D.A. de Vekki1 2 METAL COMPLEX CATALYZED …science.spb.ru/files/BulletinIT/2013/1/Articles/20/files/assets/downloads/publication.pdfIntramolecular polymerization of organosiloxanes

1 Dimitriy Andreevich de Vekki, PhD (Chem.), Associate Professor, Departament of Chemical Technology of Rubber, e-mail: [email protected] 2 Nikolai Konstantinovich Skvotsov, Dr. Sci. (Chem.), Professor, Departament of Chemical Technology of Rubber, e-mail: [email protected]

D.A. de Vekki1 and N.K. Skvortsov2

METAL COMPLEX CATALYZED HYDROSILYLATION OF VINYL- WITH HYDROSILOXANES (A REVIEW) St. Petersburg State Institute of Technology (Technical Uni-versity), Moskovskii pr. 26, St. Petersburg, 190013 Russia Abstract—The review considers the influence of the type of metal complex catalysts and the structure of unsaturated and hydrogen-containing organo-silicon compounds on the rate and route of hydrolsilylation in siloxane sys-tems under thermal and photoactivation. Key words: Hydrosilylation, hydrosiloxanes, vinylsiloxanes, platinum, rho-dium, catalyst, photoactivation.

The hydrosilylation reaction, i.e. the reaction between un-

saturated compounds and silicon hydrides, was first men-tioned in 1947 as an example of the reaction of trichlorisilane and 1-octene in the presence of acetyl peroxide [1]. In 1957 Speier et al. [2] discovered one of the most efficient homoge-neous metal complex catalysts of this reaction, viz. chloropla-tinic acid, which was given the name the Speier’s catalyst. Since then hydrosilylation has been actively studied. Initially researchers’ effort has been focused on the synthesis and application of new organosilicon compounds and later on the search and development of new catalysts for this reaction. As a result, organosiloxanes with different structrures and prop-erties have been synthesized, and also new efficient catalysts, such as the Karstedt [vinylsiloxane complexes of Pt(0)] [3]

and Wilkinson catalysts [rhodium(I) tris(triphenylphosphine) chloride] [4], as well as colloidal catalysts have been discov-ered [5−12]. Hydrosilylation of unsaturated compounds has been the subject of monographs, reviews, and books [13−23], but, however, insufficient attention has still been paid to hydrolysilation in siloxane systems.

The reaction of vinyl- and hydrosiloxanes is a particular case of the hydride addition reaction and conforms to the general regularities for such reactions [14−16]. Siloxanes, like most unsaturated compounds, are hydrosilylated to form α- or β-adducts, i.e. the reaction either follows Markovnikov’s rule (the first case) or occurs against this rule (Farmer’s rule, the second case)

The reaction route is affected, along with common factors

(reagent concentrations, reaction temperature and time, sol-vent nature, etc.), by such specific factors as the type of the catalyst and the structure of the unsaturated and hydrogen-containing organosilicon compound. The present work deals with just these specific factors.

The overwhelming part of early research on hydrosilyla-tion in siloxane systems was based on the catalysis either with chloroplatinic acid and its modifications or with vi-nylsiloxane complexes of Pt(0). Therewith, the greatest con-tribution into the synthesis of new organosilicon compounds was made by the research group of Acad. K.A. Andrianov. Other works were focused on modeling the curing process and research on the influence of the structure of metal com-plex catalysts on reaction conversion and selectivity. Certain researchers compared their proposed catalysts with the Speier’s or Karstedt’s catalysts; siloxane activity studies were also reported.

Zhdanov et al. [24] synthesized branched polyorganocy-clocarbosiloxanes by the addition of tetramethylcyclosiloxane

[Н(СН3)SiO]4 and tetraphenylcyclosiloxane [H(C6H5)SiO]4 to trimethyltrivinylcyclotrisiloxane [CH3(CH2=CH)SiO]3, tetrame-thyltetravinylcyclotetrasiloxane [CH3(CH2=CH)SiO]4, triphenyl-trivinylcyclotrisiloxane [C6H5(CH2=CH)SiO]3, and tetraphenyl-tetravinylcyclotetrasiloxane [C6H5(CH2=CH)SiO]4 in ampules in the presence of H2PtCI6⋅6H2O (100−105ºС, (0.7−1.0)⋅10−5 g/g mixture as a 0.01 N THF solution) [24]. The reaction products were optically transparent glassy substances. To prepare network polyorganocarbosiloxanes, the polymer, after the gel point had been reached, was heated at 100-105 ºС for an additional 100 h.

Andrianov et al. [25] proposed to perform hydrosilylation of 1-vinyl-1,3,3,5,5,7,7-heptamethylcyclotetrasiloxane with hydrosiloxanes of the general formula H(CH3)nSi[OSi(CH3)3]3-n (n = 0−2) by the ampule method at 20, 50, and 90ºС in the presence of Speier’s catalyst. The activity of hydrosiloxanes, measured by the relative rate constants of competitive reac-tions, was found to be the higher the less the number of bulky trimethylsiloxy groups at the silicon atom linked to hy-drogen. Consequently, according to the authors of [25], the

Si O Si H Si O Si

CH3

Si OCH Si

Si O Si CH2 Si O SiCH2Si O SiCH2 CH

+

addition

additioncatalyst

I. CHEMISTRY AND CHEMICAL TECHNOLOGY • TECHNOLOGY OF ORGANIC SUBSTANCES

activity is primarily controlled by steric factors, rather than the presence at the silicon atom of −I substituents which facilitate dissociation of the Si−H bond. Gas chromatography revealed, along with adducts, by-products whose fraction increased as the reaction temperature increased and the activity of the hydrosiloxane decreased in the order [25]

H(CH3)2SiOSi(CH3)3 > H(CH3)Si[OSi(CH3)3]2 >> HSi[OSi(CH3)3]3

Polyaddition of dihydromethylsiloxanes to divinylme-thylsiloxanes with terminated trimethylsiloxy groups in the presence of Speier’s catalyst gives rise to liquid β-adducts (1Н NMR data) with MW < 2000 [26]. Comparison of the hydrosi-lylation rates of divinylsiloxane

(CН3)3SiO[(CH3)(CH2=CH)SiO]2Si(CH3)3 showed that 1,1,3,3-tetramethyldisiloxane adds 100 times faster that (CН3)3SiO[(CH3)HSiO]2Si(CH3)3, which, too, was explained by steric factors.

The reaction of 1-vinyl-1,3,3,5,5,7,7-heptamethylcyclotetrasiloxane with 1,3,3,5,5,7,7-heptamethylcyclo- and 1-phenyl-3,3,5,5,7,7-hexamethylcyclotetrasiloxane in the presence of H2PtCl6⋅6Н2О (1⋅10-6 mol/g vinylsiloxane, 100−150ºС) was used to obtain bicyclic organosiloxanes [27]

According to IR spectral data, the exclusive reaction

products were β-adducts (10−30 %); the reaction was found to occur with small heat release.

The reactions of 1-vinyl-1,2,3,4-pentamethylcyclotrisiloxane with 1,3,5,7-tetramethyl-3,5,7-triphenyl-, 1,3,5,5,7-pentamethyl-3,7-diphenyl-, and

1,3,3,5,5,7,7-hexamethyl-5-phenylcyclotetrasiloxanes in the presence of Speier’s catalyst (0.01 М solution of THF, 1⋅10-4 mol/g hydrosiloxane, 80−120ºС) [28]. The formation of the corresponding bicyclosiloxanes occurs by Farmer’s rule (IR and 1Н NMR data)

The hydrosilylation reaction was used to synthesize or-ganocyclotetrasiloxanes in which the silicon atom is sur-rounded by the dimethylsiloxane chains with terminated

trimethylsiloxy groups [29]. In the presence of catalytic amounts of chloroplatinic acid, the reaction occurs by the β-carbon atom of the vinyl group (IR and 1Н NMR data)

The reaction of hydrosiloxanes of the general formula

H[Si(CH3)2O]nSi(CH3)3 with vinylorganocyclotrisiloxanes in the presence of Speier’s catalyst gives rise to organocyclot-

risiloxanes which are β-adducts having methylsiloxanyl ethyl groups at the silicon atom (IR data) [30, 31]

Si OCH3

CH3

OSiO

CH3

CH3

Si OCH3

CH3

SiR

H

Si OCH3

CH3

OSiO

CH3

CH3

Si OCHCH3

SiCH3

CH3CH2

Si OCH3

CH3

OSiO

CH3

CH3

Si OCH3

CH3

SiR

Si OCH3

CH3

OSiO

CH3

CH3

Si OCH2

CH3

SiCH3

CH3CH2

H2PtCl6

R = CH3, C6H5

+

Si ORCH3

OSiO

CH3

R'

Si OR'CH3

SiCH3

HO

SiO

Si OCHCH3

SiCH3

CH3CH2

CH3 CH3

Si ORCH3

OSiO

CH3

R'

Si OR'CH3

SiCH3

OSi

O

Si OCH2

CH3

SiCH3

CH3CH2

CH3 CH3

H2PtCl6

R = CH3, R' = C6H5; R = C6H5, R' = CH3; R = R ' = C6H5

+

Si ORCH3

O

Si

O

CH3

CH

Si ORCH3

SiCH3

CH CH2

CH2

Si OCH3

SiCH3

CH3

CH3R'Si OCH3

CH3

H

H2PtCl6

R = CH3; R = C6H5;

R = R' = CH3; R = R' = C6H5;

R = C6H5, R' = CH3.

CH2

Si OCH3

SiCH3

CH3

CH3R'Si OCH3

CH3

CH2

Si ORCH3

O

SiO

CH3

Si ORCH3

SiCH3

CH2Si OCH3

SiCH3

CH3

CH3R'Si OCH3

CH3

CH2

+

n

2n

nn = 0, n = 0,n = 1, n = 2,n = 3,

OSiO

Si OSi

R'R

CH

CH3

R'R

CH2

Si OHCH3

SiCH3

CH3

CH3CH3

H2PtCl6

R = R' = CH3 C6H5; R = CH3, R' = C6H5;

Si OCH2

CH3

SiCH3

CH3

CH3CH3

OSiO

Si OSi

R'R

CH2

CH3

R'R

+

n

or n = 0-2

n


The nature of the R and R' substitutents in the ring does not affect the reaction initiation temperature, and it tends to increase with increasing number of substituents n in the hydrosiloxane (70ºС at n = 0; 90ºС at n = 1, and 130ºС at n = 2). The activity order for hydrosiloxanes with terminated Si−H groups is the following [30]

HSi(CH3)3 > H(CH3)2SiOSi(CH3)3 > H(CH3)2SiOSi(CH3)2OSi(CH3)3

Further cross-linking of the above oligomers, which oc-curs in the presence of tetramethylammonium polydime-

thylsiloxanolate (20−75oС) is associated with chain propa-gation on the siloxanylethylene groups under block-polymerization conditions [31]. Therewith, the bulk of the siloxanylethylene group in the starting compounds only slightly affects the polymerization rate. Intramolecular polymerization of organosiloxanes contain-

ing both a Si−H bond and a vinyl substituent was studied in [32, 33]

The reaction was performed in CCl4 at 50-85oС in the

presence of H2PtCl6⋅6H2O [(1.4−4.2)⋅10−4 M] [33]. The prod-ucts were exclusively β-adducts (IR and 1H NMR data). Kinetic studies of this reaction showed that it is first-order in catalyst and second-order in monomer and involves intermediate for-mation of a monomer−catalyst compound. The activation

energy at the catalyst concentration 1.4⋅10−4 M is 71.23 kJ/mol [33].

