Dendrimers are highly branched macromolecules with welldefined architectures and surface functionality that are obtained from iterative stepwise procedures. They have been one of the most extensively studied materials during the last decade because of their controllable nanosize,unique topological structure,and versatile physicochemical properties,and were widely used in the fields of catalysis,medicinal chemistry,and nanotechnologies^{[1, 2, 3]}. In recent years,there has been a rapid growth in the discovery and application of new dendrimers. Carbosilane dendrimers are among the most widely used because of their high flexibility, catalytic inertness,and accessibility ^[4]. These dendrimers typically possess low glass transition temperatures and relatively low viscosities. They are kinetically and thermodynamically stable owing to the low polarity of the Si-C bond and its high bond strength (306 kJ/mol),which is very similar to that of C-C bond (345 kJ/mol) ^[5]. In addition,the availability and versatility of many synthetic reactions in organosilicon chemistry enable the facile synthesis of these dendrimers. As a result,the synthesis,characterization,and application of different types of carbosilane dendrimers have attracted wide attention ^{[6, 7, 8]}.

Carbosilane dendrimers are generally synthesized by the divergent method. They are built stepwise from the central core that possesses alkenyl groups through the reiteration of sequential hydrosilylations with chlorosilanes and alkenylations with Grignard reagents,which allow the construction of dendrimers with various generations ^[9]. The synthetic route of carbosilane dendrimers offers high flexibility and versatility as the hydrosilylation and alkenylations reagents can be varied accordingly. Especially,the hydrosilylation reaction,an addition of a hydrosilane unit (Si-H) to a double bond,has been proved to be high yielding and selective and represents a powerful tool for the rapid and efficient synthesis of carbonsilane dendrimers ^[10]. In our previous studies,a series of acetyl,N,N-dimethylaniline,and platinum-capped carbosilane dendrimers were synthesized ^{[11, 12, 13]}. However,the main drawback of these dendrimers is that such functional groups exhibit relative low reactivity,which may limit further functionalization and application of the carbosilane dendrimers. The synthesis of carbosilane dendrimers with reactive functional groups at periphery is still a great challenge.

In the present study,a series of ester-capped carbosilane dendrimers were synthesized via a hybrid divergent-convergent method and their structures were confirmed by FTIR,¹H NMR,and HRMS analyses. The peripheral reactive ester functional groups can be further functionalized using appropriate synthetic procedures to expand the application of carbosilane dendrimers. 2. Experimental Fourier transform infrared spectra (FTIR) were recorded on a Bruker Tensor 27 spectrophotometer (Bruker,Switzerland) in the range of 400-4000 cm^-1 with a resolution of 4 cm^-1 by accumulating 32 scans. The data were treated with an OPUS spectroscopy software of version 6.¹H NMR was measured in chloroform on a Bruker AVANCE-400 NMR Spectrometer,and chemical shifts were recorded in parts per million (ppm) without an internal reference. Molecular weights were determined by an Agilent Technologies 6510 Q-TOF LC-MS (HRMS). Methyl acrylate (MA),ethylenediamine (EDA),allylamine (AA),1,1,3,3-tetramethyldisiloxane (M^HM^H) and tetrahydrofuran (THF) were redistilled just before use. All other reagents used were of analyticalreagent grade. The starting materials carbosilane dendrimers G0, G1,and G2 were prepared according to the method described in Ref. ^[13]. DMAA was prepared by a Michael addition reaction ofMA with AA according to the procedure described in our previous work ^[14]. The synthetic routes of ester-capped carbosilane dendrimers are illustrated in Scheme 1.

Scheme 1

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Scheme 1.The hybrid divergent-convergent synthesis routes of ester-capped carbosilane dendrimers.

MHM-DMAA was prepared according to a similar method described in Ref. ^[14]. A suspension of 2.68 g (0.020 mol) of M^HM^H and three drops of the Karstedt catalyst were added into the flask and the mixture was heated slowly at 70 ℃ under argon atmosphere. Then 1.15 g (0.0050 mol) of DMAA was added drop wise with stirring and the reaction mixture was allowed to stir for 12 h. After the reaction completed,the excess M^HM^H was removed under reduced pressure and 1.70 g (0.0047 mol) of MHM-DMAA was obtained. Yield: 94.10%.

FTIR (KBr,cm^-1): υ 2969,2863 (CH₃,CH₂),2110 (SiH),1738 (COOCH₃),1251 (Si-CH₃),1041 (Si-O-Si); ¹H NMR (CDCl₃): δ-0.07 to 0.02 (m,12H,SiCH₃),0.30-0.35 (t,2H,SiCH₂CH₂CH₂N),1.29 (m,2H,SiCH₂CH₂CH₂N),2.29-2.33 (m,6H,SiCH₂CH₂ CH₂NCH₂),2.62-2.65 (m,4H,NCH₂CH₂CO),3.52 (s,6H,OCH₃),4.52 (s,H,SiH). 2.2. Synthesis of G0-COOCH₃

G0-COOCH₃ was synthesized via the hydrosilylation reaction between G0 and MHM-DMAA similar to the procedures described elsewhere ^[11]. A sample of 0.17 g (0.0010 mol) G0,1.34 g (0.0036 mol) MHM-DMAA,and three drops of the Karstedt catalyst were suspended in 30 mL THF under argon atmosphere. The mixture was stirred at 65 8C for 12 h,then the solvent and remaining MHM-DMAA were removed under reduced pressure, and 0.98 g (0.78 mmol) of G0-COOCH₃ was obtained. Yield: 78.0%.

