Separation and identification of oligomeric vinylmethoxysiloxanes by gradient elution chromatography coupled with electrospray ionization mass spectrometry

Journal of Chromatography A, 1395 (2015) 129–135

Contents lists available at ScienceDirect

Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Separation and identification of oligomeric vinylmethoxysiloxanes by gradient elution chromatography coupled with electrospray ionization mass spectrometry Guiying Jia, Qian-Hong Wan ∗ School of Pharmaceutical Science & Technology, Tianjin University, 92 Weijin Road, Tianjin 900032, China

a r t i c l e

i n f o

Article history: Received 14 November 2014 Received in revised form 26 December 2014 Accepted 29 March 2015 Available online 4 April 2015 Keywords: Vinylmethoxysiloxane oligomers Silsesquioxanes Hydrolytic condensation Hybrid materials Liquid chromatography–mass spectrometry

a b s t r a c t A high-performance liquid chromatography with online electrospray ionization mass spectrometry (HPLC–ESI-MS) has been used to separate and identify the reaction products resulting from controlled acid-catalyzed hydrolytic polycondensation of vinyltrimethoxysilane (VMS). The reaction products were prepared in the molar ratio of water to VMS (r1 ) ranging from 0.6 to 1.2, characterized by standard spectroscopic techniques, and subsequently analyzed by HPLC–UV absorbance detection and HPLC–ESI-MS. Linear vinylmethoxysiloxane oligomers with the number of repeat units (n) ranging from 3 to 11 are predominant species at the beginning of the reaction (for r1 = 0.6). Then they transform into monocyclic (for r1 = 1.0) and bicyclic (for r1 = 1.2) species with gradually increasing amount of water in the reaction mixture. The oligomer conversions suggest that structure growth of vinylmetoxysiloxanes proceeds by nonrandom cyclization reactions, which are favored over chain extension under the chosen reaction conditions. Direct ESI-MS, HPLC–ESI-MS and HPLC–UV were used to determine the molar mass distributions for the vinylmethoxysiloxane oligomers prepared in three different values of r1 . The molar mass averages increase slightly with the amount of water in the reaction mixture and vary somewhat with the method used. Our results indicate that with the combined capability of separation, sensitivity and identification, HPLC–ESI-MS is especially useful to study highly complex silicon-based compounds with hyperbranched, caged or cubic structures as building blocks for hybrid materials. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Hybrid materials with organic–inorganic character are advanced functional materials which represent not only a new field of basic research but offer prospects for many applications in diverse fields [1,2]. Organic–inorganic hybrid materials can be defined as nanocomposites with covalently bonded organic and inorganic components. Typically, they are obtained through hydrolysis of organically modified silicon alkoxides condensed with or without simple metallic alkoxides. Polycondensation results in a variety of structures ranging from monodisperse silica particles to polymer networks depending on the reaction conditions used. In addition, copolymerization of alkoxysilanes of variable functionality makes it possible to tailor the structure, morphology, and properties. Tetrafunctional alkoxysilanes form densely crosslinked silica structures, trifunctional monomers polymerize to branched or cagelike silsesquioxanes and

∗ Corresponding author. Tel.: +86 22 2740 3650; fax: +86 22 2740 3650. E-mail address: [email protected] (Q.-H. Wan). http://dx.doi.org/10.1016/j.chroma.2015.03.079 0021-9673/© 2015 Elsevier B.V. All rights reserved.

bifunctional alkoxysilanes yield linear polysiloxane chains increasing the elasticity of a material [3]. Hybrid materials can be classified based on the possible interactions connecting the inorganic and organic species [1]. Class I hybrid materials are those that show weak interactions between the two phases, such as van der Waals, hydrogen bonding or weak electrostatic interactions. One example for such a material is the combination of inorganic clusters or particles with organic polymers lacking a strong (e.g. covalent) interaction between the components. Class II hybrids are formed when the discrete inorganic building blocks are covalently bonded to the organic polymers. Today the main hybrid materials that find industrial applications are based mostly on class II hybrids for their improved homogeneity and durability. Organofunctional trialkoxysilane monomers of higher compatibility with organic polymers are commonly used to prepare hybrid polymers, especially as coupling agents in coating materials [3,4] and glass fillers in dental restoratives [5]. Vinyltrimethoxysilane (VMS) is an organofunctional alkoxysilane monomer that can undergo both the sol–gel polymerization of the alkoxy groups and curing of the vinyl functionality to