The result of the polyaddition of 3-vinyl-, 3-allyl, and 3-(4-vinylphenyl)-1,1,3,3-tetramethyldisiloxanes and 7-vinyl-1,1,3,3,5,5,7,7-octamethyltetrasiloxane depends on the type of the catalyst and gives rise to oligomers with molecular weights varying from 1600 to 5600 [34]

The hydrosilylation of 3-vinyl- and 3-(4-vinylphenyl)-1,1,3,3-tetramethyldisiloxanes in the presence of cis-[Pt(PhCN)2Cl2] or the platinum vinylsiloxane complex obtained by the reaction of H2PtCl6⋅6H2O with 1,3-divinyl-1,1,3,3-tetramethyldisiloxane gives both α- and β-adducts, whereas the polyaddition of allylsiloxanes in the presence of the same catalysts results in exclusive formation of β-adducts (1H NMR data) [34]. 3-(4-Vinylphenyl)-1,1,3,3-tetramethyldisiloxane, too, is hydrosilylated by Farmer’s rule, but here Со2(СO)8 should be used as catalyst (50oС, 3 h).

Study of the effect of substituents on the silicon atom linked to the vinyl group on the reactivity of linear α,ω-divinylorganosiloxanes of the general formula СH2=CH(CH3)(R)SiO[(CH3)(R')SiO]nSi(CH3)(R)CH=CH2 (n = 2, R = C6H5, R' = CH3; n = 4, R = CH3, R' = CH3 or C6H5; R =C2H5, R' = CH3) in hydrosilylation with α,ω-dihydroorganosiloxanes of the general formula

H(CH3)(R)SiO[(R')(R")SiO]nSi(CH3)(R)H (n = 2, R = CH3, R' = CH3 or C6H5, R'' = С6H5; R = C6H5, R' = R'' = CH3; n = 4, R = R = R = CH3) at 50oС in the presence of chloroplatinic acid showed that the methyl, ethyl, and phenyl substituents on silicon slow down the reaction the stronger the higher is their electron-donor power [35]. In certain cases, a steric effect of the phenyl groups was observed.

The polyaddition of α,ω-dihydromethylsiloxanes to 1,5-divinyl-1,3,3,5,7,7-hexamethylcyclotetrasiloxane under argon at an equimolar reagent ratio (100-110oС, 50−160 h, solvent toluene or none) in the presence of H2PtCl6⋅6H2O [(1−1.5)⋅10−5 g/g initial mixture as a 0.01 N solution in THF; part of the catalyst was added before reaction, and the rest in 25−140 h] gives viscous liquids with molecular weights vary-ing from 75000 to 471000, depending on the molecular weight of the starting dihydrosiloxane [36]

Si OCH3

CH3

OSiO

CH3

CH3

Si OCHCH3

SiCH3

HCH2

H2PtCl6 Si OCH3

CH3

OSiO

CH3

CH3

Si OCH2

CH3

SiCH3

CH2

75 C0

n

n

Si OCH3

CH3

SiCH3

CH3

HCH3

Si OCHCH3

CH3

Si

CH3

CH3

Si OCH2

CH3

CH3

Si

CH3

CH3

CH2m n

Si OCH3

CH3

SiCH3

CH3

H

CH3

Si OCHCH3

CH3

SiCH3

CH3

Si OCH2

CH3

CH3

SiCH3

CH3

CH2m n

Si OCH3

CH3

SiCH3

CH3

HCH3

Si OCHCH3

CH3

Si

CH3

CH3

Si OCH3

CH3

SiCH3

CH3

CH2m n CH2

3

Si OCH3

CH3

SiCH3

CH3

H

CH3

Si OCHCH3

CH3

SiCH3

CH3

Si OCH3

CH3

SiCH3

CH3

CH2m

n CH2

3 3

33

catalyst

catalyst

catalyst

catalyst


To prevent reduction of viscosity of polymers obtained by

the catalytic hydrosilylation reaction, it should be performed with H2PtCl6⋅6H2O dissolved in THF rather than isopropanol [36]. Based on the IR spectra and experiments on prolonged heating of a mixture of the starting divinylhexamethylcyclo-tetrasiloxane isomers, Zhdanov et al. [36] drew a conclusion that the polyaddition occurs with retention of the structure of

the starting compounds and involves no polymerization of divinylhexamethylcyclotetrasiloxanes; polymerization can be induced by heating of hydrosilylation products in the presence of an anionic polymerization catalyst (0.001−0.1 %).

The synthesis of polyorganosiloxanes containing both lin-ear and cyclic fragments in the chain was described in [37]

The reaction of 1,7-dimethyl-3,5,9,11-tetraphenyl-1,7-divinyltricyclohexasiloxane with α,ω -dihydromethyl(phenyl)siloxanes was performed in toluene in the presence of H2PtCl6 in THF [(0.16−2.1)⋅10−3 mol/mol active hydrogen]. The reaction was followed by monitoring the content of active hydrogen in the reaction mixture; grad-ual increase of the specific density of toluene solutions was observed. The reaction rate and depth was found to change as the chain length in the starting α,ω-dihydrosiloxane chang-es in the order: H(CH3)2SiOSi(CH3)2H >> H(CH3)2Si[OSi(C6H5)2]23OSi(CH3)2H >

> H(CH3)2Si[OSi(CH3)2]86OSi(CH3)2H > H(CH3)2Si[OSi(C6H5)2]40OSi(CH3)2H

The hydrosilylation of tetramethylvinylcyclotetrasiloxane with tetramethylcyclotetrasiloxane in the presence of H2PtCl6 (60−200 ºС, 1⋅10−6 mol/g hydrosiloxane) was used to syn-thesize 3D polymers [38], and with α,ω -dihydromethylsiloxanes Н(СН3)2SiO[(CH3)2SiO]nSi(CH3)2Н (n = 0−200) in the presence of H2PtCl6⋅6H2O (100−110ºС, (1−1.5)⋅10−5 g/g mixture as a 0.01 N solution in THF), regu-

lar network polycarbodiorganosiloxanes were obtained [39]. It was shown that the aggregation state of final products depends on the starting reagent ratio. The reaction of the above dihydrosiloxane (n = 20, 30, 60, 130) with 1,3,5-trimethyl-1,3,5-trivinylcyclosiloxane and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane in the presence of H2PtCl6⋅6H2O (100oС) produces network tri- and tetrafunc-tional polymers [40].

Network carborane-siloxane polymers can be obtained by reactions of carborane-siloxane monomers containing termi-nated vinyl and ethynyl groups with linear and branched hy-drosiloxanes (unsaturated substrate-to-branched hydrosilox-ane ratio is 1:0.5 or 1:1; the unsaturated substrate-to-linear hydrosiloxane ratio is 1:1.04, 1:0.53 , or 1:0.27) in the pres-ence of Speier’s [41] and Karstedt’s catalysis in hexane [42].

The addition of a linear polymethylhydrosiloxane to vinyl-carborane-siloxane involves the β-carbon atom and gives colorless plastic polymer networks with a high cross-linking degree [41, 42]

Si OCH3

CH3

OSiO

CH3

CH

Si OCHCH3

SiCH3

CH3CH2

CH2

Si OCH3

SiCH3

HCH3R'

Si OCH3

CH3

H

H2PtCl6 Si O

CH3

SiCH3

CH3R'Si OCH3

CH3

CH2

Si OCH3

CH3

OSiO

CH3

CH2

Si OCH2

CH3

SiCH3

CH3CH2

+

nx

n = 0, 1, 4, 5, 6, 10, 20, 27, 34, 57, 94, 150, 200

n

A CHCHCH2 CH2Si OR

SiCH3

HCH3CH3

Si OCH3

CH3

H

A (CH2)2CHCH2Si OR

SiCH3

CH3CH3

Si OCH3

CH3

A (CH2)2(CH2)2 Si Si OR

SiCH3

CH3CH3

OCH3

CH3

H

H2PtCl6+

n

m m

n m n

A =R = CH3,

R = C6H5,OSi

O

CH3 OSiOSi

Si

Si O SiO

CH3

C6H5 C6H5

C6H5C6H5

OO ,n = 0, 10, 19, 49 or 86;

n = 6, 23, 40 or 74


Vinyl- and acetylcarborane-siloxаnes are hydrosilylated by tetrakis(dimethylsiloxyl)-, methyltris(dimethylsiloxyl)-, and phenyltris(dimethylsiloxyl)silanes are hydrosilylated by Farmer’s rule to form network polymers, and, therewith, at the acetylcarborane-silane : hydrosiloxane ratio 2:1, the ace-tyl fragment takes up only one hydrosiloxane molecule, whereas at the 1:1 ratio, already two (IR data). Kolel-Veetil and Keller [42] noted that Karstedt’s catalyst (2.1−2.4 % of Pt in xylene) is compares in activity with Speier’s catalyst (2.1−2.4 % of Pt in THF); in the absence of hexane in the

reaction medium, the reaction occurs momentarily and in-volves strong heat release. The resulting polymers exhibit a high thermo-oxidative stability which depends on the struc-ture of the hydrosiloxane.

The polyhydrosilylation of monomers of the АВ2 type, con-taining both Si−H bonds and vinyl or allyl substituents, in the presence of chloroplatinic acid or P/C for 3−4 h at 50oС leads to hyperbranched polymers which, in their turn, can be modi-fied with epoxy acrylates for further light-induced cross-linking [43, 44].

The polyaddition of polyhydroorganosiloxanes to polyvi-nylorganosiloxanes in the presence of H2PtCl6 (150oС, acid concentration 1⋅10−4 mol/mol vinyl groups, 5−6 h) gave cross-linked polymers [45, 46]. It was found that replacement of methyls by phenyls on the silicon atoms linked to reacting groups of the starting polyorganosiloxanes results in a reduc-tion of the curing degree of the products for steric reasons. Moreover, replacement of the hydroxyl by trimethylsilyl

groups in branched siloxane polymers adversely affects their thermal stability, unlike what is observed in linear polymers [46].

