*3.2. Melt-Transesterification Process of DPC and PDMS*

Differences may exist in the miscibility of DPC and PDMS with different average chain lengths during the melt-transesterification stage, which may affect the equilibrium transesterification conversion of PDMS. For this reason, we reacted PDMS having different chain lengths with DPC catalyzed by KOH. Samples were extracted from the reactor at set intervals for analyses through 1H-NMR and OM to verify whether the miscibility of the mixture was improved by the transesterification between PDMS and DPC.

Figure 5 gives the OM photographs of transesterification products obtained from DPC and PDMS with different chain lengths reacted at different times. Transesterification temperature was set at 180 °C, and the molar ratio of DPC to PDMS was controlled at 1:1. Figure 5 shows that when the chain length of PDMS was shorter, more DPC was added and more DPC crystals appeared on the OM photographs. However, the results in Figure 5 revealed that the experiments of Silanol5.3 had fewer DPC crystals before KOH addition than the other two Silanols with larger average chain lengths. The 1H-NMR spectra of the transesterification products of DPC and PDMS with different average chain lengths before KOH addition are shown in Figure 6. Silanol5.3 had already undergone transesterification with DPC, and at this point, 1.8% DPC conversion had been achieved. By the time the reaction reached 3 min, 5.2% DPC was consumed, and the relatively small volume of Silanol5.3 more easily dissolved in the remaining DPC due to the change in end group, reflected in the OM photographs as the complete disappearance of DPC crystals. The DPC consumption deduced from 1H-NMR at this moment can be regarded as the critical transesterification level when DPC and PDMS reached mutual miscibility.

**Figure 5.** OM photographs of transesterification products of DPC/Silanol5.3 reacted for (**a**) 0 min, (**b**) 3 min and (**c**) 6 min, DPC/Silanol22.5 reacted for (**d**) 0 min, (**e**) 3 min and (**f**) 6 min and DPC/Silanol56.2 reacted for (**g**) 0 min, (**h**) 3 min and (**i**) 9 min at 180 ◦C.

Table 2 shows the critical transesterification level of DPC and PDMS with different average chain lengths to reach mutual miscibility. With an increased average chain length of PDMS, the critical transesterification level for the complete disappearance of DPC crystals also increased. A conversion rate of 17.0% for DPC was required with Silanol22.5 and 38.6% for DPC with Silanol56.2 for PDMS to be completely dissolved in DPC. This phenomenon indicated that PDMS with a longer average chain length required a higher critical transesterification level to be soluble in the melt with DPC, also indicating that PDMS with different chain lengths had certain solubility differences.

**Figure 6.** 1H-NMR spectra of first transesterification product of (**a**) Silanol5.3, (**b**) Silanol22.5, and (**c**) Silanol56.2 reacted with DPC under the action of 100 ppm KOH at 180 ◦C.

**Table 2.** Critical transesterification level of silanol-terminated PDMS with different chain lengths reacted with DPC to reach two-phase intermiscibility.


Figure 7 presents the change curve of the transesterification level of DPC and PDMS with different chain lengths over time. It should be pointed out that under the catalysis of high concentrations of KOH, prolonging the reaction time may cause side effects such as thermal degradation or rearrangement of PDMS, which affect the calculation of the transesterification level. Therefore, we determined a suitable duration of 100 min for the transesterification. Transesterification between Silanol56.2 and DPC already reached equilibrium by 100 min, and the conversion of DPC was constant at 38.6%. Conversely, the two sets of experiments on Silanol5.3 and Silanol22.5 did not yet reach equilibrium, and the transesterification levels of the end product were 51.5% and 45.6%, respectively. This finding explained why long-chain PDMS was more difficult to introduce into the PC backbone, affecting its conversion in the melt-polycondensation process for the preparation of PC-PDMS copolymers.

