Polymerization initiators containing thio bridges

A process for the manufacture of reactive polymers of vinyl monomers by reacting the latter, or the polymers formed therefrom, with compounds which form free radicals. The latter compounds are tetrakis-(organylthio)-methanes, 1,1,2,2-tetrakis-(organylthio)-ethanes or hexakis-(organylthio)-ethanes. The polymers obtained can readily be modified, e.g. by oxidation or quaternisation.

The homopolymerization or copolymerization of monomeric vinyl compounds, 
e.g. ethylene, styrene, vinyl chloride, acrylic acid esters and others, by 
means of initiators which form free radicals, has been disclosed. The 
initiators employed are in general either azo compounds, e.g. 
azodiisobutyronitrile, or organic per-compounds, e.g. peroxides, 
hydroperoxides, peresters, peroxydicarbonates and others. In many cases it 
is desirable to vary the properties of the polymers obtained, to make them 
suitable for particular applications. However, the vinyl polymers 
manufactured with the aid of conventional initiators as a rule do not 
undergo reactions easily and can, at best, be modified under more or less 
extreme conditions. 
It is an object of the present invention to provide a method of manufacture 
of polymers of conventional vinyl monomers, which gives polymers which can 
undergo further reactions under comparatively mild conditions. 
We have found, surprisingly, that this object is achieved by using certain 
organic compounds, containing thio bridges, as initiators during the 
polymerization and/or as modifiers after the polymerization. 
Accordingly, the invention relates to a process for the manufacture of 
polymers of vinyl compounds by polymerizing the monomeric vinyl compounds, 
individually or as mixtures with one another, in the presence or absence 
of a preformed polymer, by means of initiators consisting of compounds 
which form free radicals, with or without subsequent modification of the 
resulting polymers by reacting them with compounds which form free 
radicals, wherein the compounds, which form free radicals, which are used 
during the polymerization and/or for the modification reaction are organic 
compounds, containing thio bridges, of the general formula (I) 
##STR1## 
where R.sup.1, R.sup.2 and R.sup.3 may be identical or different and each 
is --S--alkyl, --S--cycloalkyl or --S--aryl, each substituent R.sup.1, 
R.sup.2 and R.sup.3 being of up to 14 carbon atoms, and any 2 of the 
substituents may be linked to form a ring, and R.sup.4 is --S--alkyl, 
--S--cycloalkyl or --S--aryl, each of up to 14 carbon atoms, or is 
##STR2## 
but, where R.sup.4 is 
##STR3## 
R.sup.3 may also be hydrogen or an --Si(--X).sub.3 group, where X is 
linear alkyl of 1 to 3 carbon atoms. 
The compounds containing thio bridges, which are used according to the 
invention, are tetrakis-(organylthio)-methanes, 
1,1,2,2,-tetrakis-(organylthio)ethanes and hexakis-(organylthio)-ethanes. 
S-alkyl, S-cycloalkyl and S-aryl are to be understood as meaning both the 
unsubstituted groups and groups in which the hydrocarbon radical is 
substituted. These include, for example, mixed aliphatic-aromatic thio 
groups, e.g. S-aralkyl and S-alkaryl, and mixed aliphatic-cycloaliphatic 
groups, as well as S-alkyl, S-cycloalkyl or S-aryl where the substituents 
are, e.g., alkoxy or halogen. The number of carbon atoms in S-alkyl is 
preferably from 1 to 10, especially from 1 to 4. It is particularly 
advantageous if alkyl in S-alkyl is linear; preferably, alkyl is methyl or 
ethyl. S-cycloalkyl is preferably of 5 to 10 carbon atoms, and is 
particularly advantageously S-cyclohexyl, where cyclohexyl may or may not 
be substituted by alkyl, alkoxy or halogen. S-aryl is preferably of 6 to 
10 carbon atoms. Aryl in S-aryl may be, for example, phenyl, tolyl, 
tert.-butylphenyl, xylyl, methoxyphenyl or ethoxyphenyl, 
halogen-substituted phenyl or naphthyl. If two of the substituents R.sup.1 
to R.sup.4 in the compounds containing thio bridges, of the general 
formula (I), are linked to form a ring, the link is in general via a 
methylene bridge or particularly via an ethylene bridge between the two 
sulfur atoms of the particular adjacent substituents R.sup.1 to R.sup.4. 
If the substituent R.sup.3 is --Si(--X).sub.3, it is in particular 
trimethylsilyl. 
One category of compounds containing thio bridges, of the general formula 
(I), which are suitable for use according to the invention is that where 
all substituents R.sup.1 to R.sup.4 are S-alkyl, S-cycloalkyl and/or 
S-aryl. Examples of this category of tetrakis-(organylthio)-methanes are 
tetrakis-(methylthio)-methane (CH.sub.3 S--).sub.4 C and 
tetrakis-(phenylthio)-methane (C.sub.6 H.sub.5 S--).sub.4 C. The use of 
tetrakis-(phenylthio)-methane has proved particularly advantageous. 