Hydrosilylation of pentamethylvinylcyclotrisiloxane and heptamethylvinylcyclotetrasiloxane with dihydrosiloxanes in the presence of H2PtCl6 (1x10−5 g/g vinylsiloxane) at 50−100oС was described in [47]

C CSiSi

OSi

OSi

SiO

SiO

Si

H

n

Si

ROO

OSi HSi

Si

H

H

SiO

SiO

Si n

SiO

SiO

Sin

C

C

Si

Si

O

Si

O

Si

R = CH3, C6H5, OSi(CH3)2H

C

C

Si

Si

O

Si

O

Si

C CSiSi

OSi

OSiSi

R'OO

OSiSi

Si

C CSiSi

OSi

OSi

R' = CH3, C6H5, OSi(CH3)2 C CSiSi

OSi

OSi

Speier'scatalyst

Karsted'scatalyst

SiCH3

H

O

O

Si

Si

CH3CH2

CH3

CH3CH2

CH3

CH

CH

CH2

CH2

Monomer of the AB2 type

SiCH3

R

O

O

Si

Si

CH2CH3R'

CH3

CH3R'

CH2CH3

Si

O

OR

Si

SiO

OSi

SiO O

SiR'

Si

R'

Si

SiO OSi Si

R' R'

Si

SiO

Si

SiO O

Si

R'

OSi

Si O

OSi

Si R'Si

R'R'

H2PtCl6

R = H, R' = CH=CH2; R = CH=CH2, R' = H

or Pt/C


Analysis of the IR, 1H NMR, and GLC data showed that

the reaction is primarily involves the vinyl β-carbon atom, and its selectivity is virtually temperature-independent. The reac-tion of dihydrosiloxanes with vinylcyclosiloxane at a 1:2 molar ratio involves initial formation of monoadducts, and, only after 50% of dihydrosiloxane has reacted, the second hydro-gen starts to react. The hydrosilylation rate decreases as the siloxane chain length increases in the order:

[H(CH3)2Si]2O > H[(CH3)2SiO]3Si(CH3)2H >> >>H[(CH3)2SiO]4Si(CH3)2H

Petrov and Vdovin [48] described the H2PtCl6-catalyzed reaction of 1,3-diethyl-1,3-dimethyldisiloxane with 1,3-diethyl-1,3-dimethyl-1,3-divinyl- and 1,3-diallyl-1,1,3,3,-tetramethyldisiloxanes to form a polymer with the molecular weight 800−900 and various ratios of the number of Si−C bridges and the number of siloxane groups (n = 0, 1):

The hydrosilylation of α,ω-divinyl- and α,ω-

dialyldimethylsiloxanes with α,ω-dihydromethylsiloxanes, catalyzed by Pt/C, too, yields linear polymers: R(CH3)2Si{[OSi(CH3)2]n(CH2)m}xSi(CH3)2[OSi(CH3)2]nR' (R = R' = CH=CH2 or H, m = 2, n = 2, 4, 6; R = R' = CH2−CH=CH2 or H, m = 3, n = 1, 2; x = 3−250) [49, 50].

Shi et al. [51] have studied the effect of the structure of hydrosiloxanes H[(CH3)2SiO]5Si(CH3)2H, (CH3)5SiO[(CH3)HSiO]10Si(CH3)3, (CH3)3SiO[(CH3)HSiO]10[(CH3)2SiO]17Si(CH3)2H, C6H5Si[OSi(CH3)2H]3, and (C6H5)2Si[OSi(CH3)2H]2 on the rate of hydrosilylation of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane

by means of 1H NMR spectroscopy. Giraud and Jenny [52] showed that the selectivity of hydrosilylation of divinyl com-pounds in the presence of [Pt2(CH2=CH2)2Cl4] in benzene under reflux depends on the distance between the silicon atoms in silanes HSi(CH3)2X(CH3)2SiH (X = O, NH, CH2, Si(CH3)2, C6H4).

The synthesis of dumbbell methylphenylbicyclocarbosilox-anes containing phenyl groups both in the polydimethylsilox-ane and in terminated fragments of the molecule was per-formed by hydrosilylation under argon in the presence of H2PtCl6 (120oС, acid concentration 1⋅10−4 mol/mol hydride, 8 h) [53]

The reaction of a linear vinylsiloxane and a silicon mono-mer having active Si−H groups (0.8−3 SiH/alkene radical) in the presence of H2PtCl6 at 150oС was considered in [54]. The curing of linear polyorganosiloxanes having vinylsiloxy units in the chain members in the presence of structuring agents (or-ganohydrosiloxanes) at 50−100oС leads to disilylethylene

groups which then act as cross-linking moieties to form linear diorganosiloxane macromplecules [55].

Meladze et al. [56] have described the reaction of vinylcy-closiloxanes with hydromethylsiloxanes in the presence of H2PtCl6 (however, complete conversion of Si−H groups could not be achieved)

Si O

CH3

Si

CH3

H

CH3CH3

H

SiO

CH3

CH3

SiO

CH3

CHCH2

H2PtCl6 SiO

CH3

CH3

SiO

CH3

CH2 CH2 Si O

CH3

Si

CH3

CH3CH3

SiO

CH3

CH3

SiO

CH3

CH2CH2

n

2m

+

mn

m

n = 1, 3, 4; m = 2, 3

Si OH Si H O Si CH2 CH CH2 H2PtCl6

O Si CH2 CH2 Si O SiCH2 2

+n 2n m

Si OCH3

CH3

O

Si

O

R

CH

Si OCH3

CH3

Si

CH3

CH3

CH2

Si O

R'

Si

CH3

H

CH3R''Si O

CH3

CH3

H

H2PtCl6Si OCH3

CH3

OSiO

CH3

CH2

Si OCH3

CH3

Si

CH3

CH3

SiCH3

CH3

CH2 Si O

R'

SiCH3

CH3R''

O Si O(CH2)2

CH3

OSiO

CH3

CH3

Si OCH3

CH3

Si

CH3

CH3

+n

2 n

n = 1-3, R = R' = CH3, R'' = C6H5;n = 1, R = CH3, R' = R'' = C6H5;n = 1 or 2, R = R'' = CH3, R' = C6H5;

Si OCH3

SiCH3

OCH3H

SiCH3

OCH3

CH3 SiCH3

CH3

CH3

SiO

RR

SiO

CH3

CHCH2

H2PtCl6

SiCH3

SiCH3

OCH3

CH3 OCH2

CH3

OCH3

SiCH3

CH3

CH3

CH3

HO SiSi

SiO

RR

SiO

CH2

CH3

n

p

+

m x

m

n

p

k xm-k

m ~ 35, n ~ 0.23, p = 2-4,R = Me, Ph


Silicone rubbers were proposed to be obtained by hydrosi-lylation in the presence of metal oxides (for example, ZnO) as fillers [57]. The reaction mixture, along with polydime-thylsiloxanes with terminated vinyl and methyl groups (viscos-ity 15 and 17 MPa at 25oС, respectively), hydro-methylpolysiloxane, and the Pt catalyst, contained oc-tamethylcyclotetrasiloxane and polydimethylsiloxane with terminated OH groups. Patents have been granted for hydros-ilylation of siloxanes with unsaturated groups (≥ 2) or orga-nopolysiloxanes (SiO4/2)a[(CH2=CH)R2SiO1/2]b(R3SiO1/2)c [where R is a substituted or an unsubstituted hydrocarbon group (other than alkenyl); a, b, and c fulfill the following conditions: 0.2 ≤ а/(а + b + c) ≤ 0.6 and 0.001 ≤ b/(a + b + c) ≤ 0.2], siloxanes with Si−H groups (≥ 2) in the presence of platinum catalysts [58, 59]; hydrosilylation siloxanes with terminated vinyl groups by hydrovinylsiloxanes (viscosity of each of the components ≥ 300 mPa⋅s) in the presence of a 1% solution of disperse platinum [60]; synthesis of thermally cured polysiloxane compositions containing aliphatic com-pounds and polysiloxanes with Si−H groups or polysiloxanes with unsaturated and Si−H groups, as well as therma stable fillers and a dialkyl platinum catalyst [61].

Compositions with a low linear distribution coefficient and high shock resistance, hardness, strength, and refractive in-dex can be obtained from phenyl-containing polyorganosilox-anes (C6H5)aRbR

1cSiO(4-a-b-c)/2 [R = unsaturated substituent; R1

= alkenyl group; a/(a + b + c) = 25−95 %; (b + c)/(a + b + c) = 5−75 %; c/(a + b + c) ≤ 10 %; a + b + c = 1.0−1.9, per silicon], a linear polyorganosiloxane with alkenyl (≥ 2), phe-nyl, and methyl groups, polysiloxane with Si−H (≥ 3) and Si−CH3 groups, a platinum vinylsiloxane complex [62].

Organosilicon compositions comprising a mixture of poly-organosiloxanes with terminated vinyl groups (CН2=СН)(CH3)2SiO[(CH3)2SiO]mSi(CH3)2(CH=CH2) (m = 103−104; viscosity 0.1−1 and 102−103 Pa⋅s), poluorga-nosiloxanes with terminated and internal Si–H groups [H(CH3)2SiO{(CH3)2SiO}xSi(CH3)2H (х = 1−100; viscosity 10−103) and (CH3)3SiO{H(CH3)SiO}y{(CH3)2SiO}zSi(CH3)3 (y, z = 1−100; viscosity 10−103)], SiO2 as a filler (specific surface area 100−200 m2/g), platinum-containing catalyst, and an alkene-containing reaction modifier show improved strength and flexibility [63]. Organosilicon compositions containing highly viscous polyorganosiloxanes with terminated (viscosity 5000−1000000 mPa⋅s) and side-chain vinyl groups (viscosity ≥ 2000000 mPa⋅s), a Si−H-containing filler, and a platinum complex show a high breaking extension [64].

The composition comprising organopolysiloxanes with sili-con−alkenyl groups (≥ 2), polysiloxane (R2HSiO1/2)k(R3SiO1/2)r(RHSiO2/2)m(R2SiO2/2)n(RSiO3/2)p(SiO4/2)q (where R = unsaturated hydrocarbon radical; k, r ≥ 0; m = 30−200; n =10−100; p = 0−10; q = 1; k + r = 1−10; 0.5 ≤ (k + m)/ (k + r + m + n + p + q) ≤ 1; viscosity 5−1000 mPa⋅s) and Karstedt’s catalyst deposited on a polyethylene-coated paper, was proposed for manufacturing antiadhesive paper [65]. A mixture of methylhydrosiloxanes, divinylpolydi-methylsiloxanes, and a platinum catalyst was proposed as a component of hair shampoos [66].

