**2.3 ROP of siloxanes**

Taking the advantage of NHCs silicophilicity [32], the ROP of cyclic (carbo) siloxanes has been investigated [33]. A rapid polymerization of 2,2,5,5-tetramethyl-1-oxa-2,5-disilacyclopentane (TMOSC) occurred in less than a minute [34]. The polymerization was activated by electron-rich NHC in toluene (**Figure 7**). The product, poly(carbosiloxane), the molar mass of 10,200 g mol−1 was controlled with dispersity equal to 1.19. This was observed if the polymerization stopped in high conversion otherwise broadening was detected due to undesired transesterification side reactions.

The authors reported a decrease in the polymerization rate when bulky, and less basic NHC is involved. Also, they revealed through mechanistic studies that the polymerization process is activated by hydrogen bonding instead of nucleophilic ring-opening of TMOSC by the NHC.

**Figure 7.** *the proposed mechanism of ROP of TMOSC.*

## **2.4 ROP of epoxides**

An attempt to activate the ROP of ethylene oxide (EO) by NHC was recorded by Raynaud et al. [34]. In this work, NHC succeeded to accelerate the ROP of ethylene oxide as a direct initiator and combined with chain regulators of the NuE-type. 1,3-diisopropylimidazol-2-ylidene initiate alone ROP of EO in DMSO at 50°C. linear difunctionalized PEOs were produced, unlike cyclic polymers that formed by ZROP of LA which was previously discussed.

## **2.5 ROP of carbonates**

The ROP of cyclic carbonates was reported in the melt or bulk polymerization [34]. Nederberg et al. investigated the ring-opening polymerization of trimethylene carbonate using varieties of organocatalysts. NHC catalysis yield a quantitative conversion in 30 min and a polymer dispersity of only 1.06, when less electron-rich NHC is employed (**Figure 8**). This study showed that the increased electron-rich nature of NHC leads to an increase in the polymerization rate at the expense of molecular weight control (99% conversion in seconds).

**Figure 8.** *The ring-opening polymerization of trimethylene carbonate.*