130

G. Jia, Q.-H. Wan / J. Chromatogr. A 1395 (2015) 129–135

form a hybrid network with covalent bonds between organic and inorganic phases. This property of VMS has been exploited for preparation of flexible thin films with variable tensile strengths [6–8] and hybrid monolithic capillary columns for applications in separation science [9–11]. Condensation products of acid-catalyzed hydrolysis of VMS with various molar equivalents of water in the reaction mixture have been characterized by size exclusion chromatography (SEC), infrared (IR), 29 Si nuclear magnetic resonance spectroscopy (29 Si NMR) [6–8]. A recent report by Zhang et al. describes the use of matrix-assisted laser desorption/ionization mass spectrometry (MALD–MS) to identify complex oligomeric species present in the hydrolytic reaction [12]. They confirmed the presence of polyhedral cagelike structures and incompletely hydrolyzed/condensed ladder structures, containing 5–20 silicon atoms. However, since single stage mass spectrometry does not provide any information about the specific arrangement of atoms, it is impossible to distinguish structural or geometric isomers which are known to be abundant based on experimental and theoretical investigations [4,13]. Tandem mass spectrometry experiments or chromatographic separation methods prior to soft ionization mass spectrometry analysis is generally required to resolve isomers for better control of the hydrolytic condensation reaction and understanding of the chemical pathways for cage or ladder formation. Up to date there is no published report on the use of liquid chromatography other than SEC for separation of the oligometic reaction products from hydrolytic condensation of trialkoxysilanes. Molar mass averages and molar mass distributions (MMDs) are important parameters for characterizing oligomeric/polymeric precursors as they provide the basics for chemical and physical properties as well as the mechanism of polymerization. These parameters are conventionally determined by SEC based on polystyrene standards for molar mass calibration. The last decade, however, has witnessed an increased use of the MS-based methods for oligomer/polymer characterization, primarily because MS provides much greater resolution than SEC and does not rely upon polystyrene standards [14–16]. Nevertheless, their potential in characterizing silsesquioxane precursors has yet to be demonstrated. In this work, we describe a high-performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC–ESI-MS) method for the separation and identification of vinylmethoxysiloxane oligomers as intermediates to polyhedral silsesquioxanes. We start with the preparation of the oligomers of different morphologies by controlled acid-catalyzed hydrolytic condensation of vinyltrimethoxysilane and their characterization by standard spectroscopic techniques. Then we proceed with the HPLC method development for the separation of oligomers and their online MS identification. Finally, we compare the molar mass averages and distributions determined by direct probe ESI-MS, HPLC–ESI-MS and HPLC–UV absorbance to demonstrate the relative performance of these methods.

