Polymerization of Aryl Polyisocyanides Catalyzed by N-Heterocyclic Carbene-Ligated Scandium Trialkyl Complex

. Polyisocyanide has attracted continuous attention because of its stable helical rod structure and its wide application, it has a unique π-conjugated system in which the carbon-nitrogen double bond in each unit is twisted along the polymeric backbone to form a helical configuration. In this paper, a series of aryl isocyanides polymers are prepared using N-heterocyclic carbene-ligated scandium trialkyl complex 1 (IPr)Sc(CH 2 SiMe 3 ) 3 as the catalyst and 2 equiv. [Ph 3 C][B(C 6 F 5 ) 4 ] as the co-catalyst. The catalyst can convert 96% of 100 equiv. monomer in 5 minutes and the activity of the polymerization can reach 329 kg of polymer mol Sc-1 h -1 . Two optically active polymers are obtained by using chiral monomers, they show strong CD signal at 364 nm and the two signals are opposite.


Introduction
In recent years, inspired by the perfect helical structure of biological macromolecules [1] , chemical researchers have devoted themselves to the development of artificial polymers with stable helical structure, so as to achieve the purpose of simulating the structure and function of natural helical polymer and manufacturing new performance materials.
Polyacrylates [2] , polyaryl alkenes [3] , polyisocyanides [4] , polyalkynes [5] , etc., have been extensively studied. Among them, polyisocyanide has attracted continuous attention because of its stable helical rod structure and its wide application. Polyisocyanide has a unique π-conjugated system in which the carbonnitrogen double bond in each unit is twisted along the polymeric backbone to form a helical configuration [6] . Depending on the direction of the helix, isocyanide polymers can be divided into left-handed polymers and right-handed polymers [7] . Isocyanides can spontaneously change into polymers during storage, but this process is very slow [8] , and the composition of the polymer is very complex, so we need to find an efficient catalyst to polymerize them. At present, the main catalytic systems for isocyanide polymerization are metal-free catalytic system, transition metal catalytic system and rare-earth metal catalytic system [9] . Compared with post-transition metal, rare-earth metal has more abundant resources. Its structure has a high coordination number due to the presence of electrons in the f shell, therefore it has the characteristics of strong positivity, chemical reactivity and strong Lewis acidity. What is more, rare-earth metal generally does not undergo oxidation addition or reduction elimination because of their positive trivalent state.
Herein, we synthesized an N-heterocyclic carbeneligated scandium trialkyl complex 1 (IPr)Sc(CH 2 SiMe 3 ) 3 and use it as the catalyst, with [Ph 3 C][B(C 6 F 5 ) 4 ] as the cocatalyst. By using this catalytic system, we successfully polymerized 6 kinds of isocyanide monomers. The polymers obtained have high M n , besides, the M w /M n are lower than the reported rare-earth metal complexcatalyted monomers. Two optically active polymers are obtained by using chiral monomers. The two polymers have strong CD signals at 364 nm, the signals are opposite.  The synthetic route of complex 1 is shown in Figure 1. In glovebox, LiCH 2 SiMe 3 (0.15 g, 1.62 mmol) is dissolved in toluene and put in refrigerator. 3 hours later, take LiCH 2 SiMe 3 out from the refrigerator and react it with 1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-3-ium (0.63 g, 1.47 mmol) for 30 minutes in a 25 mL bottle. After the reaction, filtrate the mixture, add the liquid to Sc(CH 2 SiMe 3 ) 3 (THF) 2 (0.77 g, 1.47 mmol), 30 minutes later, remove the solvent, wash the remaining solid with n-hexane for 5 times, the obtained solid is the product. Yield: 68%. Complex 1 (IPr)Sc(CH 2 SiMe 3 ) 3 was characterized by 1

Synthesis of monomer a to f
All the monomers are synthesized according to literature [10] .

Results and Discussion
The polymerization of aryl isocyanide monomers a-f catalyzed by 1 are shown in Table 1. The experiment result shows that complex 1 has a good polymerization effect on all of the 6 monomers. When the steric hindrance of monomer is large, the efficiency of polymerization is relatively low. This may be because when the substituent steric hindrance is large, the monomer insertion efficiency will be lowered. The polymerization efficiency of monomer with aliphatic side chain is lower than that of monomers with aromatic groups. This may be caused by an electronic effect. Monomer b has the highest polymerization activities. 100 equiv. monomer can be converted 96% in 5 minutes. The activity can reach 3.29 × 10 5 g of polymer mol Sc -1 h -1 . M w /M n are relatively low, which are within 1.53 to 2.65.  The optical activity polymer a and polymer b are characterized by CD spectrum (Figure 3). As is showed in Figure 3, polymer a and polymer b all have a strong signal at 346 nm, and the two peaks are at the opposite directions, means that the two polymers have an opposite handness, which are consistent with the handness of the two chiral monomers.

Conclusion
In summary, we synthesized a rare-earth metal complex and a series of aryl isocyanide monomers with different structures. Using this complex as the catalyst, we catalyzed the polymerization of the 6 isocynaides. The results show that this rare-earth complex can be an efficient catalyst when combined with 2 equiv.
[Ph 3 C][B(C 6 F 5 ) 4 ] as the co-catalyst. The catalytic activity is high and M w /M n is relatively low. Optically active polymers were obtained by using chiral monomers. Therefore, we can exploit more functional isocyanide monomers, and use this efficient catalyst to prepare polymers with various function and wide application.