Multicomponent silicone rubbers are synthesized from a mixture of organopolysiloxanes containing ≥ 2 unsaturated groups (viscosity ≥ 1000 mPa⋅s), organopolysiloxanes having ≥ 2 hydroxy groups or other hydrolysable groups, or their

mixture (average polymerization degree 2−50), a filler with a hydrophilic surface, organopolysiloxanes containing Si−H bonds (≥ 2 Si−H bonds per 1 mol of siloxane with a viscosity of 10 mPa⋅s) and a hydrosilylation catalyst, which are fed to the reaction zone from different feeders [67].

The reactions of 1,1,3,3-tetramethyldisiloxane, oligohy-dromethylsiloxane (CH3)3SiO[(CH3)HSiO]50Si(CH3)3, dihy-drooligodimethylsiloxane (viscosity 1⋅10−5 m2/s), and oli-gomethylhydrodimethylsiloxane (viscosity 3.4⋅10−5 m2/s) with α,ω-divinyloligosiloxane (СН2=СH)(CH3)2SiO[(CH3)2SiO]87Si(CH3)2(CH=CH2), 1-vinyl-1,1,3,3-pentamethyldisiloxane, and 3-vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane can be accomplished in the presence of the platimum complexes {[(CH2=CH)(CH3)2Si]O}3Pt2, (EtOCp)2Pt(OEt)2, [Pt(Сp)2Cl2], and Pt(cod)Fe(CO)6S2 at 50−80оС [68]. Therewith, the reaction rate depends on the structure of the starting reagents and the catalyst. For exam-ple, in the presence of (EtOCp)2Pt(OEt)2 hydrosilylation occurs faster in the system oligomethylhydrodimethylsiloxane – [(CH3)3SiO]2Si(CH3)СH=СH2, in the presence of Pt(cod)Fe(CO)6S2, in the system oligomethylhydrodime-thylsiloxane – (CH3)3SiOSi(CH3)2СН=СН2, whereas in the presence of [PtCp2Cl2] the reactions in both the above-mentioned systems occur at almost the same rates. The au-thors of [68] note that the reactivity of hydrosilanes follows common laws, whereas the reactivity of vinylsiloxanes pri-marily depends on the structure of the platinum catalyst: in the presence of (EtOCp)2Pt(OEt)2 more active are vinylsilox-anes with an internal vinyl group, in the presence of Pt(cod)Fe(CO)6S2, with an terminated vinyl group, whereas with [Pt(Сp)2Cl2] both groups of vinylsiloxanes compare with each other in reactivity. The observed regularities were ex-plained by different formation − dissociation rates of transient metal complexes. The activity of the studied complexes de-creases in the order:

{[(CH2=CH)(CH3)2Si]O}3Pt2[Pt(Сp)2Cl2]> (EtOCp)2Pt(OEt)2 > Pt(cod)Fe(CO)6S2

Simpsom et al. [69, 70] performed a kinetic study of hy-drosilylation of the divinylsiloxane (CH2=CH)(CH3)2Si[OSi(CH3)2]15OSi(CH3)2CH=CH2 with an equimolar amount or a 3.2-fold molar excess of the polysilox-ane (CH3)3Si[OSi(CH3)H]25OSi(CH3)3 in the presence of a plat-inum catalyst and 1-ethynylcyclohexanol (catalysis inhibitor) at 25−120ºC. The hydrosilylation kinetics were measured in 1.5−27-µm thin films deposited by spin coating freshly pre-pared reaction mixtures on silicon wafers. It was shown that the total reaction time linearly decreases with increasing film thickness. The reaction was found to be accompanied by side processes: hydrolysis of Si–H groups and reactions of newly formed Si−OH groups with each other or with Si−H groups to form new siloxane bonds.

Platinum and palladium complexes with silylpropoxynitrile ligands were studied as catalysts in the reaction of 1-vinyl-1,1,3,3,3-pentamethyldisiloxane and 1,1,3,3,3-pentamethyldisiloxane at 80oС [71]. It was found that palladi-um complexes are inactive in this reaction, whereas all plati-num catalysts give β-adducts in 94−95% yields. However, the selective of the addition in their presence is lower than in the presence of Karstedt’s catalyst.

Novel copolymers of 1,9-dihydropermethyl- and 1,9-

dihydro(trifluoropropyl)methylpentasiloxanes with 1,9-divinylperphenylpentasiloxanes [72] and of 1,9-divinyl-1,1,3,3,5,5,7,7,9,9-decaphenylpentasiloxane with α,ω-

O CN(CH2)3R MCl2

M = Pt, Pd; R = Me3SiOSiMe2, (Me3SiO)2MeSi, Me3SiO(Me2SiO)3SiMe2, Me3SiO(Me2SiO)25SiMe2.

CHC6H13N Pt

Cl

ClO

OR

OMe


dihydropermethyloligosiloxanes and studied hydrosilylation of 1-vinyl-1,1,3,3,3-pentamethyldisiloxane with excess 1,1,3,3,3-pentamethyldisiloxane in the presence of Karstedt’s catalyst in

a xylene solution [73]. The resulting copolymers are stable up to 400oС under nitrogen and up to 300oС in air; at higher temperatures, Si−C bond cleavage or depolymerization occur.

Podoba et al. [74] in their study on the reactions of dime-

thylvinylsilicone rubber (brand SKTV-1) and dimethylsilicone rubber (brand SKT) with oligoethylhydrosiloxane and oli-gomethylhydrodimethylsiloxane in the presence of Speier’s catalyst obtained evidence showing that curing of rubbers by the hydrosilylation reaction is accompanied by side reactions. The authors suggested that the major side reaction is dehy-drocondensation of hydrosiloxanes, which involves Si−H con-sumption (IR data) for forming Si−O−Si bonds and plays an important role in building a 3D network structure during the silicone rubber curing process. Oligoethylhydrosiloxane was found to be more reactive in the hydrosilylation reaction than oligomethylhydrodimethylsiloxane. Detailed study of the side reactions accompanying hydrosilylation revealed dispropor-tionation of hydrosiloxanes, which occurs concurrently with dehydrocondensation [75]; vinylsiloxanes, too, undergo dis-proportionation under the reaction conditions [76].

The disproportionation of hydrosiloxanes in the presence of Pt(II) complexes involves the following reactions [75]:

2 (HMe2Si)2O Me2HSiOSi(Me)2OSiHMe2 + Me2SiH2 2 HMe2SiOSiMe3 (HMe2Si)2O + (Me3Si)2O

Hydrosilylation reactions in the presence of sulfoxide or cyclooctadiene platinum(II) complexes which are highly active siloxane hydrosilylation catalysts give the greatest amounts of hydrosiloxane disproportionation and dehydrocondensation products; sulfide and pyridine sulfoxide complexes rank lower in this respect, and phosphine platinum(II) complexes exert the weakest effect (1H and 13С NMR, IR, GC−MS data) [75].

Formation of Si−Si bonds is observed with cycloоctadiene, bis-sulfoxide, and sulfide catalysts, but the contribution of Si−Si compounds into side reactions associated with hydrosi-lylation processes is no more than 1% (NMR data). In terms of the rate effect on side reactions associated with hydrosi-lylation, the above-considered ligands in platinum catalysts can be ranked as follows:

phosphine < pyridine sulfoxide < sulfide ≈ ≈ sulfoxide < cyclooctadiene.

The mechanism of disproportionation of hydrosiloxanes depends on the type of the catalyst and, according to [75], involves inner-sphere transformations of coordinated hy-drosiloxanes; for example, the reaction with (HMe2Si)2O in-volves formation of a 4-hydrosiloxane complex.

Cl

ClPt

Si

MeH

Me

OSi

HMe

Me

The disproportionation of vinylsiloxanes

(СH2=CH)MenSi(OSiMe3)3-n (n = 0, 1, 2) in the presence of [Rh(cod)(м-OSiMe3)]2 leads to geometric isomers of silyleth-ylene [76]

The yield and ratio of the isomers formed depends on the

structure of the starting vinylsiloxane: n = 0, yield 5%, trans-isomer is a single product; n = 1, yield 50%, trans/cis ratio 11.3; n = 2, yield 44%, trans/cis ratio 1.6 (90оС, 24 h).

The disproportionation of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane in the presence of [Rh(cod)(μ-OSiMe3)]2 gives a mixture of three isomeric products (80оС, 5 days, yield

31%, cyclotetrasiloxane : trans-silylethylene : cis-silylethylene ratio 6:3:1), and the same reaction in the presence of [Rh(cod)Сl]2 gives a divinylsiloxane cyclization product in 30% yield (3 weeks, 130оС). The final product of the dispropor-tionation of divinylsiloxane in the presence of [Rh(cod)Сl]2 is a linear polymer with MW 1815 (130оС) [76]

SiCH3

CH3

O SiCH3

RO Si

CH3

CH3

H H

Si O Si O Si

Ph

Ph

Ph

Ph Ph

PhSiCH3

CH3

O SiCH3

RO Si

CH3

CH3 SiPh

PhO Si

Ph

PhO Si

Ph

Ph

R = CH3, CH2CH2CF3

3

+Speier's catalyst3 3

3y

SiPh

PhO Si

Ph

PhO Si

Ph

Ph

SiCH3

CH3

O SiCH3

CH3

O SiCH3

CH3

H H

SiCH3

CH3

O SiCH3

CH3

O SiCH3

CH3 SiPh

PhO Si

Ph

PhO Si

Ph

Ph 3

+Karstedt's catalystm

m = 0-3

3m

n

[Rh(cod)(-OSiMe3)]2CH2=CHSiMen(OSiMe3)3-n

SiMen(OSiMe3)3-n

SiMen(OSiMe3)3-n

(Me3SiO)3-nMenSi

SiMen(OSiMe3)3-n

+

n = 0, 1, 2


Hydrosilylation of poly(methylvinylsiloxane)

Me3SiO([CH2=CH]MeSiO)x(Me2SiO)nSiMe3 with poly(methylhydrosiloxanes) Me3SiO(HMeSiO)m(Me2SiO)nSiMe3 (x, m and n = are integers;100–280oC; СCat 10-100 ppm) can be successfully carried out in the presence of rhodium siloxide complexes [Rh(cod)(μ-OSiMe3)]2, [Rh(cod)(μ-OSiMe2{CH=CH2})]2 and [Rh(cod)(PR3)(μ-OSiR'3)2] (R = Cy or Ph ; R' = Me or i-Pr) [77]. The advantage of these catalysts is that they are inactive at room temperature.

Marciniec et al. [78] proposed to perform hydrosilylation of 1,1,5,5-tetramethyl-3-trimethylsiloxy-3-vinyltrisiloxane with 1,1,1,3,5,5,5-heptamethyltrisiloxane in the presence of im-mobilized rhodium phosphine complexes [Rh(cod)(PR3)☼] (R = Cy, Ph, i-Pr; ☼ = Aerosil 200 silica) (100oC, toluene, 1 h, [SiH]/[H2C=CHSiMe2(OSiMe3)2]/[Rh] ratio = 1:1:10–4). Such catalysts withstand 5 runs of use without changing the activi-ty (yield 99%), followed by a decrease in the yield of prod-ucts of hydrosilylation (22–25% after 10 runs).