mixture of VMS (12.7 g, 0.085 mol) and methanol (7 mL) was added to a three-necked round bottom flask equipped with a mechanical stirrer, a nitrogen inlet and an outlet. This mixture was cooled in an ice bath for 10 min. The water and hydrochloric acid were mixed in the molar ratio of H2 O to VMS, r1 , in the range of 0.60–1.2 and the molar ratio of HCl to VMS, r2 , fixed at 0.035 and added dropwise over a period of 5 min to the methanol solution of VMS under vigorously stirring. Upon completion, the final mixture was brought to room temperature and stirring was continued for 10 min. A regulated nitrogen gas flow was supplied and the mixture was heated at 70 ◦ C for 3 h with continued stirring at the rate of 150 rpm. Viscous liquids weighed 7.5, 7.1, and 6.5 g were obtained for r1 = 0.6, 1.0 and 1.2, respectively. 2.3. Structural characterization The infrared (IR) spectra of vinylmethoxysiloxanes were acquired on a Tensor 27 FTIR spectrophotometer (Bruker Optics, Ettlingen, Germany) using KBr pellets. 29 Si NMR spectra were acquired on a Varian InfinityPlus-300 MHz spectrometer equipped with a 7.5 mm double-resonance MAS probe operating at 59.56 MHz with a 90◦ pulse width (Palo Alto, CA, USA). The neat liquids of vinylmethoxysiloxanes were transferred into PE tubes for spectral measurements. Direct ESI mass spectra were acquired on an Agilent 6310 ion trap mass spectrometer equipped with an electrospray ionization interface (Waldbronn, Germany) operated in the positive ionization mode. The ionization parameters were set as follows: dry temperature, 350 ◦ C; nebulizer gas, 30 psi; dry gas flow, 8 L/min; HV capillary voltage, 3.85 kV. The capillary exit block and skimmer voltages were set at 142 and 40 V, respectively. The ESI-MS instrument was controlled by Agilent ChemStation and the signals were acquired and processed by Bruker Compass DataAnalysis 4.0. The mass spectra were recorded in the m/z range of 200–2200. A 10-␮L aliquot of the methanol solutions of vinylmethoxysiloxanes at 100 mg/ml was introduced directly into the ionization source at 0.4 mL/min with flow splitting ratio at 1:2. 2.4. Chromatographic separation An Agilent 1200 Series HPLC system consisting of a degasser, a quaternary pump, column oven, autosampler, UV–visible wavelength absorbance detector, and ESI-ion trap mass spectrometer was used throughout. Separations of the vinylmethoxysiloxane oligomers were carried out on a BaseLine C18 column (250 mm × 4.6 mm I.D., 5 ␮m particle size, 100 A˚ pore size) at 30 ◦ C (BaseLine Chromtech Research Center, Tianjin, China). A binary solvent system consisting of water (solvent A) and methanol (solvent B) was used in linear gradient mode from 80% to 100% B over 40 min and then held at 100% B for 10 min. The flow rate was 1 mL/min. The UV wavelength was set at 215 nm. An injection volume of 10-␮L was used for HPLC analysis. The online ESI-MS conditions were the same as for direct ESI-MS experiments.

2. Experimental 2.1. Chemicals Methanol (HPLC grade) and hydrochloric acid (Analytical grade, 37 wt% HCl) were purchased from Concord Chemical Reagents (Tianjin, China). Vinyltrimethoxysilane (VMS) was obtained from WD Silicone (Wuhan, Hubei, China).

2.5. Data analysis The number- and weight-average molar masses, Mn , Mw , of vinylmethoxysiloxanes synthesized under various conditions and their corresponding degrees of polymerization, nn , nw , are calculated by the following formulas: Mn =

 c  i

(1)

Mw =

 (c M ) i i

(2)

2.2. Preparation of oligomeric vinylmethoxysiloxanes The vinylmethoxysiloxanes were synthesized by following the literature procedure with minor modification [7]. Briefly, a

(ci /Mi )

ci

G. Jia, Q.-H. Wan / J. Chromatogr. A 1395 (2015) 129–135

131

Fig. 1. Controlled acid-catalyzed hydrolysis and condensation of vinylmethoxysilane with water to produce vinylmethoxysiloxane oligomers where n represents, respectively, the number of repeat units in the linear species and the sum of the silicon atoms x, y and z in the cyclic species.

nn =

Mn Mo

(3)

nw =

Mw Mo

(4)

