Anionic polymerization of 2,2,4,4‐tetramethyl‐6,6‐diphenylcyclotrisiloxane a model siloxane monomer of heterogeneous composition of a reactive grouping

Abstract

The anionic polymerization of 2,2,4,4-tetramethyl-6,6-diphenylcyclotrisiloxane (l), its copolymerization with hexamethylcyclotrisiloxane (2), and the homopolymerization of 2 were studied. It was found that the polymerization of 1 initiated with potassium trimethylsilanolate proceeds with the formation of 2,2,4,4,6,6-hexamethyl-8,8-diphenylcyclotetrailoxane (3). When using lithium butyldimethylsilanolate/HMPT as initiator an inversion of the reactivity order in the homopolymerization of 2 and 1 was observed. The results indicate that both propagation and depropagation occur with a high degree of regioselectivity, which is dependent on the applied catalyst.

Selectivity of siloxane‐siloxane copolymer synthesis by ring opening polymerization

“…We have studied the polymerization of 1 using various types of initiator and solvent Results are presented in Table 2 The "Si NMR spectrum of the copolymer synthesized using the common initiator-solvent system, lithium silanolate in THF, is shown as an example in Fig. 2 The spectrum does not contain any pentad forbidden for the cheinoselective process which exclude the occurrence of any polymer cleavage reactions However, apart from pentads DDXDD and DXDDX characteristic for the alternate structure of the copolymer, there are considerable amounts of XDXDD and four D centred pentads forbidden for the regioselective reaction It is worth to notice that both observed X-centred pentads belong to the same triad DXDcharacteristic for the alternate copolymer Thus, on the bases of the observation of X-centlred triads alone this process could be erroneously assurned to be regioselective I, I -d~plienyltetramethylcyclotrisiloxane was found to be inore reactive inonninei-in the anionic copolymei-ization than its f~illy substituted analogue (Ref 19), which pointed to the preferential attack at the Ph,SiO unit giving rise to the addition according to route I This unexpected lack of regioselectivity requires comment Monomer 1 has a complex structure and sevei-al factors may affect its reactivity lfthe decisive factor in the monomer addition to the polyniet-end is the formation of the new bond, then the nucleophilic attack should indeed be directed t o the most electrophilic center which is silicon bonded to phenyl groups Eq (5) It may, however. happen that the cleavage ofthe bond in cyclotrisiloxane is more important and thc ring will be ruptured in such a place where the best leaving group from silicon is formed This factoigives preference to route 2 where the most stable rilanolate is formed, Eq 6…”

“…I, I -d~plienyltetramethylcyclotrisiloxane was found to be inore reactive inonninei-in the anionic copolymei-ization than its f~illy substituted analogue (Ref 19), which pointed to the preferential attack at the Ph,SiO unit giving rise to the addition according to route I This unexpected lack of regioselectivity requires comment Monomer 1 has a complex structure and sevei-al factors may affect its reactivity lfthe decisive factor in the monomer addition to the polyniet-end is the formation of the new bond, then the nucleophilic attack should indeed be directed t o the most electrophilic center which is silicon bonded to phenyl groups Eq (5) It may, however. happen that the cleavage ofthe bond in cyclotrisiloxane is more important and thc ring will be ruptured in such a place where the best leaving group from silicon is formed This factoigives preference to route 2 where the most stable rilanolate is formed, Eq 6 Finally, we should remember that the propagation centre is an ion pair Thus, a coordination of the countei--ion to tnonoiner may be important (Ref 13) …”

“…19,20) The most probable place of the coordination is oxygen linking two methyl substituted silicon atoms as this oxygen is the centre of the highest electron density. Such coordination would facilitate the cleavage of the ring according to route 3 , Eq.…”

“…In contrast to larger siloxane rings which interact with counter-ion inultidentally (Ref,12), the interaction with cyclotrisiloxanes is localized (Refs. 19,20) The most probable place of the coordination is oxygen linking two methyl substituted silicon atoms as this oxygen is the centre of the highest electron density. Such coordination would facilitate the cleavage of the ring according to route 3 , Eq.…”

Anionic Polymerization of Siloxanes Non-Trivial Model System for Studies of Common Phenomena

“…H,e. C.H, 2 It was found in the polymerization of 2,2,4,4-tetramethyl-6,6-diphenylcyclo -trisiloxane (1) with two different catalytic systemspotassium silanolate and lithium silanolate with HMPT that the polymerization pattern is rather different in the two cases [32]. When the lithium catalyst was employed, pol~mer of pretty regular structure was obtained, as indicated by C NMR.…”