Series of surface rhodium siloxide complexes

[Rh(diene)(☼)] (diene = cod, norbornadiene, tetrafluoroben-zobarrelene; ☼ = Aerosil 200 and SBA-15 silica) has ap-peared to be the effective catalysts for hydrosilylation of 3-

vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane by 1,1,1,3,5,5,5-heptamethyltrisiloxane (100oC, 1 h, argon atmosphere, [SiH]/[H2C=CHSiMe2(OSiMe3)2]/[Rh] ratio = 1:1:10–4, yield of product 56–92 %) even after 21 runs without a decrease in the yield and selectivity [79–81]. At the same time, the con-tent of rhodium, for example, in the sample of [Rh(cod)( Aer-osil)] after 20 runs in this reaction was 72% of the initial amount of the metal [80, 81]. The best result occurs when using as a carrier Aerosil (Aerosil less porous than SBA-15, and therefore easier access of reactants to active centers), and the best ligand is norbornadiene [79].

Hydrosilylation of 1,1,5,5-tetramethyl-3-trimethylsiloxy-3-

vinyltrisiloxane with 1,1,1,3,5,5,5-heptamethyltrisiloxane in the presence of [Ir(cod)(μ-OSiMe3)]2, [Ir(cod)(PCy3)(OSiMe3)], [Ir(CO)(PPh3)2(OSiMe3)] and [Ir(CO)(PCy3)2(OSiMe3)] (80–120оС) leads to the formation of hydrosilylation product, which is accompanied by product of the dehydrogenative silylation and, in some cases, side prod-ucts. The side products are result of preliminary H/vinyl ex-change between reactants, followed by their hydrosilylation and dehydrogenative silylation [82].

Catalytic properties of the complexes depend on the na-

ture and conditions of the process. The dimeric iridium silox-ide complex shows high yield of hydrosilylation products un-der oxygenfree conditions but the hydrosilylation reaction is a strongly accompanied by the H/vinyl exchange of the sub-strates. Interestingly, exposure of the complex to air com-pletely stops the hydrosilylation process. Contrary to the di-meric complex, all phosphine monomeric complexes show much higher catalytic activity in the air than in oxygen-free conditions. This effect is a result of facile oxygenation and dissociation of phosphine ligand (more PCy3 than PPh3). Kownacki et al. [82] suggest that a dissociation of M–P bond

via oxygenation of tertiary phosphine ligand is the crucial step for generation of Ir(cod)H(vinylsiloxane) species, which is a key intermediate of hydrosilylation in the presence of iridium siloxide complexes. At the same time, the curing process of polysiloxanes catalyzed by [Ir(cod)(PCy3)(OSiMe3)] and [Ir(CO)(PCy3)2(OSiMe3)] occurs at a higher temperature (about 200oC) than the same system catalyzed by Karstedt-diallylmaleate system (130oC), and does not require an inhibi-tor to maintain low viscosity of the reaction mixture at room temperature for several days.

Brand et al. [83] performed a detailed kinetic study of the reaction between low-molecular hydro- and vinylsiloxanes

SiO

SiCH3

CH3

CH3

CH3

- CH2=CH2 [Rh]

SiCH3

CH3

SiO

SiCH3

CH3

CH3

CH3O

Si

CH3

CH3

SiO

Si

CH3

CH3

CH3

CH3

SiCH3

CH3

OSi

CH3

CH3 SiO

SiO

Si Si

CH3

CH3CH3

CH3

CH3 CH3

CH3CH3

Si

CH3

CH3

OSi

CH3

CH3

SiO

Si

CH3

CH3

CH3

CH3

SiCH3

CH3

OSi

CH3

CH3

x

y

x + y = 1-20

Si SiO OH

RhPR3

R = Cy, Ph, i-Pr

Si SiO OH

Rh

diene = cod, norbornadiene,tetrafluorobanzobarrelene

(Me3SiO)3Si

HMeSi(OSiMe3)2

[Ir](Me3SiO)3Si

SiMe(OSiMe3)2 (Me3SiO)3SiSiMe(OSiMe3)2

(Me3SiO)3SiSi(OSiMe3)3 (Me3SiO)3Si

Si(OSiMe3)3+

+

+ +


R(СH3)3-nSi[OSi(CH3)3]n (n = 1, 2; R = H or СН2=СН) in the presence of Karstedt’s catalyst (3−64 ррm, 20-500С) and identification of the reaction products by GC−MS and 1H, 13С, and 29Si NMR. Spectral analysis of the reaction mixtures showed that hydrosilylation in the considered systems gives mostly β-adducts (~ 97%) and only a little α-adducts. The activity of hydrosiloxanes decreases as their molecular weight increases; with vinylsiloxanes, a reverse dependence is ob-served.

Initially, the hydrosilylation rate follows zero-order kinet-ics, then, until 50% conversion, the reaction is first-order in catalysis, and above 50% conversions the following kinetic equation is valid: v = k·[SiH]·[SiVinyl]·[Pt]. The obtained kinetic results were explained in terms of the Chalk−Harrod mechanism, assuming different rate-limiting stages at differ-ent conversions. When the hydrosilylation rate is independent on catalyst concentration and temperature, the limiting stage involves olefin (vinylsiloxane) insertion into the М−H bond; the fact that the reaction order increases to 2 is explained by decelerated formation of an olefin complex and redox addition of hydrosiloxane. However, the mechanism proposed Brand et al. [83] is valid only for hydrosilylation in the presence of vinylsiloxane Pt(0) complexes, as, according to published data, square-planar Pt(II) complexes initially react with hy-drosilanes [15, 18, 84].

Dvornic et al. [85–87] have described the reaction of 3-vinyl-1,1,3,3-tetramethylsiloxane with 1,1,3,3-tetramethylsiloxane in the presence of Karstedt’s catalyst ([3.1–70]⋅10−5 mol of Pt/mol of CH=CH2, 25-56oC, toluene) for the synthesis of high molecular weight linear polymer (the molecular weight of up to 75900). The best results were ob-served when using a catalyst concentration of about 2⋅10−5

mol of Pt/mol of CH=CH2 and a reaction temperature of about 50oC. It was found that this polymer shows increased inher-ent molecular flexibility and reduced thermal and thermooxi-dative stability compared to the closely related poly(dimethylsiloxane) [86].

Kinetic studies of this reaction showed typical first order for the product formation [87]. The activation energy of hy-drosilylation is 78.2 kJ/mol. Rate of hydrosilylation increases with increasing concentration of catalyst and temperature of reaction, and depends on the equilibrium between binuclear Karstedt’s catalyst and these activated mononuclear Pt-complexes.

Nanush'yan and Gol'tsev [88] studied the catalytic activity of divinylsiloxane platinum complexes in hydrosilylation of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane with 1,1,1,3,5,5,5-heptamethyltrisiloxane. It was found that the reaction is first-order in catalyst and that the catalytic activity is more affect-ed by additional metal−ligand bonding than the inductive effect of substituents in the siloxane molecule [88].

Souchek et al. [89] made use of the competitive reaction technique to study the reactivity of vinylsiloxanes of the gen-eral formula (СH2=СН)(СH3)3-nSi[ОSi(CH3)3]n (n = 0−3) and 1-vinyl-1,3,3,5,5,7,7-heptamethylcyclotetrasiloxane in their reactions with 1,1,3,3,3-pentamethyldisiloxane in the pres-ence of [Rh(PPh3)3Cl] and, for comparison, in the presence of H2PtCl6. It was shown that the reactivity increases as the number of the (CH3)3SiO groups increases in the order:

(СH2=СН)(СH3)3Si > (СH2=СН)(СH3)2SiOSi(CH3)2 > > (СH2=СН)(СH3)Si[ОSi(CH3)3]2 >

(СH2=СН)Si[ОSi(CH3)3]3 Furthermore, the deviation of the dependence of the

log(relative addition rate constant) on the number of the (CH3)3SiO groups from linearity shows that the activity of the vinyl group is controlled by electronic factors. With increasing number of the electron-acceptor (CH3)3SiO groups, the posi-tive charge on silicon increases, thereby decreasing the elec-tron density of the carbon atoms of the vinyl group. Analysis of the relative rate constants of hydrosilylation of a cyclic

siloxane and 3-vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane led the authors of [89] to a conclusion that the activity of the vinyl group is affected by electronic rather than steric factors.

The major hydrosilylation products are β-adducts [80], and, at the same time, Wilkinson’s catalyst exhibits a higher selectivity compared to Speier’s catalyst (5⋅10−5 mol/mole hydrosiloxane, 90oС, 2 h).

The reactivity of dihydrosiloxanes H(CH3)2SiO[(CH3)2SiO]nSi(CH3)2H (n = 0−3) in hydrosi-lylation of hept-1-ene and 1-vinyl-1,3,3,5,5,7,7-heptamethylcyclotetrasiloxane was considered in [90]. The reaction of siloxanes at 90oС for 2 h in the presence of [Rh(PPh3)3Cl] (1⋅10−4 mol/mole vinylsiloxane) occurs by Farmer’s rule and forms mono- and diadducts. Analysis of the relative hydrosilyation products showed the reactivity of dihy-drosiloxanes decreases as their molecular weight increases

[H(СH3)2Si]2O > [H(СH3)2SiO]2Si(CH3)2 > > H(СH3)2SiO[(CH3)2SiО]2Si(CH3)2H >

H(СH3)2SiO[(CH3)2SiО]3Si(CH3)2H Andrianov et al. [91] studied the addition of 1,1,3,3,3-

pentamethyldisiloxane to hex-1-ene, 3-vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane, and 1-vinyl-1,3,3,5,5,7,7-heptamethylcyclotetrasiloxane in the presence of platinum, rhodium, palladium, cobalt, and nickel complexes. Palladium and nickel complexes, specifically [Ni(cod)2] and [Pd(C6H5CN)2Cl2], effectively catalyze hydrosilylation of conju-gated diene systems, but proved to be unsuitable for siloxane systems because of the fast reduction to metals which did not work as catalysts. Even though hydrosilylation conditions were not optimized for each catalyst, it was found [100oС, Сcat (1−10) ⋅10−5 M] that rhodium complexes ([Rh(PPh3)Cl], [Rh(CO)2(acac)], and [Rh(PPh3)2(CO)Cl]) are more selective than platinum complexes (H2[PtCl6], [Pt(PPh3)4], and [(MeCN)2PtCl2]); with the palladium complex [Pd(PPh3)2Cl2], traces of products were only detected after 2 h, and the bisphosphine nickel(II) complex was completely inactive. The hydrosiloxane addition to the linear vinylsiloxane was found to occur more selectively than to the cyclic vinylsiloxane. The [Ni(cod)2] complex doped with phosphines (PPh3 and PPh2Me), too, was inactive. The highest catalytic activity in hydrosilylation of siloxanes was found to be characteristic of the rhodium acetylacetonate complex.