Figure 8 reveals the 29Si-NMR spectra of pristine PDMS with different chain lengths and their transesterification products with DPC. Figure 8 shows that a new peak appeared at around δ −13.72 ppm (*α*), which was believed to originate from DPC-PDMS oligomers formed by transesterification. Moreover, according to the literature, some cyclic siloxanes may be present during the preparation of silanols, and the main structures would be D3, D4, D5, and D6, showing peaks at −9.2, −20, −22.6, and −23.0 ppm in that order [31,32]. Figure 8 shows that PDMS with different chain lengths did not display any peak at the above positions, and no cyclic siloxanes formed in the final transesterification products after reacting with DPC at 180 ◦C for 100 min. This finding meant that PDMS itself did not suffer

from side reactions such as cyclic degradation during the melt-transesterification stage within 100 min. However, the unconverted long-chain PDMS during transesterification may be more susceptible to side reactions at high temperatures and strong bases, reducing PDMS conversion. The experimental results confirmed that improving PDMS conversion in melt polycondensation and suppressing possible side reactions required the selection of PDMS with lower viscosity, shorter chain length, and higher activity as the raw material. This selection would ensure the enhancement in PDMS conversion in the transesterification stage and the elimination of the effect of terminal reactive groups on the side reactions, such as chain tailoring or self-condensation.

**Figure 7.** Time dependence of transesterification level of silanols with different average chain lengths reacted with DPC catalyzed by KOH at 180 ◦C.

**Figure 8.** 29Si-NMR spectra of (**a**) Silanol5.3, (**b**) Silanol22.5, and (**c**) Silanol56.2, as well as the final transesterification products of (**d**) Silanol5.3, (**e**) Silanol22.5, and (**f**) Silanol56.2 reacted with DPC for 100 min at 180 ◦C.

#### *3.3. Influence of Silanol Feeding on Conversion*

The study on the transesterification process between DPC and PDMS helped us select Silanol5.3, which had a lower viscosity, higher hydroxyl content, and shorter chain length, for further transesterification with DPC and BPA. Due to the difference in reactivity between BPA and Silanol5.3, competition existed during the transesterification with DPC, so we expected that the feed amount of Silanol5.3 affected the PDMS conversion. Hence, we prepared a series of silanol-based copolycarbonates by varying the silanol feed amount.

The sequence distribution of BPA and Silanol5.3 moieties was determined by the chemical shifts at around δ 150.5–151.2 ppm in the 13C-NMR spectrum (Figure 9). The carbonyl carbons split into three peaks corresponding to *C*<sup>1</sup> (PDMS-PDMS), *C*<sup>2</sup> (BPA-PDMS), and *C*<sup>3</sup> (BPA-BPA). The chemical shifts of the three peaks detected at δ 150.8, 151.0, and 151.1 ppm were assigned to the carbon atoms. The central carbon atom of BPA linked within the chain also split into three peaks (*C*4, *C*5, and *C*6) as a result of the copolymerization [33]. Molar ratios of the *C*1, *C*2, and *C*<sup>3</sup> sequences of Silanol5.3-based copolycarbonates can also be calculated by the integral ratio of the three different types of peaks [34].

**Figure 9.** 13C-NMR spectra of PC-PDMS copolymer with different feed ratios of BPA to Silanol5.3.

Table 3 summarizes the trends of the sequence distribution of Silanol5.3-based copolycarbonates with Silanol5.3 feed amount. Table 3 shows that the contents of *C*<sup>1</sup> and *C*<sup>2</sup> structures gradually increased with increased Silanol5.3 feed amount, whereas the content of *C*<sup>3</sup> structures gradually decreased. The number-average sequence length of the BPA segment (*L*nBPA) decreased with increased Silanol5.3 feeding, whereas *L*nPDMS gradually increased. When the molar ratio of BPA/PDMS was 90/10, *L*nPDMS was equal to 2, meaning that the PDMS segments in the copolymer were primarily in the form of

Silanol5.3–O–C(=O)–O–Silanol5.3. Changing the feed amount of PDMS affected the distribution of PDMS segments in the copolymer.


