Bicationic initiator

Figure 3.15 TGA traces of pure clay, clay intercalated with bicationic initiator, and pure bicationic initiator. Reproduced from [62] with permission.

Fan, X. Xia, C. Advincula, R.C. Grafting of pol3mers from clay nanoparticles via in-situ free radical surface-initiated polymerization monocationic versus bicationic initiators. Langmuir 2003, 19, 4381-4389. 

Representation of the polymerization from the surface approach with bicationic initiator bound to the clay surface. It should be noted that the bicationic initiator may be attached either to one platelet or to two platelets. (Reproduced from Mittal, V., in Encyclopedia of Polymer Composites, ed. M. Lechkov and S. Prandzheva, Nova Science Publishers, New York, 2009, pp. 76-99. With... 

On further analysis, the d spacing values seemed to be inconsistent with the steric sizes of the two initiators. The d spacing of bicationic intercalated clay (1.52 nm) is substantially smaller flian diat of the monocationic intercalated clay although their molecular dimensions are comparable (both chain length values are 2.20 nm, as estimated by Chem 3D software). The bicationic initiator molecule possesses charged groups on bofti ends that can have two intercalation possibilities ... 

This observed result is quite unexpected. We would anticipate diat these adjacent clay layers in the clay/bicationic initiator system will be gradually... 

By comparison, an intercalated monocationic initiator is more easily delaminated than a bicationic initiator. The monocationic initiator molecule is also more organophilic. The weaker van der Waals interaction between the alkyl headgroups of the monocationic initiator and clay surfaces makes the intercalated clay more easily swelled by the solvent and monomer. Once the clay intercalated with monocationic initiator is exfoliated by sonicating and stirring, the attached initiators have more accessibility to monomer and thus results in better monomer intercalative diffusion. 

The synthetic schemes and structures of these initiators are shown in Figs 5.7(a) and 5.7(b). They are both AIBN-analogue initiators for free radieal polymerization. The use of another symmetric bicationic azo compound, 2,2 -azobis(isobutyramidine hydrochloride) (AIBN), has also been proven to be feasible for styrene SIP on high surface area mica powder. " However, no structural information for these SIP products has been reported. Asymmetric azo initiators in the form of silanes have also been successfully employed to free radically polymerize styrene from spherical silica gel surfaces. To the best of our knowledge, there have been no reports on a direct free radical SIP approach from surface-bound monocationic azo initiators on individual clay nanoparticles. 

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