Comparative experiments on the addition of hydrosilox-anes to vinylsiloxanes in the presence of carbonyl complexes of nickel and cobalt revealed an appreciable activity of Co2(CO)8 at high concentrations (1⋅10−2 mol/mole hydrosilox-ane, 20oС, 36 h), whereas [Rh(CO)2(acac)] worked already at the concentration of 1⋅10−4 mol/mole hydrosiloxane; [Ni(CO)4] did not catalyze the reaction even at 50oС.

Analysis of the data in [91] shows that the linear vi-nylsiloxane is more active that the cyclic vinylsiloxane. The activity of the catalysts, assessed by the conversion 1,1,3,3,3-pentamethyldisiloxane − 3-vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane decreases in the order [Rh(PPh3)3Cl] > [Rh(CO)2(acac)] > [Pt(PPh3)4] > H2[PtCl6] >>

Co2(CO)8 In the system 1,1,3,3,3-pentamethyldisiloxane − vinylcy-

closiloxane, the following catalyst activity order was obtained [Pt(PPh3)4] > [Rh(CO)2(acac)] > [Rh(PPh3)3Cl] >

[Rh(PPh3)2(CO)Cl] >> > H2[PtCl6] > [(MeCN)2PtCl2] >> [Pd(PPh3)2Cl2] The work of Gorshkov and co-workers [92] who explored

the behavior of silicon hydrides, as well as polyfunctional hydro- and vinylsiloxanes in the presence of H2PtCl6⋅6Н2О and tetraorganylammonium salts of the general formula ([C8H17]3RN)2[Pt(X)n] (X = NO2, n = 4, R = C6H5CH2, p-ClC6H4CH2, CH2CH=CH2, CH2CH=CH2C(CH3)3, C8H17; X = Cl, n = 4 or 6, R = C8H17) deserves mentioning. Analysis of the hydrosilylation kinetics (toluene, 50−120oС, СCat 4⋅10−6 −2⋅10−4 M) showed that the reactivity of cyclic vinylsiloxanes


(CH2=CHMeSiO)n (n = 3-5) and a linear vinylsiloxane Me3SiO(CH2=CHMeSiO)20SiMe3 toward HEtSiCl2 decreases with enhancing steric hindrance to reaction: (CH2=CHMeSiO)3 > (CH2=CHMeSiO)4 > (CH2=CHMeSiO)5 >

> Me3SiO(CH2=CHMeSiO)20SiMe3 It was suggested that the reactivity of hydrosiloxanes to-

ward (EtO)3Si(CH=CH2), too, is associated with the steric demand in the siloxane, and Me3SiO(HMeSiO)20SiMe3 is less reactive than (HMeSiO)4 [92]. The authors noted that residual Si–H groups strongly decelerated hydrosilylation at conver-sions higher than 75% and explained this phenomenon by steric hindrance of the newly formed CH2CH2Si(OEt)3 groups and its enhancement with increasing conversion. The prevail-ing reaction pathway was β-addition. It was also found that tetraorganylammonium salts are slightly less active than chlo-roplatinic acid. The greatest difference was observed in the system (CH2=CHMeSiO)4 − 4(EtO)3SiH, on which grounds the R radicals in the ammonium ligand were arranged in the fol-lowing order in terms of their effect on the catalytic activity:

C8H17 > C6H5CH2 > CH2CH=CH2C(CH3)3 > p-ClC6H4CH2 > CH2CH=CH2

The lowest catalytic activity of [(C8H17)3(CH2CH=CH2)N]2[Pt(NO2)4] was explained by addi-tional coordination of the allyl group with platinum. The na-ture of the anionic ligand was found to only slightly affect the catalytic activity, whereas the increase of the oxidation de-gree of the central atom from 2 to 4 decreased the reactivity

by a factor 1.2. The least selective addition was observed in the presence of H2PtCl6. The lowest Еа was observed with [(C8H17)3(C6Н5СН2)N]2[PtСl6] (74.6 kJ/mol) and the highest, with [(C8H17)3(C6Н5СН2)N]2[Pt(NO2)4] (134.0 kJ/mol).

Proceeding with the research on the catalytic activity of tetraorganylammonium platinates (R4N)2Pt(X)m (X = Cl, NO2, I, Br, NCS; m = 4-6; R = CnH2n+1 [n = 1–8], C6H5, C6H5CH2, C8H17, p-ClC6H4CH2, CH2CH=CH2, CH2CH=CH2C(CH3)3, C8H17), Gorshkov and co-workers [93] performed curing of the dime-thyl vinyl silicone rubber (brand SKTV-M) with oligoethylhy-drosiloxane by the hydrosilylation reaction. The general regu-larities established previously for hydrosilylation in low-molecular systems were found to be also valid for the compo-sition in study, but the metal complexes proved to larger vary in activity. The rate effect of the acido ligand decreased in the order: Cl– NO2

– > Br– > I– > NCS–. Monitoring of the rate of cross-linking and Si−H consumption showed that the catalyst efficiency increases to approach that of H2PtCl6 as the tem-perature is raised from 100 to 200ºС.

Kopylov and co-workers [92, 93] suggested, based on va-por osmometry data and hydrosilylation kinetics, that the salt form of the catalyst remains unchanged in the course of the reaction, whereas chloroplatinates(IV) are first reduced and then catalyze hydrosilane addition. The proposed mechanism of catalysis involves the formation of a platinum hydride com-plex as the main stage

L = Cl, Br, I, NO2, SCN; R = C6H5, C6H5CH2, C8H17, p-ClC6H4CH2, CH2CH=CH2, CH2CH=CH2C(CH3)3, C8H17; R' = Н or (CH2=CHSiMeO)n,

Me3Si(OSiMeCH=CH2)19(OSiMe)OSiMe3; R'' = (CH2=CHSiMeO)n, Me3Si(OSiMeCH=CH2)19(OSiMe)OSiMe3 or Н; n = 2-4. We performed a comparative study of the catalytic activity

of bornyl, menthyl, and triethylammonium platinates (Am)2[PtCl6] [Am = (+)-BornylNH3, (-)-MenthylNH3, Et3NH], and (+)-(BornylNH3)2[PtCl4] using the bifunctional siloxanes 1,1,3,3-tetramethyldisiloxane and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane at a 1:1 molar ratio [94]. Hydrosilylation

involves both one and two active groups (Si–H or Si–СH=СН2), leading to α-, β-, and diadducts (NMR, IR, GCMS, and GLC data) which fact transform into higher molecular adducts (viscosity > 0.34 Pa·s). Initially, one 1,1,3,3-tetramethyldisiloxane molecule is added, largely by Farmer’s rule:

Am = (+)-BornylNH3, (-)-MenthylNH3 or Et3NH

The resulting monoadducts then fast transform into into

β,β-adducts (major products) and α,α- and α,β-adducts (minor products). The activity of the

catalyst, as measured by the time required for [(СH2=СН)Me2Si]2O to reach 50% conversion decreases in the following order:

(–)-(MenthylNH3)2[PtCl6] > (Et3NH)2[PtCl6] > (+)-(BornylNH3)2[PtCl4] > (+)-(BornylNH3)2[PtCl6].

The full conversion of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane is the most rapid (470 min) in the pres-ence of (Et3NH)2[PtCl6], and it is the slowest (>10 h) in the presence of (+)-( BornylNH3)2[PtCl4]. The selectivity of β-addition (1Н NMR data) varies in the following order:

(+)-(BornylNH3)2[PtCl4] (96%) > (+)-(BornylNH3)2[PtCl6] (93%) ≈ (–)-(MenthylNH3)2[PtCl6] (92%).

The above siloxanes were also hydrosilylated in the pres-ence of Pt(II) and Pt(IV) complexes of the general formulas

PtLL

L L

R4N 2 Pt

LL

L L

HSi

R4N 2 Pt

HL

L L

R4N 2 Pt

HL

L L

R4N 2 SiH

-LSi

R'HC=CHR''

R'HC=CHR''

PtCHL

L L

R'CH2R''

R4N 2 SiH

L

L L

R'CH2R''

HSi

CHPt

R4N 2 Pt

HL

L L

R4N 2

R'

CH2R''CHSi+

H

Si

O

Si

H

CH3 CH3

CH3 CH3

CH

Si

O

Si

CH

CH3 CH3

CH3 CH3

CH2

CH2

CH2 Si O Si CHCH3

CH3

CH3

CH3

CH2H Si O SiCH3

CH3

CH3

CH3

CH2

Si O Si CHCH3

CH3

CH3

CH3

CH2H Si O Si

CH3

CH3

CH3

CH3

CHCH3

+ +(Am)2[PtCln]

n = 4, 6


R2-n[Pt(L)nX6-n] and R2-n[Pt(L)nX4-n] [n = 0, 1; R = K+, Na+, H+, NH4

+, Et3NH+; L = Me2SO, Et2SO, i-Pr2SO; X = Cl–, NCS–], K[Pt(D-Alanine)Cl2], cis-[Pt(L′)2Cl2] and trans-[Pt(L′)2X′4] (L′ = Me2SO, Me2S, Et=S; X′ = Cl–, Br–) and octamethylcyclotetra-siloxane as internal reference to show that the major reaction pathway involves the β-carbon atom of the vinyl group [95].

The efficiency of nitrogen-containing catalysts, as meas-ured by the reaction rate and inductive period, decreases in the order:

Et3NH[Pt(i-Pr2SO)Cl5] > (Et3NH)2[PtCl4] > (Et3NH)2[PtCl6] > (NH4)2[PtCl4] > (NH4)2[PtCl6]

Unlike what is observed with ammonium salts, with potas-sium salts the influence of oxidation state of the central atom on the catalytic activity of the Pt(II) and Pt(IV) complexesis not so obvious. Thus, potassium Pt(IV) complexes having no neutral ligands in the inner sphere are more active than Pt(IV) complexes. At the same time, if an inner sphere dime-thyl sulfoxide ligand is present, Pt(IV) complexes are more active than Pt(II) complexes. The most catalytically active in the reaction in focus was sodium hexachloroplatinate(IV). The catalytic activity of Pt(IV) complexes in the following order of outer-sphere cations [95]:

Na+ > H+ > Et3NH+ > NH4+ > K+

The catalytic activity of octahedral and square-planar plat-inum complexes with inner-sphere ligands has some special features. The rate of hydrosilylation in the presence of Pt(IV) complexes with inner sphere sulfoxide ligands is higher com-pared to similar Pt(II) complexes. At the same time, bis-sulfoxide platinum(IV) complexes are less efficient hydrosi-lylation complexes than their square-planar analogs. There-with, the nature of the acido ligand has almost no effect on the activity and selectivity of the catalysts [95].

