Microstructure of the Copolymer Chain Generated by Anionic Ring-Opening Polymerization of a Model Cyclotrisiloxane with Mixed Siloxane Units

Abstract

Sequencing of siloxane units was studied in the copolymer chain formed by anionic polymerization of 2,2,4,4-tetramethyl-6,6-diphenylcyclotrisiloxane. The study involved three polymerization systems: lithium silanolate in THF, potassium silanolate complexed with 18-crown-6 in toluene, and tetramethylammonium silanolate in toluene. The sequencing proved to be a very good diagnostic tool for the studies of polymerization chemoselectivity. In all the three mentioned systems the chemoselectivity was high. In particular, processes leading to chain cleavage, such as backbiting and chain fragment interchange, occurred at a much lower rate than propagation, which allowed for quantitative studies of regioselectivity in polymerization. The pentad analysis, performed using the first-order Markov statistics, completed by the determination of the unit sequence at chain extremities, permitted the determination of the contributions from the monomer ring openings at three nonequivalent sites. The regioselectivity is poor as the ring is opened in each of these three places with a significant probability. The results provide important information on the mechanism of the chain propagation. They point to the role of the counterion assistance to the monomer ring opening. The generation of a more stable silanolate propagation center may also be important for choosing the site of the ring-opening. Ab initio calculations suggest the direct interaction of the phenyl group with the metal cation.

Controlled Synthesis of Siloxane Polymers and Siloxane-Siloxane Block Copolymers with 3-Chloropropyl Groups Pendant to the Siloxane Chain

“…The ordered arrangement of units, where chloropropyl appears at every third silicon, would give rise to one Me 2 SiO centered pentad DXDDX and only one chloropropylsiloxane centered pentad DDXDD. [25,26] The spectrum presented in Figure 4 shows a significant abundance of other pentads which indicates that the regioselectivity of propagation is poor.…”

Synthesis of gradient copolysiloxanes by simultaneous copolymerization of cyclotrisiloxanes and mechanism for kinetics inverse between anionic and cationic ring‐opening polymerization

“…Polysiloxanes are important inorganic/organic materials possessing unique set of properties that make them useful in numerous applications . Siloxane chains present in polysiloxanes exhibit unusual static flexibility which in combination with low intermolecular forces leads to exceptionally low glass transition temperatures ( T g ) and small viscosity changes with temperature of polysiloxanes .…”

“…Incorporation of polysiloxane segments into common organic polymers or different types of polysiloxane segments lead to the fabrication of the copolymers . Up until now, the copolymers with different siloxane units in the main chain have always attracted significant attention because different siloxane units in polysiloxanes are often necessary to obtain materials with tunable microstructures, physical properties, and required performances . For instance, incorporation of fluorinated polysiloxane blocks into conventional vinyl polysiloxanes results in the formation of the block copolymers with thermal resistance, superior damping properties, and fuel resistance.…”

Synthesis, characterization, and properties of vinyl‐terminated copolysiloxanes containing trifluoropropyl and 4‐trifluoromethylphenyl groups

“…9,[12][13][14] Currently, the copolymers with different siloxane units in main chain have attracted significant attention because different siloxane units in polysiloxanes are often necessary to obtain materials with tunable microstructures, physical properties, and required performances. 12,[14][15][16][17][18][19][20][21][22][23][24] For example, the thermal stability and oil resistance of conventional silicone rubber have been improved by the partial substitution of some methyl groups with phenyl and trifluoropropyl groups, 16,18,[25][26][27] respectively. Moreover, the composition and sequences distribution of monomer units, namely, block, gradient, and random, significantly affect the performance of these copolymers.…”

“…Moreover, the composition and sequences distribution of monomer units, namely, block, gradient, and random, significantly affect the performance of these copolymers. 16,18,19,21,28 However, the comonomers exhibit unusually large reactivity difference toward polymerization; 16,18,23,29 therefore, generation of these copolysiloxanes with well-defined structures, especially for random sequence distribution, is extremely difficult to achieve. 18,23 The monomer, 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane (F3) exhibits higher reactivity than hexamethylcyclotrisiloxane (D3) because of the increased electronegativity of the trifluoropropyl substituent and the enhanced negative induction effect on the electropositive ring silicon.…”