FTIR (KBr,cm^-1): υ 2952,2873 (CH₃,CH₂),1737 (COOCH₃), 1251 (Si-CH₃),1041 (Si-O-Si); ¹H NMR (CDCl₃): δ 0.00-0.10 (m,39H,SiCH₃),0.38-0.48 (m,12H,SiCH₂CH₂CH₂Si),0.50-0.59 (m,6H,SiCH₂CH₂CH₂N),1.24-1.45 (m,12H,SiCH₂CH₂CH₂Si, SiCH₂CH₂CH₂N),2.36-2.53 (m,18H,SiCH₂CH₂CH₂NCH₂),2.73-2.91(t,12H,NCH₂CH₂CO),3.65 (s,18H,OCH₃); HRMS: m/z 1256.7007 [MH]⁺ (calcd. 1255.6864). 2.3. Synthesis of G1-COOCH₃

Under argon atmosphere,a suspension of 0.24 g (0.0004 mol) G1,1.34 g (0.0036 mol) MHM-DMAA,and three drops of the Karstedt catalyst in 30 mL THF was stirred at 65 ℃ for 16 h. After the reaction is complete,the purification procedures similar to those of G0-COOCH₃ were used to produce 0.39 g (0.143 mmol) G1-COOCH₃. Yield: 72.1%.

FTIR (KBr,cm^-1): υ 2953,2873 (CH₃,CH₂),1738 (COOCH₃), 1251 (Si-CH₃),1040 (Si-O-Si); ¹H NMR (CDCl₃): δ 0.02-0.10 (m,84H,SiCH₃),0.38-0.45 (m,24H,SiCH₂CH₂CH₂Si),0.51-0.59 (m,24H,SiCH₂CH₂CH₂N),1.25-1.35 (m,18H,SiCH₂CH₂CH₂Si), 1.37-1.45 (m,12H,SiCH₂CH₂CH₂N),2.40-2.46 (m,36H, SiCH₂CH₂CH₂NCH₂),2.75-2.79 (t,24H,NCH₂CH₂CO),3.66 (s, 36H,OCH₃); HRMS (FAB): m/z 2724.5228 [MH]⁺ (calcd.2723.5150). 2.4. Synthesis of G2-COOCH₃

A solution of 0.65 g (0.0005 mol) G2,3.63 g (0.01 mol) MHMDMAA, five drops of the Karstedt catalyst in 100 mL THF was stirred under argon atmosphere at 65 ℃ for 24 h. After the reaction completed,the purification procedures similar to that of G0-COOCH₃ were used to produce 1.97 g (0.348 mmol) G2-COOCH₃.Yield: 69.5%.

FTIR (KBr,cm^-1): υ 2953,2873 (CH₃,CH₂),1738 (COOCH₃), 1251 (Si-CH₃),1041 (Si-O-Si); ¹H NMR (CDCl₃): δ -0.03 to 0.08 (m,174H,SiCH₃),0.38-0.47 (m,84H,SiCH₂CH₂CH₂Si),0.51-0.62 (m,24H,SiCH₂CH₂CH₂N),1.24-1.34 (m,42H,SiCH₂CH₂CH₂Si), 1.40-1.43 (m,24H,SiCH₂CH₂CH₂N),2.37-2.46 (m,72H, SiCH₂CH₂CHvNCH₂),2.74-2.79 (t,48H,NCH₂CH₂CO),3.65 (s,72 H,OCH_{3/sub>); HRMS (FAB): m/z 5660.1801 [MH]⁺ (calcd. 5659.1722). 3. Results and discussion}

In general,the syntheses of carbosilane dendrimers with different peripheral functional groups were commonly achieved by the divergent method. Based on this strategy,ester-capped carbosilane dendrimers could also be synthesized according to the procedures described in Scheme 2 (G0-COOCH₃ was chosen as a representative). As shown in Scheme 2,G0-NH₂ can be first synthesized according to the procedures that described in our previous studies ^{[11, 13]},and then the Michael addition of MA to the -NH2 group in G0-NH₂ leads to the formation of G0-COOCH₃ ^[14]. However,the divergent synthesis strategy can be hindered by side reactions that yield incomplete or imperfect dendrimers ^{[13, 15]}. During the hydrosilylation process of ATMS with M^HM^H and G0 with MHM-ATMS,the amino groups should be protected by a trimethylsilyl group to reduce the side reactions between the amino group and the silane. An alcoholysis reaction should be employed to deprotect the silyl group in order to obtain G0-NH₂ ^[11]. Moreover,ATMS must be added in excess to ensure the completion of the reaction with M^HM^H due to its relative low activity. To overcome these drawbacks and the tedious protection- deprotection procedures,a hybrid divergent-convergent method was adopted. As illustrated in Scheme 1,G0,G1,and G2 were first synthesized by the divergent method,then the MHM-DMAA was convergently attached to the allyl group in G0-G2 to give the corresponding G0-COOCH₃-G2-COOCH₃. Using this method, DMAA was obtained by the Michael addition of MA with AA in nearly 100% yield ^[14]. The intermediate ester-capped compound MHM-DMAA was prepared by the hydrosilylation of DMAA with M^HM^H in the presence of the Karstedt catalyst in excellent yield (94.1%). At last,G0-COOCH₃-G2-COOCH₃ can be produced by a hydrosilylation reaction of MHM-DMAA with G0,G1,and G2 in good yields (78.0%,72.1%,and 69.5%,respectively). Therefore,the hybrid divergent-convergent method provides simpler synthetic procedures and higher yields compared to that of the divergent method,and was suitable for the synthesis of ester-capped carbosilane dendrimers.

