2,2,4,4,6,6,8,8-Octamethylcyclotetrasiloxane polymerization

A scheme of this variant of electron transfer from the naphthalene radical anion was first proposed by Morton et al. to describe the initiation of octamethylcyclotetrasiloxane polymerization [191], Initiation according to scheme (47) is rapid with oxirane and slow with octamethylcyclotetrasiloxane. 

Detailed procedures for the synthesis ofa,o>-organofunctionally terminated siloxane oligomers with well defined structures have been given 50,66-67). Tables 6 and 7 provide the data on the synthesis and characteristics of aminopropyl and hydroxybutyl terminated polydimethylsiloxane oligomers prepared via anionic and cationic ringopening polymerization of octamethylcyclotetrasiloxane (D in the presence of appropriate disiloxanes, respectively. 

As already discussed in Section 2.2, crystalline dimethylsilanediol 53 can be prepared by hydrolysis from hexamethylcyclotrisilazane 51, from dimethoxydimethyl-silane [40], and from octamethylcyclotetrasilazane (OMCTS) 52. The most simple preparation of 53 is, however, controlled hydrolysis of dimethyldichlorosilane 48 in the presence of (NH4)2C03 or triethylamine [41]. Likewise, hydrolysis of hexam-ethylcyclotrisiloxane 54 and of octamethylcyclotetrasiloxane 55 eventually gives rise to dimethylsilanediol 53. In all these reactions the intermediacy of the very reactive dimethylsilanone 110 has been assumed, which can be generated by pyrolytic [42, 43] and chemical methods [44—46] and which cyclizes or polymerizes much more rapidly, e.g. in contact with traces of alkali from ordinary laboratory or even Pyrex glassware [40, 47] to 54, 55, and 56 than trimethylsilanol 4 polymerizes to hexamethyldisiloxane 7. Compound 111 is readily converted into dimethylsilanone 110 and MesSil 17 [46] (Scheme 3.6). 

Ring-opening polymerization of cyclic monomers, usually by anionic or cationic catalysts, is another route to elastomers. These include the polymerization of octamethylcyclotetrasiloxane... 

The ceiling temperature phenomenon is observed because AH is highly exothermic, while AS is mildly exoentropic. The opposite type of phenomenon occurs in rare instances where AS is endoentropic (AS = +) and AH is very small (either + or —) or zero. Under these conditions, there will be a floor temperature Tf below which polymerization is not possible. This behavior has been observed in only three cases—the polymerizations of cyclic sulfur and selenium octamers and octamethylcyclotetrasiloxane to the corresponding linear polymers (Secs. 7-lla). AH is 9.5, 13.5, and 6.4 kJ mol-1, respectively, and AS is 27, 31, and 190 J K-1 mol-1, respectively [Brandrup et al., 1999 Lee and Johannson, 1966, 1976]. 

Ring-opening polymerization of cyclic siloxanes with cationic initiators allows the possibility of introducing stable end groups by the use of suitable chain transfer agents. Thus, polysiloxanes with trimethylsilyl end groups are formed when the cationic polymerization of octamethylcyclotetrasiloxane is carried out in the presence of hexamethyldisiloxane as transfer agent ... 

Boileau and coworkers11 have used a novel trimethylsilylmethyl lithium initiator MesSiCTDLi (1), in combination with a cryptand [211], for the ring-opening polymerization of cyclosiloxanes. Initiation of hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) polymerization has been followed by H, 7Li, 13C and 29Si NMR. 

Octalactin A, via ring-closing diene metathesis, 11, 230 Octamethylcyclotetrasiloxane, in polymerization, 3, 654 Octamethyl[3]ruthenocenophane, preparation, 6, 637... 

Polymer III was made by copolymerizing a mixture of octamethylcyclotetrasiloxane, (Me2SiO)i , and 2,4,6,8-tetramethyl-2,4, 6,8-tetra-y-mercaptopropyl-cyclotetrasiloxane in the presence of MesSiOSiMes using trifluoromethane sulfonic acid, CF3SO3H, as a catalyst. The polymerization procedure is the same as that of Polymer I. 