<sup>1</sup> Molar content of BPA-BPA, BPA-PDMS, and PDMS-PDMS sequences in the carbonyl carbon region; <sup>2</sup> block length of BPA and PDMS measured by 13C-NMR (Figure S1 in Supplementary Materials); <sup>3</sup> degrees of randomness (*B*) of copolymers calculated by 13C-NMR (Figure S1).

> Table 4 demonstrates the effect of the initial feed ratio of BPA to Silanol5.3 on the molecular-structure characteristics of the Silanol5.3-based copolycarbonates. BPA and DPC were well miscible at the molecular level, but BPA was less reactive than Silanol5.3, and the poor miscibility between Silanol5.3 and DPC may need to be improved through transesterification. This phenomenon may affect the performance of the polycondensation product. For instance, the highest *M*η of synthesized products was achieved when the molar ratio of DPC to diols was 1:1. With decreased relative content of DPC, the conversion of Silanol5.3 was supposed to decrease. However, PDMS conversion increased when the BPA/PDMS feed ratio was 90/10 probably because the short-chain, high-reactivity Silanol5.3 fed in higher amounts was more likely to produce the *C*<sup>1</sup> structure. Consequently, conversion was higher due to the inhibition of cyclic degradation by the adjacent PC segments. The short-chain Silanol5.3 was used as a third monomer in the copolymerization with BPA and DPC, and the PDMS conversion decreased with increased PDMS feed amount. With further increased PDMS feed amount, PDMS conversion remained over 65%.

**Table 4.** Molecular-structure characteristics of silanol copolycarbonates with different feed ratios of BPA and Silanol5.3 obtained from 1H-NMR.


<sup>1</sup> Based on the initial molar percentages of BPA and PDMS; <sup>2</sup> based on the initial feeding ratios of DPC and diols; <sup>3</sup> calculated by 1H-NMR analysis according to Equation (3); <sup>4</sup> measured by DSC at a heating rate of 10 °C min–1 (2nd scan); <sup>5</sup> degradation temperature for 5% weight loss was determined by TGA at a heating rate of 10 °C min–1 (with N2).

#### *3.4. Influence of Chain Length on Conversion*

After the investigation of transesterification in Section 3.2, we found differences in the miscibility between DPC and PDMS with different chain lengths, which required improvement through transesterification. The equilibrium transesterification levels of PDMS with different chain lengths were dissimilar, and the residual PDMS oligomers increased the tendency of side reactions to occur in the polycondensation stage. Furthermore, PDMS conversion in the transesterification stage directly determined PDMS conversion in the final products. Therefore, we selected PDMS with different chain lengths as substrates and controlled the same feeding ratio of BPA to PDMS to investigate the effect of the average chain length of PDMS on its conversion.

Figure 10 shows the 13C-NMR spectra of PC-PDMS copolymers prepared from different-chain-length silanols under the same conditions. A longer PDMS block length in copolymers caused the *B* to approach 1, and random copolymers were more easily obtained. Table 5 presents the molecular structure of PC-PDMS copolymers prepared from PDMS with different chain lengths. PDMS conversion significantly decreased when longer-chain PDMS was added (~68% vs. 13%).

**Figure 10.** 13C-NMR spectra of PC-PDMS copolymers using (**a**) Silanol5.3, (**b**) Silanol22.5, and (**c**) Silanol56.2 as raw materials with a feeding mole ratio of BPA/PDMS = 94/6.

**Table 5.** Molecular-structure characteristics of PC-PDMS copolymers with different chain lengths of silanol-terminated PDMS obtained from 1H-NMR and 13C-NMR.


<sup>1</sup> Based on the initial molar percentages of BPA and PDMS; <sup>2</sup> determined by 1H-NMR analysis; <sup>3</sup> calculated by 13C-NMR.