The hydrosilylation of (СH2=СН)Me2SiOSiMe3, 3-vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane, 1,3-divinyl-1,1,3,3-tetramethyldisiloxane, and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane with 1,1,1,3,5,5,5-heptamethyltrisiloxane, 1,1,3,3-tetramethyldisiloxane, and 1,1,3,3-tetramethyldisiloxane in the presence of square-planar platinum complexes [Pt(LL’)X2] [L = RR′SO; R = Me, Et; R′ = Me, Et, p-Tol; L′ = RR′SO, Py, α-Pic, α-aminopyridine (α-NH2Py), CH2=CH2, Ph3PS; LL’ = cod, 1-methylcycloocta-1,5-diene (MeCOD); X = Cl–, Br–, NO3

–; X2 = C2O42–] yields β-

and α-adducts whose ratio depends on the structure of the reagents and the thermodynamics of the process, as well as on the type of the catalyst [84]

L = RRSO; R = Me, Et; R = Me, Et, p-Tol; L = RRSO, Py, -Pic, -NH2Py, CH2=CH2, Ph3PS;

LL’ = сod, MeCOD; X = Cl–, Br–, NO3–; X2 = C2O4

2–

The reaction in a bifuctional siloxane medium (two Si–H

or Si–СН=СН2 groups) involves either one or both active groups to form β-, α-, or diadducts. Therewith, with all the complexes studied, monoadducts underwent further trans-formations to give β,β-adducts (major products) and α,α- and α,β-adducts (minor products), as well as dehydrocondensa-tion products. Analysis of the kinetics of the catalytic hydrosi-lylation of siloxanes showed that the reactivity of vinyl- and hydrosiloxanes is much dependent on the electronic and ste-ric characteristics of the substituents on the silicon atom. In the presence of sulfoxide Pt(II) complexes, the reactivity of hydrosiloxanes at an equimolar reagent ratio decreases in the order [84]

(HMe2Si)2O >> HMe2SiOSiMe3 > HMe2Si(OSiMe3)2 The reactivity of hydrosiloxanes decreases in parallel with

increasing positive charge on the silicon atom, which favors shortening of the Si−H bond and its strengthening. Moreover, steric hindrance, too, enhaces in going from (HMe2Si)2O to HMe2Si(OSiMe3)2, and this further decreases the activity of HMe2Si(OSiMe3)2 compared to disiloxanes.

The reactivity of vinylsiloxanes in the presence of plati-num complexes increases with increasing electron density on the silicon atom at the vinyl double bond [84]

(СH2=СН)Me2SiOSiMe3 >> (СH2=СН)Me2Si(OSiMe3)2 > [(СH2=СН)Me2Si]2O

Study of the effect of anionic ligands on the catalytic ac-tivity of bis-sulfoxide Pt(II) complexes is favored by weakly coordinated ligands. The reaction rate decreases in the fol-lowing order [84]

C2O42- > NO3

- > Cl- >> Br-.

It was found that the effect of neutral ligands on the cata-lytic activity of platinum complexes in hydrosilylation of low-molecular siloxanes decreases in the following order [84]

Ph3PS > Me-p-TolSO > MeCOD > CH2=CH2, cod > Et2SO > Et2S >> Me2SO >> -NH2Py > Py > -Pic.

This order corresponds to decreasing σ-donor and in-creasing π-acceptor power of the neutral ligands. Analysis of the catalytic activities of the geometric isomers of the (-)[Pt(Me-p-TolSO)PyCl2] complex revealed a higher activity of the cis-isomer in hydrosilylation in siloxane systems, com-pared to the trans-isomer.

It was found that the reaction of sulfoxide Pt(II) complex-es with hydro- and vinylsiloxanes leads to isomerization of the complexes and dissociation of the sulfoxide, whereas similar reactions with bis-sulfoxide complexes result in deoxygenation of the coordinated sulfoxide or to formation of a platinum colloid. The whole body of evidence gave us grounds to sug-gest that the first stage of the catalytic run with all types of sulfoxide complexes involves the reaction of the cis-isomer of the metal complex with a hydrosiloxane, leading to incorpora-tion of the latter into the coordination sphere of platinum via replacement of the sulfoxide ligand [84]. The trans-isomers first undergo isomerization and then sulfoxide replacement takes place. Side processes, specifically deoxygenation and isomerization of the cis-isomers of sulfoxide-containing metal complexes, decelerate hydrosilylation and sometimes delay the reaction onset.

“Colloidal” catalysts which are produced by the reactions of hydrosilanes or hydrosiloxanes with Pt(0) or Pt(II) com-plexes in aprotic solvents can form a good alternative for homogeneous catalysts in the case of hydrosilylation in silox-ane systems [96]. The activity of hydrosiloxanes in the pres-ence of the platinum colloid obtained by the reaction of hex-yldiphenylphosphine platinum complex with triethoxysilane decreases in the order [12]

Me3SiOSiMe2H > (Me3SiO)2SiMeH > (Me3SiO)3SiH

Si OCH3

SiCH3

R''CH3

R H CH SiCH3

R'CH3

Si OCH3

R''CH2

CH2 SiCH3

R'CH3

Si O

CH3

R''Si OCH3

SiCH3

R''CH3

R CH2 SiCH3

R'

CH3

Si OCH3

R''Si OCH3

SiCH3

R''CH3

R CHCH3

R = H, R'' = CH3; R = CH3, R'' = CH3, OSi(CH3)3; R' = CH=CH2, R'' = CH3; R' = CH3, R'' = CH3, OSi(CH3)3

[PtLL'X2]+

+


The activity of vinylsiloxanes in their reaction with silox-anes in the presence of the same platinum colloid depends on the electronic environment of the silicon atom attached to the vinyl group, decreasing in the order:

Me3SiOSiMe2СН=СН2 > (Me3SiO)2SiMeСН=СН2 > (Me3SiO)3SiСН=СН2

Dihydrosiloxanes slower react with Me3SiOSiMe2(СН=СН2) than siloxanes with one Si−H bond [12]

(Me3SiO)2Si(H)OSiMe3 > Me3Si(OSiMeH)2OSiMe3 Me3SiOSiMe2H > HMe2SiOSiMe2H

Faltynek [97] proposed zero-valent nickel complexes of the general formula NiXG (X = bidentate cyclic alkenes С8−12; G = mono- and bidentate phosphorus-containing groups con-taining hydride bonds; substituted or unsubstituted hydrocar-bon radicals or phosphorus-containing groups with different atoms attached to phosphorus) as catalysts of hydrosilylation of polysiloxane systems. Such complexes successfully catalyze this reaction, and it occurs already at room temperature to give organosilicon elastomers.

Meuser and Mignani [98] described the synthesis of cata-lysts for hydrosilylation of siloxanes, specifically platinum complexes (n = 1, 2; R, R′, R′′, R′′′ = alkyl and/or acyl radi-cal, for instance, СН3 or С1-С18; L = ligands like maleic acid or its derivatives: amides, imides, or anhydrides bearing differ-ent substituents or bridged to each other, as well as free and substituted benzo- and naphthoquinones). The starting mate-rials were a toluene solution of Karstedt’s catalyst and a dou-ble molar excess of ligand L. Metal complexes of a similar structure or containing olefin ligands (norbornene, cyclohex-ene, or ethylene) instead of 1,3-divinyldisiloxane, too, are catalytically active in siloxane systems and allow preparation of network polymers and modification of siloxane siloxane oils [99].

Fang et al. [100] reported a readily recoverable platinum

catalyst immobilized on an inorganic support with SiEt groups, which exhibits a high catalytic activity in hydrosilylation with hydrosilanes or hydrosiloxanes of unsaturated silanes or si-loxanes.

The reaction of 1,1,3,3-tetramethyl-1,3-divinyldisiloxane in the presence of Pt(II) immobilized on polymethylene sulfide was studied in [101]. Hydrosilylation initially formed α- and β-adducts of 1,1,3,3-tetramethyldisiloxane at one vinyl group of 1,3-diviny-1,1,3,3-tetramethylldisiloxane, the β-adduct pre-vailing. Then the resulting monoadducts fast converted into α,α-, α,β-, and β,β-adducts (major product) which, in their turn, underwent further hydrosilylation (in 5 h at 50ºС the molecular weight of the monomers was 850−900). The tem-perature regime of the catalysis with Pt(II) chemisorbed on polymethylene sulfide (> 50ºС) depends both on the concen-tration of immobilized platimun (optimally, 8.5⋅10-5 mol/g sorbent) and on the concengtration of platimun in the reac-tion mixture (optimally, 5.95⋅10-5 M). The selectivity of addi-tion is 87% at СPt 8.5⋅10-5 mol/g polymethylene sulfide. The activation energy of the hydrosilylation reaction catalyzed by

immobilized platinum is 82.2 ± 1.5 kJ/mol (lnk0 25.6) [101], which is 3−10 times higher than in the hydrosilylation of si-loxanes in the presence of homogeneous catalysis.

One of the possible ways to affect the activity of catalysts

is to expose them to photo- irradiation. Two photoactivation techniquies are applied with metal complexes: photoassis-tance and photoinduction. Photoassistance involves continu-ous irradiation of a precatalyst (a real catalyst is considered to be formed directly in the reaction medium) for maintaining its catalytic activity.

A typical example of the inductive effect of light is the re-ductive photodissociation of the ligand in a Pt(II) acety-lacetonate complex (50 ppm) on exposure to visible light, forming a highly active 14-electron platinum center, which was studied on an example of gel formation (84 s) in a mix-ture of 37.5 parts of polydimethylsiloxane with vinyl terminat-ed groups and 2.5 parts of polymethylhydrosiloxane with trimethylsilyl terminated groups [102].

Another interesting example of photoactivation of metal complexes, related to the inductive effect of light, is the near-UV irradiation of (η5-cyclopentadienyl)trialkylplatinum(II) in the presence of polyaromatic sensitizers absorbing visible light, leading to formation of colloidal platinum which exhibits a high catalytic activity in the reaction of hydrosiloxanes RnSi[(OSiR2)pR]4-n (R = H, cycloalkyl, Ph, ≤ 50% R = H; n = 0-3, p = 1–3000) with vinylsiloxanes RmSi[(OSiR2)tR]4-m (R′ = halogenated cycloalkyl groups, Ph, halogenated alkenyl groups; t = 1–3000; m = 0–3) [103].