The classical examples are the polymerization of styrene in NH3, initiated by KNH2 [176], and the polymerization of octamethylcyclotetrasiloxane initiated by alkali metal hydroxides [177]. At the present time, simple strong mineral bases are no longer used for the initiation of anionic polymerizations. 

Fig. 30. Conversion curves for the anionic polymerization of (a) dodecamethylcyclotetrasiloxane and (b) octamethylcyclotetrasiloxane at 523 K.

Siloxane-containing block copolymers are often prepared by step-growth or condensation polymerization of preformed diftmctionalized siloxane oligomers with other diftmctionalized monomers or oligomers. Our current work (3, 4, 6-8) on siloxane chemistry includes the preparation of a number of functionalized oligomers, with emphasis on equilibration processes with the commonly available cyclic tetramer, octamethylcyclotetrasiloxane (D4), in the presence of a functionalized disiloxane or end blocker. 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

The manufacture of pure octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), which arc either marketed as such or are used as raw materials in the production of polydimethylsiloxanes by the polymerization process, is carried out by the so-called cyclization process. The hydrolysis or methanolysis product is heated in a suspension of potassium hydroxide and an inert liquid (e.g. mineral oil). This method is chosen to hinder polymerization of the siloxanes to highly viscous liquids. The potassium hydroxide catalyzes an equilibrium reaction in which the Si-O-Si bonds are cleaved and newly made (equilibration). Since in this process the, by comparison with the linear siloxanes, more volatile octamethylcyclotetrasiloxane and decamethylcyclopenta-siloxane are continuously distilled off from the siloxane mixture, the equilibrium is shifted in a direction favoring the desired cyclic siloxane thereby enabling all of the siloxane to be so converted. 

Equilibrium polymerization, which can be anionic or cationic, is utilized to convert cyclic organosiloxanes into polydiorganosiloxane polymer chains. In the chemical industry octamethylcyclotetrasiloxane is preferred as such, or as a mixture with other siloxanes for chain termination and/or production of copolymers for specific applications. Particularly indu.strially important is anionic polymerization with basic catalysts such as alkali hydroxides, whereby the activity falls off in the order Cs > Rb > K > Na > Li. KOH is most frequently used e.g. as a suspension in octamethylcyclotetrasiloxane at 140°C, the catalyst being active from a concentration of several ppm. According to the assumed mechanism of this catalytic process, potassium siloxanolate is initially formed, which leads to cleavage of the Si-O-Si bonds and chain formation ... 

In their pioneering work on the kinetics of the base catalysed polymerization of octamethylcyclotetrasiloxane (D4) Grubb and Osthoff2 concluded that the rate of... 

The use of alkali metal hydroxides to catalyse the polymerization of cyclodiorganopolysiloxanes was first disclosed by Hyde13 in 1949. He observed that initially, the metal hydroxide reacts with the cyclosiloxane to form a metal silanolate which he found to be capable of catalysing the polymerization. Grubb and Osthoff2 demonstrated that the rates of polymerization of octamethylcyclotetrasiloxane (D4) catalysed by equimolar amounts of potassium hydroxide and potassium silanolate were... 

A series of well characterized a,w-hydrogen difunctional polydimethylsiloxane oligomers were prepared as shown in equation 6 by the cationic ring opening polymerization of 2,2,4,4,6,6,8,8-octamethylcyclotetrasiloxane (D4) in the presence of tetramethyldisiloxane as a chain stopper (10). 

The commercial importance of polysHoxanes (silicone polymers) was discussed in Sec. 2-12f. The higher-MW polysiloxanes are synthesized by anionic or cationic polymerizations of cyclic siloxanes [Bostick, 1969 Kendrick et al., 1989 Noll, 1968 Saam, 1989 Wright, 1984]. The most commonly encountered polymerizations are those of the cyclic trimers and tetra-mers, for example, for octamethylcyclotetrasiloxane... 

Ways have been investigated to reduce or avoid leaching of plasticizers. Barreto et al. (2012) deposited a barrier coating onto PVC, resulting in a reduction of more than 80% of leaching of the plasticizer. The barrier was established by means of plasma-activated CVD used to polymerize octamethylcyclotetrasiloxane (OMCTS) and hexamethyldisiloxane (HMDSO). Covalent bonding of the plasticizer to PVC... 

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