Fig. 2 shows the IR spectra of hydrolytic condensation products of VMS obtained at three different values of r1 . The absorption bands at 3022–2844 cm−1 assigned to methoxy groups decrease as the molar ratio r1 increases, indicating a gradual loss of the methoxy

where i is the increment index; ci refers to the weight concentration of oligomer/polymer molecules with molar mass Mi ; and Mo is the formula weight of the repeating unit in the macromolecules. 3. Results and discussion 3.1. Structural characterization The controlled acid-catalyzed hydrolytic condensation of vinyltrimethoxysilane (VMS) proceeds as illustrated in Fig. 1, resulting in soluble products comprising various amounts of linear/branched (A-series), monocyclic (B-series) and bicyclic (Cseries) oligomeric vinylmethoxysiloxanes depending upon the reaction conditions employed. The hydrolysis of the terminal or pendant methoxy groups yields oligomers with silanol groups corresponding to D-, E- and F-series. The key processes to produce soluble vinylmethoxysiloxanes without gelation lay in the tight control of the molar ratio of H2 O to VMS (r1 ) in the reaction mixture. In this work, the value of r1 was varied from 0.6 to 1.2 while r1 = 1.5 corresponds to stoichiometric conditions for the total reaction of hydrolysis and condensation. The sol–gel process was catalyzed with hydrochloric acid the molar ratio of which to VMS was kept unchanged. The viscous liquids obtained were characterized by IR, 29 Si NMR and direct ESI-MS.

Fig. 2. IR spectra of oligomeric vinylmethoxysiloxanes prepared at various molar ratios of water to vinylmethoxysilane, r1 .

132

G. Jia, Q.-H. Wan / J. Chromatogr. A 1395 (2015) 129–135

Fig. 3. 29 Si NMR spectra of oligomeric vinylmethoxysiloxanes prepared at various molar ratios of water to vinylmethoxysilane, r1 .

r1 = 0.6, the T1 and T2 fractions account for a total of 97%, indicating that linear species with little branching predominate in the reaction products. At r1 = 1.0, the T2 and T3 fractions increase considerably accompanying by down-field shifts of the signals. All these suggest the formation of branching or cyclic species. At r1 = 1.2, a further increase in T3 fraction from 9% to 24% is observed, suggesting an increase in the fractions of the branching or cyclic species with the increasing amount of water in the reaction mixture. Fig. 5 shows the direct ESI mass spectra of vinylmethoxysiloxanes prepared at various values of r1 . The molecular ion peaks observed for each oligomer appear as doublets due to the formation of singly charged ammonium- and sodium-adducts. They can be assigned to the oligomers of the molecular weights calculated by the generic formulas presented in Fig. 1, with appropriate modifications for ammonium- and sodium-adducts. For r1 = 0.6, linear species (A-series) appear to be predominant with n ranging from 3 to 9. The nominal mass separation between two major peaks is 102 Da, which is equal to the formula weight of the repeat unit [RCH3 OSiO], where R is vinyl group. The loss of 14 Da from the corresponding A-series oligomers indicates the formation of demethoxylated oligomers or D-series. For r1 = 1.0, monocyclic (Bseries) oligomers with n ranging from 3 to 10 are the major species in addition to the A-series. The hydrolysis of methoxy groups is evident by the formation of D- and E-series of oligomers. For r1 = 1.2, bicyclic (C-series) oligomers with n ranging from 3 to 9 are identified as predominant species together with the B-series whereas the A-series are virtually undetectable. With increasing amount of water in the reaction mixture, the cyclic species with more ring structures are produced at the expense of the linear species while the number of repeat units remains essentially unchanged. This confirms previous reports that the formation of small rings (threeto six-membered rings) is kinetically favored over chain extension in conditions typical of acid catalyzed hydrolysis and condensation of alkoxysilanes [18].