Under pulse irradiation (hν 710 nm) trimethyl(methyl-η5-cyclopentadienyl)platinum(II) successfully catalyzes microfab-rication of 3D polydimethylsiloxanes [104]. Having absorbed two photons, the catalyst preserved activity even when the radiation source was removed; such two-photon initiation led to formation of a poorer resolved network but favored better cross-linking of the polysiloxane surfaces with each other. Unlike what is observed with the platinum catalyst, the reac-tion in the presence of isopropyl thioxanthone (radical polymerization initiator, hν 780 nm) is a multiphoton process involving no dark hydrosilylation, which provides access to a greater diversity of high-resolution microstructured materials [104].

An example of the inductive effect of light on catalysts is provided by the hydrosilylation of -1,3-divinyl-1,1,3,3-tetramethyldisiloxane and hept-1-ene with 1,1,3,3-tetramethyldisiloxane and heptamethylcyclotetrasiloxane in the presence of photoactivated rhodium(I) complexes, pri-marily [Rh(PPh3)3Cl] (light with λ 300−450 nm) [105]. It was found that hydrosilylation is appreciably accelerated com-pared to the dark process and the irradiation efficiency is much enhaced by air oxygen due to oxidation of the dissoci-ated phosphine into a weakly coordinating phosphine oxide.

The [HRh(Ph3P)2Cl(SiМе2Сl)] complex proved to be the

most efficient among the rhodium catalysts studied, and its efficiency did not depend on temperature (25−70ºС) and irradiation and compared with the efficiency of irradiated Wilkinson’s catalyst (70ºС) [105]. The reaction catalyzed by

[Rh(PPh3)3Cl] was suggested to occur via a mononuclear oxo complex [Rh(PPh3)3(О2)Cl]⋅nCH2Cl2 rather than a dimeric oxo complex [Rh(PPh3)3(О2)Cl⋅CH2Cl2]2, even though in the latter case the key stage of catalyst generation, specifically phos-phine dissociation, is more facile. Therewith, after completion

SiO

Si

R'''

R''

R'R

Pt L

n

SCH2

S CH2

CH2

SCH2

SCH2

CH2

Pt

n

[Rh(R3P)3Cl] [Rh(R3P)2Cl] + R3PO2[Rh(R3P)2Cl]R3PO +

hArh


of irradiation, air oxygen oxidized phosphine into phosphine oxide, thereby preventing the reverse reaction and providing a high concentration of the rhodium catalyst. The reduced

efficienct of this catalyst in the subsequent dark process was associated with the formation of a silicon−rhodium complex.

We performed a GLC and 1Н NMR study of the reaction of

1,1,1,3,5,5,5-heptamethyltrisiloxane and 1,1,3,3-tetramethyldisiloxane with 3-vinyl-1,1,1,3,5,5,5-heptamethyltrisiloxane and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane in the presence of the rhodium(I) com-plexes [Rh(Ph2PCH=CH2)2(CO)Cl] and [Rh(PPh3)3Cl] and thermo- and photoactivated platinum(II) complexes [Pt(R2R′Х)2Cl2] (Х = P, R = Me, R′ = Ph; Х = P, As, Sb, R = R′ = Ph or Bu) [106]. The effect of neutral ligands on the activity of Pt(II) catalysts in the thermally induced hydrosilyla-tion in low-molecular siloxane systems decreases in the fol-lowing order:

SbPh3 > AsPh3> PPh3 > PPhMe2 > PBu3

The hydrosilylation rate changes inconsiderably but slight-ly decreases in the following order of complexes [106]:

cis-[Pt(PMe2Ph)2Cl2] > cis-[Pt(PPh3)2Cl2] > cis-[Pt(PBu3)2Cl2]

The addition rate in the presence of cis-[Pt(PPh3)(Py)Cl2] is lower than in the presence of cis-[Pt(PPh3)2Cl2]; the least active was Wilkinson’s catalyst. The activation energy of the reaction of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane with a triple molar excess of 1,1,3,3-tetramethyldisiloxane in the presence of cis-[Pt(PPh3)2Cl2] in the fast hydrosilylation region was 10.2 ± 0.8 kJ/mol (lnk0 0.89).

The photoinduced addition of hydrosilanes to vinylsilox-anes in the presence of bis-phosphine Pt(II)-complexes (wide-band light, Pyrex filter) occurs at a slightly higher rate compared to the dark process and has an appreciably shorter induction period [106]. Detailed study of hydrosilylation in the presence of a photoactivated complex cis-[Pt(PPh3)2Cl2] showed that 1 min irradiation is the most favorable in terms of the hydrosilylation rate.

Phosphine ligands quite differently affect the efficiency of photoactivated catalysts in siloxane systems compared to the dark process; for example, the induction period decreases in the following order of phosphine ligands

PPh3 > PBu3 >> PMe2Ph This order parallels the order of the ability of organosili-

con compounds to reduce the phosphine complexes HMe2SiOSiMe3 > (HMe2Si)2O >> MeSiHCl2 >>

[(СH2=СН)Me2Si]2O The mechanism of hydrosilylation in the presence of

phosphine Pt(II) complexes suggests an attack of the hy-drosiloxane on the cis-complex, which results in expulsion of the neutral ligand from the inner coordination sphere of Pt(II) to form a η2-silicon hydride complex. Subsequently, coordina-tion of the vinylsiloxane takes place and reaction with a fur-ther hydrosilane molecule to give reaction products and re-generate the catalyst.

[Rh(PPh3)3Cl] [Rh(O2)(PPh3)3Cl]

[RhH(PPh3)2(SiR3)Cl][Rh(PPh3)2Cl]

[Rh(PPh3)2Cl]2 [RhH(PPh3)2(SiR'3)Cl]

O2

O2

(O2, h

HSiR3,

HSiR3

HSiR'3R

Rh(PPh3)2Cl

R

Rh(PPh3)Cl

H SiR'3

R

RSiR'3

HSiR'3

PPh3 PPh3

PPh3

R

)h


The mechanism of hydrosilylation in the presence of pho-

toactivated catalysts involves initial photodissociation of the ligand, leading to formation of a coordinately unsaturated platinum center, and the latter then reacts with the hy-

drosiloxane. Further conversions of the η2-hydrosiloxane complex occur, according to [106], like those shown in the dark hydrosilylation scheme.

One more example of the inductive effect of light is pro-

vided by the hydrosilylation of divinyltetramethyldisiloxane with pentamethyl- and tetramethyldisiloxane in the presence of photoactivated alkene- and sulfoxide-containing Pt(II) complexes [Pt(MeCOD)Cl2], (–)-cis-[Pt(Mе-p-TolSO)2Cl2], cis- and trans-(–)-[Pt(Me-p-TolSO)PyCl2], and (–)-cis-[Pt(Me-p-TolSO)(CH2=CH2)Cl2] [107]. Detailed study of the effect of photoactivation time on the rate and selectivity of hydrosilyla-tion showed that hydrosilylation selectivity is almost inde-pendent of photoactivetion time, whereas the rate of β,β-adduct formation is directy related to photoactivation time. The strongest effect of irradiation was observed with the bis-sulfoxide complexes in vire of their higher sensitivity to light quanta. Photolysis of these complexes results in the dissocial-tion of the neutral lugand to form a coordinately unsaturated center which, in their turn, immediately reacts with siloxanes, yielding real hydrosilylation catalysts. At the same time, pho-

toactivation of trans-complexes was unsuccessful, which is associated with their inability for photodissociation [106].

The strongest effect of the rate of photoinduced hydrosi-lylation is observed in the case of the bis-sulfoxide complexes containing the second photosensitive center (the oxalate lig-and). The effect of the anionic ligand environment of the activity of photoactivated bis-sulfoxide catalysts decreases in the order [107]

C2O42– > NO3

– > Cl– >> Br– The practical application of photoactivation in hydrosilyla-

tion has been recently reported. For example, Mistele [108] has patented a hermetic with improved adhesion to plastic, containg a silicon fluid, Si−H and vinyl-containing polysilox-anes, and a hydrosilylation photocatalyst.

Thus, the catalytic hydrosilylation considered in the pre-sent review is a convenient synthetic approach to a great variety of siloxanes, including high-molecular siloxanes, which are attracting permanent interest. The reactivity of hydro-

L'

SiH

Si SiCH2CH2

PtLHlg

HlgL'SiH

PtHlgL

HlgL'

HlgL

HlgSiH

PtSiH SiSi

HlgL

HlgHH

Pt

HOSi

OH2

HlgL

HlgSiH

Pt

SiCHCH2

SiCHCH2

Si CH CH2HlgL

HlgSiH

Pt

SiH

SiH

SiCHCH3

Si

L, L' = ligands

- L'

H-SiPt

HlgL

HlgL'

XL

XSiH

PtPtHlgL

HlgSolv

hSolv


and vinylsiloxane os controlled by three main factors: molecu-lar weight, steric effect, and electron-donor power of the substituent on the silicon atom (their increase have a nega-tive reactivity impact); the reactivity of linear vinylsiloxanes is higher compared to cyclic vinylsiloxanes, and, moreover, it is dependent of the structure of the catalyst. The contribution of side reactions associated with hydrosilylation, i.e. dispropor-tionation of hydro- and vinylsiloxanes and dehydrocondensa-tion of hydrosiloxanes, depends on the activity and structure of the catalyst (the lower activity the less side reactions), but sometimes side reactions play a great role in the formation of the 3D network of the resulting siloxane oligomers and poly-mers.

Most researchers prefer to experiment with commercial Speier’s and Karstedt’s catalysts. However, such approach has certain drawbacks associated with difficultly controlled reac-tion rate and a long induction period of hydrosilylation reac-tions in the presence of those catalysts. The alternative effi-cient catalysts for hydrosilylation in siloxane systems are al-kene-, phosphorus-, and sulfur-containing platinum complex-es which allow the curing regime of siloxane compositions to be varied over a wide range and enhance the addition selec-tivity in most cases. Rhodium(I), nickel, cobalt and iron com-plexes are less efficient in siloxane systems, whereas palladi-um complexes are almost inactive. Of undeniable interest is hydrosilylation in the presence of photoactivated metal com-plexes; however, this catalyst activation technology has not yet been commercialized, probably, because of the complicat-ed instrumentation and lack of commercial efficiency. At the same time, the works concerning the mechanism of hydrosi-lylation in siloxane systems are few in number, and their re-sults are controversial. However, over the past years a certain tendency of researchers is observed to agree that the key stage stage of hydrosilylation is the coordination of hy-drosiloxane, leading to formation of a real catalyst of the pro-cess.

In any case, hydrosilylation in siloxane systems is far form being understood, and the potential of possible catalysts is far from being exhausted. The most promising approach seems to consist in the search for new platinum catalysts with lig-ands capable of coordinating silicon hydrides.

The work was financially supported by the Russian Foun-dation for Basic Research (project nos. 13-03-00890a).

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D.A. de Vekki1 2 METAL COMPLEX CATALYZED …science.spb.ru/files/BulletinIT/2013/1/Articles/20/files/assets/downloads/publication.pdfIntramolecular polymerization of organosiloxanes