(522f) Scale-up of the Production of Monofunctional Polysiloxanes

Monofunctional PDMS are interesting building blocks for various inorganic-organic hybrid materials such as block copolymers, core-shell-particles and similar compounds.^{1, 2} However, the production of these materials on an industrial scale is limited.

Main target of this project is the development of a continuously operated reactor system for the production of monohydrido-substituted polysiloxanes with a narrow molecular weight distribution. The catalyst-free ring opening polymerization (ROP) of hexamethylcyclotrisiloxane (D₃) with tert-butyllithium (t-BuLi) is a promising route for this process (Figure 1).

In a first step, the kinetics and thermodynamics of the individual reactions ? initiation, propagation and termination ? were investigated. The ROP is highly selective towards the monomer D₃. This substance is selectively polymerized in the presence of other cyclic oligosiloxanes, such as octamethylcyclotetrasiloxane (D₄) or higher analogs.

Figure 1: Ring opening polymerization (ROP) of hexamethylcyclotrisiloxane (D₃) with tert-butyllithium (t-BuLi) and endcapping with chlorodimethylsilane (CDMS).

In a second step a lab-scale plug flow reactor (PFR) was successfully operated. Priliminary experiments in a stainless steel pipe with static mixers showed a higher degree of polymerization than expected. An interaction of the polymerization reaction with the braze joint at the reactor inlet is assumed.

To solve this problem a PTFE plug flow reactor was used. Because of laminary flow conditions the tubing was coiled on a steel pipe to create radial mixing due to Dean vortices. To provide isothermal polymerization the reactor setup was placed in a temperature controlled water bath. With this experimental setup monofunctional PDMS with desired chain lengths and narrow molecular weight distributions (PDI ≤ 1,2) were accessible with high reproducibility.

¹ P. J. Miller, K. Matyjaszewski, Macromolecules 1999, 32(26), 8760-8767

² J. Pfeiffer 5^th European Silicon Days 22.09.09, Vienna/Austria, Oral Presentation