3.2. Oligomeric separation and identification

Fig. 4. Fractions of T1 , T2 , and T3 as a function of the molar ratio of water to vinylmethoxysilane, r1 .

groups due to hydrolysis. The absorption band at 1076 cm−1 assigned to siloxane groups (Si–O–Si) indicates the formation of vinylmethoxysilone oligomers [17]. The broad band at 3418 cm−1 observed for r1 ≥ 1.0 is ascribed to hydrogen-bonded silanol groups, which is essentially absent for r1 = 0.6. The concentration of the silanol groups increases with the amount of water in the reaction mixture. The 29 Si NMR spectra of these samples show three groups of peaks (Fig. 3), which are labeled as T1 (−64 ppm), T2 (−69 to −72 ppm), and T3 (−79 to −82 ppm) [6,7]. These peaks represent silicon atoms containing one, two, and three siloxane (Si–O–Si) bonds formed by condensation of VMS, corresponding to a terminal, linear and branching/cyclic structure, respectively. The integrated peak areas can be used to demonstrate the structural evolution in the hydrolysis and condensation of VMS. As shown in Fig. 4, with increasing r1 or the amount of water in the reaction mixture, the T2 and T3 fractions increase at the expense of the T1 fraction. At

A separation procedure for vinylmethoxysiloxanes was developed on the basis of gradient elution reversed-phase liquid chromatography. Using a binary gradient consisting of methanol and water, a multitude of oligomers were separated on a C18 column and detected by UV absorbance detection. Fig. 6 shows chromatograms for the separation of vinylmethoxysiloxanes obtained at various r1 values. For r1 = 0.6, about twelve main peaks are detected and peak areas decrease with the retention time. Similar trends are observed for oligomers prepared at the r1 = 1.0. For r1 = 1.2, however, the earlier eluted peaks become smaller and the predominant species are shifted towards those with intermediate retention times. HPLC with online ESI-MS detection was employed for the identification of the oligomers eluting from the column. Fig. 7 shows the total-ion chromatograms (TIC) of the three samples of vinylmethoxysiloxanes. Despite the noisy appearance of the TIC traces, individual mass spectra obtained are generally of high quality. The extracted ion chromatograms (EIC) at the m/z values corresponding to each linear, monocyclic and bicyclic species (see Figs. S1–S3 in Supplementary data) allow the peaks in UV traces to be identified. For r1 = 0.6, the vinylmethoxysiloxanes comprised of linear species with repeat units up to 11 are detected. For r1 = 1.0, the monocyclic oligomers appear and coexist with the linear species. For r1 = 1.2, the bicyclic oligomers become the dominant species. These results are consistent with those observed by direct ESI-MS, except that the isomeric compounds which cannot be distinguished in direct ESI-MS can now be partially or completely resolved.

G. Jia, Q.-H. Wan / J. Chromatogr. A 1395 (2015) 129–135

133

Fig. 5. Direct ESI-mass spectra of oligomeric vinylmethoxysiloxanes prepared at various molar ratios of water to vinylmethoxysilane, r1 .

3.3. Molar mass averages and distributions To demonstrate the feasibility of using MS-based methods to characterize the precursors of silsesquioxanes, direct ESI-MS and HPLC–ESI-MS were used to determine the MMD parameters of oligo(vinylmethoxysiloxane)s and the resulting data were compared with those obtained by HPLC–UV as shown in Table 1. For calculating these parameters, peak areas are taken as measures of the weight concentrations of the respective oligomers except for

HPLC–UV where the peak heights are used because the oligomer peaks are not baseline resolved in the latter case. The average molecular masses, Mw and Mn , increase slightly with r1 over the range of 0.6–1.2, consistent with the trend determined by SEC [6]. The MMD parameters show some variations with the measuring method used, probably because of the different detection mechanisms involved. Since the oligomers are readily ionized under ESI conditions, they can be detected by the MS methods with high sensitivity. In contrast, the UV detection does not provide a

Fig. 6. HPLC separations of oligomeric vinylmethoxysiloxanes prepared at various molar ratios of water to vinylmethoxysilane, r1 . Conditions: column, BaseLine C18 (250 mm × 4.6 mm I.D., 5 ␮m particles); column temperature, 30 ◦ C; mobile phase, water (solvent A) and methanol (solvent B) gradient elution from 80% to 100% B over 40 min and held at 100% B for 10 min; UV detection, 215 nm.