Scheme 2

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Scheme 2. The divergent synthesis routes of G0-COOCH3.

In order to ensure the hydrosilylation reaction proceeds readily and completely,excess MHM-DMAA was used in the reaction. In addition,the content of C=C groups and the steric hindrance of the dendrimer increase in later generations,and thus the amount of MHM-DMAA,the Karstedt catalyst,and the reaction time need to be increased to complete the reaction. Previous studies showed that allyl derivatives allowed the synthesis of carbosilane dendrimers up to the fifth generation with unified molecular structures ^[16]. Montealegre also successfully synthesized the third generation carboxylate-terminated carbosilane dendrimers possessing 32 terminal groups with a molecular weight of 6586 ^[17]. The structures of G0-COOCH₃-G2-COOCH₃ and the intermediate compounds were confirmed by FTIR,¹H NMR,and HRMS analyses. The FITR spectrum of MHM-DMAA showed the characteristic adsorption bands of υ(Si-H) and υ(COOCH₃) at 2110 and 1738 cm^-1,and the adsorption bands at 1251 and 1041 cm^-1 corresponding to δ(Si-CH₃) and υ(Si-O-Si),respectively. After the hydrosilylation reaction of MHM-DMAA with G0-G2,the vibration peaks of υ(Si-H) at 2110 cm^-1 and υ(C=C) at 1630 cm^-1 disappeared in the FTIR spectrum of G0-COOCH3-G2- COOCH3,and new adsorption peaks at 1041 and 1738 cm^-1 attributed to υ(Si-O-Si) and υ(COOCH₃) appeared,indicating that G0-COOCH₃-G2-COOCH₃ had been successful synthesized. The 1H NMR spectrum of G0-COOCH₃ exhibited resonances for Si-CH₃ protons in the region of 0.00-0.10 ppm and for ester protons (OCH₃) at 3.65 ppm. The resonances appeared in the range of 0.38- 0.48,0.50-0.59,1.24-1.45,2.36-2.53,and 2.73-2.91 ppm can be assigned to the protons of SiCH₂CH₂CH₂Si,SiCH₂CH₂CH₂N, SiCH₂CH₂CH₂Si,SiCH₂CH₂CH₂NCH₂,and NCH₂CH₂CO,respectively. The ¹H NMR spectrum of G1-COOCH₃ and G2-COOCH₃ are almost identical to that of G0-COOCH₃ for analogous protons,although broader resonances are observed with the increased generations. The reason for this can be ascribed to the slightly different chemical environments for the protons in different generations and the restricted mobility of the respective protons in the outer shells ^[14]. In the ¹H NMR spectrum of G0-COOCH₃-G2-COOCH₃, the ratio of ester end groups (OCH₃) to CH₃Si groups was consistent with the expected value based on the formula. Also,the complete disappearance of C=C group resonances in the spectrum of G0-G2 further confirmed the formation of unified molecular dendrimers of G0-COOCH₃-G2-COOCH₃. Furthermore,the molecular weights of G0-COOCH₃-G2-COOCH₃ were determined by HRMS,which were in good agreement with the theoretical calculation. This unambiguously confirmed the formation of G0-COOCH₃-G2- COOCH₃,further demonstrating that the desired ester-capped carbosilane dendrimers had been successfully synthesized. 4. Conclusion

In conclusion,a series of novel ester-capped carbosilane dendrimers (G0-COOCH₃-G2-COOCH₃) were successfully synthesized via a hybrid divergent-convergent method in good yield. All the targeted dendrimers were confirmed by FTIR,¹H NMR,and HRMS analyses. The synthesized dendrimers possess reactive ester functional groups,which can be further functionalized using appropriate synthetic procedures to expand the application of carbosilane dendrimers. Acknowledgments

The authors are grateful for the financial support by the National Natural Science Foundation of China (No. 21307053), China Postdoctoral Science Foundation Funded Project (No. 2013M541911),Promotive Research Fund for Excellent Young and Middle-Aged Scientists of Shandong Province (No. BS2013CL044),and Natural Science Foundation of Ludong University (No. LY2011004).