To improve the conversion of long-chain PDMS in the polycondensation process and to suppress side reactions such as cyclic degradation, the critical transesterification level of DPC and PDMS must be reached to achieve mutual miscibility. The equilibrium conversion rate of PDMS must also be increased to introduce as much PDMS into the PC oligomers as possible at a lower temperature. Accordingly, we proposed a countermeasure to overcome this problem of low equilibrium conversion of PDMS in the transesterification stage by increasing the amount of KOH used as catalyst (Table 6).

Figure 11 shows the curves of transesterification level of long-chain Silanol56.2 and DPC catalyzed by different amounts of KOH as a function of time. With increased KOH amount, the equilibrium transesterification level of long-chain Silanol56.2 increased (~49.0% vs. 38.6%), and this idea can be carried over to the preparation of the polycondensation products. Table 7 provides information about the molecular structure of the PC-PDMS copolymers prepared from long-chain Silanol56.2 catalyzed by different amounts

of KOH. Increased equilibrium conversion at the transesterification stage significantly increased PDMS conversion throughout the polycondensation (90.4% vs. 13.1%).

**Table 6.** Polymerization of silanol copolycarbonates with different chain lengths of PDMS and BPA as two different diols and DPC as the carbonate source.


<sup>1</sup> Based on the initial feeding ratios of BPA and PDMS; <sup>2</sup> based on the initial molar percentages of DPC and diols; <sup>3</sup> measured by an Ubbelohde viscometer using CHCl3 as the solvent through Equations (4) and (5); <sup>4</sup> determined by UV-*vis* spectrometer through Equation (7); <sup>5</sup> measured by DSC at a heating rate of 10 °C min−<sup>1</sup> (2nd scan); <sup>6</sup> degradation temperature for 5% weight loss was measured by TGA at a heating rate of 10 ◦C min−<sup>1</sup> (with N2).

**Figure 11.** Time dependence of transesterification level of Silanol56.2 reacted with DPC under the catalysis of different amounts of KOH at 180 ◦C.

**Table 7.** Molecular-structure characteristics of PC-PDMS copolymers prepared by different amounts of KOH obtained from 1H-NMR and 13C-NMR.


<sup>1</sup> Based on the initial molar percentages of BPA and PDMS; <sup>2</sup> determined by 1H-NMR analysis; <sup>3</sup> calculated by 13C-NMR; <sup>4</sup> a KOH amount of 200 ppm to diols on a mole basis was used.

#### **4. Conclusions**

We investigated the transesterification process between DPC and PDMS with different chain lengths using KOH as catalyst and successfully prepared a series of high-molecularweight PC-PDMS copolymers with low color difference through melt polycondensation. Transesterification experiments confirmed that with increased chain lengths of PDMS, the difficulty of miscibility between PDMS and DPC increased. Consequently, a higher critical transesterification level was needed to dissolve in DPC than PDMS with lower chain lengths. Furthermore, the conversion rate of Silanol56.2 was only 38.6% when the transesterification with DPC reached equilibrium, and unreacted PDMS oligomers were more prone to undergo side reactions in the polycondensation stage. The feed amount of PDMS and its chain length affected the conversion of PDMS throughout melt polycondensation. The

short-chain Silanol5.3, as the third raw material in the copolymerization with BPA and DPC, was more likely to produce a *C*<sup>1</sup> structure, and PDMS conversion decreased with increased PDMS fed. With further increased average chain length of PDMS, conversion clearly decreased. With increased KOH amount, the equilibrium transesterification level of Silanol56.2 and DPC was promoted, thereby enhancing the conversion rate of long-chain Silanol56.2 in the final product (90.4% vs. 13.1%). These results can serve as a reference for the preparation of high-conversion and high-quality PC-PDMS copolymers through melt polycondensation.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/polym13162660/s1, Figure S1: Typical 13C-NMR spectrum (CDCl3, 150 MHz) of PC-PDMS copolymer and calculation of *L*nBPA, *L*nPDMS and *B*.

**Author Contributions:** Writing–Original Draft Preparation, Z.Z.; Writing–Review and Editing, G.W. and Z.Z. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

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