134

G. Jia, Q.-H. Wan / J. Chromatogr. A 1395 (2015) 129–135

Fig. 7. Total ion chromatograms of oligomeric vinylmethoxysiloxanes prepared at various molar ratios of water to vinylmethoxysilane, r1 .

linear response to the mole fraction of the oligomer as the molar absorptivity or extinction coefficient varies nonlinearly with the degree of polymerization in the oligomer region [19]. Considering that mass discrimination effect has not been reported for the compounds of this type, it is reasonable to assume that HPLC–ESI-MS is more suitable to quantitatively determine MMDs of oligomers because it combines high separation power of HPLC with high sensitivity and selectivity of MS. To obtain accurate MMDs, the response factors of each oligomers must be determined and calibration methods be used for data generation. Unfortunately, purified single vinylmethoxysiloxane oligomers to be used as standards are

Table 1 Molar mass distribution parameters for oligomeric vinylmethoxysiloxanes determined by direct ESI-MS, HPLC–ESI-MS, and HPLC–UV methods. Analytical method

r1

Mw

Mn

Mw /Mn

nw

nn

Direct ESI-MS

0.6 1.0 1.2

555.8 586.1 594.2

511.4 538.4 557.5

1.09 1.09 1.07

5.45 5.75 5.83

5.01 5.28 5.47

HPLC–ESI-MS

0.6 1.0 1.2

663.2 664.6 672.6

600.2 610.6 628.7

1.11 1.09 1.07

6.50 6.52 6.60

5.88 5.99 6.16

HPLC–UV

0.6 1.0 1.2

454.1 521.9 542.2

420.4 469.1 476.3

1.08 1.11 1.14

4.40 5.12 5.31

4.12 4.60 4.70

not available for this purpose. Work is underway in our labs to optimize and scale up the HPLC process for preparation of purified compounds.

4. Conclusions Separations of vinylmethoxysiloxane oligomers prepared in various molar ratios of water to vinyltrimethoxysilane (r1 ) have been achieved on a C18 reversed-phase stationary phase using gradient elution with a binary mobile phase of methanol and water. The resolved oligomer species are identified by online ESI-MS with their respective molecular structures. Under the chosen reaction conditions, the structure growth of the oligomers proceeds from linear to monocyclic and to bicyclic oligomers with increasing amount of water in the reaction mixture. To our knowledge, this is the first time that HPLC has been used to resolve vinylmethoxysiloxane oligomers and to follow the hydrolytic condensation reactions. This work provide evidence in support of nonrandom cyclization theory proposed by McCormick et al. that favors ring closure over chain extension [18]. Being able to resolve and detect isomers with high sensitivity, the HPLC–ESI-MS approach to oligomer analysis may be extended to investigations of hydrolytic condensations of a variety of alkoxysilanes involved in the preparation of hybrid materials.

