*Article* **Preparation of Bisphenol-A and Polydimethylsiloxane (PDMS) Block Copolycarbonates by Melt Polycondensation: Effects of PDMS Chain Length on Conversion and Miscibility**

**Zibo Zhou and Guozhang Wu \***

Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science & Engineering, East China University of Science & Technology, Shanghai 200237, China; s13867700926@163.com **\*** Correspondence: wgz@ecust.edu.cn

**Abstract:** This study aimed to improve polydimethylsiloxane (PDMS) conversion in the preparation of polycarbonate (PC)–polydimethylsiloxane (PDMS) copolymer through melt polycondensation. We examined the transesterification process of PDMS with diphenyl carbonate (DPC) and its copolymerization products with bisphenol-A (BPA) for different chain lengths of PDMS. The key factors affecting PDMS conversion were investigated. Results showed that long-chain PDMS required a higher critical transesterification level (38.6%) to improve miscibility with DPC. During polycondensation, side reactions were more prone to occur when the equilibrium transesterification level of long-chain PDMS was lower. PDMS conversion was also lower when more short-chain PDMS was fed. Increasing the chain length of PDMS also reduced PDMS conversion. Notably, increasing the amount of KOH can significantly improve PDMS conversion throughout the polycondensation stage by increasing the equilibrium transesterification level of long-chain PDMS, thereby inhibiting the occurrence of side reactions.

**Keywords:** PC-PDMS copolymer; melt polycondensation; miscibility; equilibrium transesterification level; conversion

#### **1. Introduction**

Polycarbonate (PC) is widely applied in various fields such as electronics, automotive parts, constructions, and medical equipment due to its excellent optical and mechanical properties, including transparency and impact strength [1–3]. However, its utilization still faces challenges because new designs tend to emphasize more on key application areas requiring thin-wall parts, complex geometries, and high-impact products that can withstand very low temperatures with no compromise on flow and processability. Meanwhile, PC shows a V-2 rating in the UL-94 test, meaning it does not meet fire-safety requirements. For this reason, PC modification with other components through copolymerization and melt blending has been extensively studied over the last four decades [4–6].

Polydimethylsiloxane (PDMS) molecules have relatively flexible polymer backbones (or chains) with a low glass-transition temperature, resulting in low temperature ductility, high thermal stability, and versatile functionality. Thus, PDMS is an ideal flame retardant or impact modifier for PC. These disadvantages can be compensated for through copolymerization with PDMS, which generally shows higher polymer-chain flexibility [7,8]. The introduction of PDMS into the PC main chain enables an easy process of PC and improves its low-temperature toughness, weatherability, hydrolytic stability, and flame retardancy [9–15]. Compared with traditional interfacial phosgenation, melt-condensation polymerization is a green process not requiring the use of hazardous phosgene and chlorinated solvent. Thus, this efficient and friendly method can be applied in preparing PC-PDMS copolymer for industrial production.

The preparation of PC-PDMS copolymers by melt polycondensation is greatly hindered by the difficulty of increasing PDMS conversion. PDMS and DPC are immiscible with

**Citation:** Zhou, Z.; Wu, G. Preparation of Bisphenol-A and Polydimethylsiloxane (PDMS) Block Copolycarbonates by Melt Polycondensation: Effects of PDMS Chain Length on Conversion and Miscibility. *Polymers* **2021**, *13*, 2660. https://doi.org/10.3390/polym 13162660

Academic Editors: Edina Rusen and Agnieszka Tercjak

Received: 18 July 2021 Accepted: 3 August 2021 Published: 10 August 2021

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each other due to the large difference in their solubility parameters (*δ*DPC = 10.53 cal1/2 cm−3/2 vs. *δ*PDMS = 7.3 cal1/2 cm<sup>−</sup>3/2) [16,17]. Thus, PDMS cannot be well-dispersed in DPC or PC oligomers in the melt state, thereby suppressing the transesterification reaction of PDMS to some extent [18]. King et al. [19] demonstrated that hydroxyl-terminated PDMS with long chains cannot achieve high conversion through melt polycondensation (only 3–5%), as experimentally observed by Koenig et al. [20] Moreover, the immiscibility between PC and PDMS is a key factor affecting the heat resistance, impact resistance, and optical transparency of PC-PDMS copolymers. Zhang et al. [21] and Zhou et al. [22] reported that PC-PDMS copolymer formed in situ in PC/PDMS blend under the action of transesterification catalyst helps improve the miscibility between the two. Therefore, we consider that PDMS and DPC should be reacted first in the transesterification stage. Afterwards, the end group of PDMS changes, which is beneficial to improving the miscibility with DPC or PC oligomers, and then it reacts with BPA or PC oligomers to obtain the desired products. Thus, the change in miscibility influences PDMS conversion during transesterification.

A series of siloxane equilibrium reactions occur during transesterification under the action of strong bases, including the linear condensation between PDMS and PDMS or the cyclic degradation of PDMS itself, thereby affecting the transesterification ratios of PDMS [23]. The former increases the average chain length of PDMS, and the latter decreases its average chain length. Either raising temperature or lowering pressure can break the equilibrium state and affect the form of PDMS in the melt. GE and Bayer have found that the siloxane chain is severely decomposed by siloxane-chain scission followed by siloxane depolymerization to give cyclic siloxanes during the typical melt process [24,25]. Cyclic siloxanes formed in this process remain in the polymer and have an exceptionally disruptive effect on applications in the electrical/electronic sector [26–28]. Another potential problem regarding PC-PDMS copolymer synthesis, which is relatively neglected often, is the stability of the hydroxyl end groups in PDMS oligomers. Hydroxyl-end groups can backbite the terminal silicon atoms in PDMS oligomers, leading to the formation of cyclic siloxane and the loss of end-group functionality, thereby inhibiting the introduction of PDMS into the PC chain. To improve PDMS conversion, the equilibrium conversion rate of PDMS in the transesterification stage must be increased to promote the miscibility of PDMS with PC oligomers while reducing the residual amount of PDMS and inhibiting the occurrence of side reactions.

Accordingly, the present study initially examined the transesterification process between PDMS and DPC and investigated the degree of transesterification reaction required for different chain lengths of PDMS and DPC to reach a critical miscibility state, as well as the final equilibrium transesterification level. The effects of the feed amount of PDMS and the average chain length on PDMS conversion during melt polycondensation were then investigated. We found that PDMS conversion in the final product depended on the equilibrium conversion rate at the transesterification stage. Finally, a solution strategy was provided to effectively increase PDMS conversion.
