, these propagation radicals can undergo chain transfer to a hydrogen donor (H-X, in this study using toluene/isopropanol (1:1)) with a higher chain transfer constant, forming P\({}_{\text{n}}\)-H. However, in practice, side reactions are unavoidable. Propagation radicals can undergo disproportionation termination or bimolecular termination to produce P\({}_{\text{n}}\)-X or P\({}_{\text{n}}\)-P\({}_{\text{n}}\), resulting in a small number of polymers with doubled molecular weight, leading to an increase in the molecular weight [27]. Although bimolecular termination meets the requirement of the RAFT terminal group removal, a more effective and optimized reaction condition needs to be explored for disproportionation termination.  
Figure 2: GPC traces of (**A**) PTBM-0, (**B**) PTBM-4, and (**C**) PBM corresponding to different polymerization time points.