G. Jia, Q.-H. Wan / J. Chromatogr. A 1395 (2015) 129–135

Acknowledgements The authors thank Mr. Kongying Zhu and Dr. Xinghua Jin from Tianjin University for their technical assistance with the 29 Si NMR and LC–ESI-MS measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.chroma.2015.03.079. References [1] C. Sanchez, P. Belleville, M. Popall, L. Nicole, Applications of advanced hybrid organic–inorganic nanomaterials: from laboratory to market, Chem. Soc. Rev. 40 (2011) 696–753. [2] G.L. Drisko, C. Sanchez, Hybridization in materials science—evolution, current state, and future aspirations, Eur. J. Inorg. Chem. 32 (2012) 5097–5105. [3] L. Matejka, O. Dukh, J. Brus, W.J. Simonsick Jr., B. Meissner, Cage-like structure formation during sol–gel polymerization of glycidyloxypropyltrimethoxiysilane, J. Non-Cryst. Solids 270 (2000) 34–47. [4] Y. Abe, T. Gunji, Oligo- and polysiloxanes, Prog. Polym. Sci. 29 (2004) 149–182. [5] M. Farahani, W.E. Wallace, J.M. Antonucci, C.M. Guttman, Analysis by mass spectrometry of the hydrolysis/condensation reaction of a trialkoxysilane in various dental monomer solutions, J. Appl. Polym. Sci. 99 (2006) 1842–1847. [6] Y. Abe, K. Taguchi, H. Hatano, T. Gunji, Y. Nagao, T. Misono, Hydrolytic polycondensation of vinyltrimethoxysilane and formation of vinylpolysiloxane films, J. Sol-Gel Sci. Technol. 2 (1994) 131–134. [7] N. Takamura, T. Gunji, H. Hatano, Y. Abe, Preparation and properties of polysilsesquioxanes: polysilsesquioxanes and flexible thin films by acid-catalyzed controlled hydrolytic polycondensation of methyl- and vinyltrimethoxysilane, J. Polym. Sci. Part A: Polym. Chem. 87 (1999) 1017–1026.

135

[8] Y. Abe, K. Kagayama, N. Takamura, T. Gunji, T. Yoshihara, N. Takahashi, Preparation and properties of polysilsesquioxanes. Function and characterization of coating agents and films, J. Non-Cryst. Solids 261 (2000) 39–45. [9] M.H. Wu, R.A. Wu, F.J. Wang, L.B. Ren, J. Dong, Z. Liu, H.F. Zou, One-pot process for fabrication of organic-silica hybrid monolithic capillary columns using organic monomer and alkoxysilane, Anal. Chem. 81 (2009) 3529– 3536. [10] J. Ma, G. Yang, C. Yan, Y. Gu, L. Bai, Y. Duan, J. Li, The preparation of a novel organic–inorganic hybrid monolithic column with sonication-assist and its application, Anal. Methods 4 (2012) 247–253. [11] J. Ou, H. Lin, Z. Zhang, G. Huang, J. Dong, H. Zou, Recent advances in preparation and application of hybrid organic-silica monolithic capillary columns, Electrophoresis 34 (2013) 126–140. [12] X. Zhang, L. Hu, Y. Huang, D. Sun, Y. Sun, Three-dimensional configurations of organic/inorganic hybrid nanostructural blocks (I). A quantum mechanical investigation for ladder-like structure of vinylsilsesquioxane, Sci. China Ser. B: Chem. 47 (2004) 388–395. [13] R. Baney, M. Itoh, A. Sakakibara, T. Suzuki, Silsesquioxanes, Chem. Rev. 85 (1995) 1409–1430. [14] R. Saf, C. Mirtl, K. Hummel, Electrospray ionization mass spectrometry as an analytical tool for non-biological monomers, oligomers and polymers, Acta Polym. 48 (1997) 513–526. [15] D.J. Aaserud, L. Prokai, W.J. Simonsick Jr., Gel permeation chromatography coupled to fourier transform mass spectrometry for polymer characterization, Anal. Chem. 71 (1999) 4793–4799. [16] G. Montaudo, F. Samperi, M.S. Montaudo, Characterization of synthetic polymers by MALDI-MS, Prog. Polym. Sci. 31 (2006) 277–357. [17] T.R. Crompton, Analysis of organosilicon compounds, in: S. Patai, Z. Rappoport (Eds.), Organic Silicon Compounds, vols. 1–2, John Wiley & Sons, Chichester, UK, 1989 (Chapter 6). [18] L.V. Ng, P. Thompson, J. Sanchez, C.W. Macosko, A.V. McCormick, Formation of cagelike intermediates from nonrandom cyclization during acid-catalyzed sol–gel polymerization of tetraethyl orthosilicate, Macromolecules 28 (1995) 6471–6476. [19] A.A. Grinev, S.D. Ittel, M. Fryd, A caveat when determining molecular weight distributions of methacrylate oligomers, J. Polym. Sci. Part A: Polym. Chem. 33 (1995) 1185–1188.