In addition to the tetrakis-(organylthio)-methanes, the 
1,1,2,2-tetrakis-(organylthio)-ethanes and above all the 
hexakis-(organylthio)-ethanes, i.e. the compounds of the general formula 
(I), where the substituent R.sup.4 is 
##STR4## 
are of importance for use as polymerization initiators in accordance with 
the invention. The 1,1,2,2-tetrakis-(organylthio)-ethanes and 
hexakis-(organylthio)-ethanes accordingly correspond to the general 
formula (II) 
##STR5## 
where the substituents R.sup.1 to R.sup.3 have the initially stated 
meaning and any two of these substituents may be bonded to one another to 
form a ring. The following are specific examples of this category of 
product which are suitable for use according to the invention: 
the hexakis-(arylthio)-ethanes of the general formula (III) 
##STR6## 
where Z is hydrogen (IIIa), p-methyl (IIIb), o-methyl (IIIc), 
p-tert.-butyl (IIId), p-methoxy (IIIe) or fluorine or chlorine (IIIf and 
IIIg); hexakis-(.beta.-naphthylthio)-ethane; 
hexakis-(cyclohexylthio)-ethane; hexakis-(methylthio)-ethane; 
1,1,2,2-tetrakis-(phenylthio)-1,2-bis-(methylthio)-ethane of the formula 
(IV) 
##STR7## 
1,1,2,2-tetrakis-(methylthio)-1,2-bis-(trimethylsilyl)-ethane of the 
formula (V) 
##STR8## 
and compounds with cyclic sulfur atoms, e.g. of the type of the general 
formula (VI) 
##STR9## 
The compounds, containing thio bridges, of the general formula (I), to be 
used, according to the invention, as polymerization initiators, are known 
per se and described in the literature, e.g. in Chemische Berichte 105 
(1972), 3,892-3,904, Tetrahedron Letters 1970, 1,933-1,936, Angewandte 
Chemie 79 (1967), 468-469 and J. Am. Chem. Soc. 93 (1971), 2,258-2,268. 
According to these publications, the tetrakis-(organylthio)-methanes may 
be obtained, for example, by reacting tris-(organylthio)-methyl-lithium 
with an appropriate diorganyldisulfide. The hexakis-(organylthio)-ethanes 
of the general formula (II) are formed in good yield when 
tris-(organylthio)-methyl-lithium, in solution, is reacted with iodine at 
low temperature, in accordance with the general equation: 
EQU 2 R.sub.3 C--Li + I.sub.2 R.sub.3 C--CR.sub.3 + LiI 
where R is organylthio. Further details regarding the manufacture of the 
compounds, containing thio bridges, which are to be used according to the 
invention are to be found in the literature. 
It is true that the literature already disclosed the dissociation of such 
compounds of the general formula (I), to form free radicals. However, it 
was surprising that the radicals, containing thio bridges, which are thus 
formed are suitable initiators for the polymerisation of vinyl monomers 
and permit the manufacture of high molecular weight polymers, since it has 
been disclosed that sulfur-containing compounds as a rule act as 
polymerization inhibitors or polymerization regulators. In contrast, when 
using the compounds, containing thio bridges, of the general formula (I) 
as polymerization initiators, no inhibiting, retarding or regulating 
effect is observed when polymerizing vinyl monomers, and using these 
compounds as initiators under otherwise comparable polymerization 
conditions results in polymers formed in the same yield, and having the 
same molecular weight, as when using conventional peroxides or azo 
compounds as initiators. 
Not only are the compounds, containing thio bridges, of the general formula 
(I) advantageous initiators -- because of their advantageous half-life, 
e.g. of about from 0.1 to 10 minutes at 100.degree. C. (depending on the 
medium) -- for the free-radical polymerization of vinyl monomers and the 
grafting and/or crosslinking of previously produced polymers, but they 
also permit the simultaneous introduction of thio groups into the polymer. 
The compounds, containing thio bridges, of the general formula (II) 
decompose, for example, in accordance with the following equation: 
##STR10## 
Since the initiator radicals are built into the polymer during 
polymerization, the resulting polymers also contain sulfur bridges. 
Because of the presence of the sulfur groups, these polymers are capable 
of further reactions and can be modified by such reactions, e.g. oxidation 
or quaternization, under comparatively mild conditions. 
The compounds, containing thio bridges, of the general formula (I) may be 
employed as initiators for the polymerization of vinyl monomers, for which 
they are used in amounts of from 0.05 to 5% by weight, preferably from 0.1 
to 2.5% by weight, based on the monomers to be polymerized. Suitable vinyl 
monomers for polymerization by means of the said initiators are, for 
example, monoethylenically or diethylenically unsaturated olefins, 
preferably of 2 to 6 carbon atoms, especially butadiene and isoprene; 
styrene and side chain-substituted and nuclear-substituted styrenes, e.g. 
.alpha.-methylstyrene, vinyltoluene and p-chlorostyrene; alkyl acrylates 
and alkyl methacrylates, where alkyl is preferably of 1 to 8 carbon atoms, 
e.g. the acrylic acid esters or methacrylic acid esters of methanol, 
ethanol, butanol or 2-ethylhexanol; vinyl esters of carboxylic acids, e.g. 
vinyl acetate and vinyl propionate; acrylonitrile and methacrylonitrile; 
vinyl chloride, vinylidene chloride and others. The vinyl monomers may be 
used individually or as mixtures with one another. The polymerization of 
the vinyl monomers or vinyl monomer mixtures may, if desired, be carried 
out in the presence of a previously produced polymer; this is preferably a 
rubbery polymer having a glass transition temperature of below 0.degree. 
C., especially of below -20.degree. C. Examples of suitable rubbery 
polymers are those based on conjugated dienes or alkyl acrylates. The 
polymerization of the vinyl monomers by means of the compounds, containing 
thio bridges, of the general formula (I) as initiators may be carried out 
as a mass polymerization, solution polymerization, suspension 
polymerization or emulsion polymerization, under the conventional 
conditions. The known conventional additives and auxiliaries for the 
various polymerization processes may be employed, e.g. protective 
colloids, emulsifiers, buffer systems, antioxidants, lubricants, slip 
agents and others. Mass polymerization or solution polymerization are 
preferred. In the latter case, inert diluents are employed, preferably in 
amounts of up to 50% by weight, based on the monomers. Examples of 
suitable inert diluents are alcohols, ketones, esters, ethers, e.g. 
tetrahydrofuran, and aliphatic, cycloaliphatic or aromatic hydrocarbons. 
The polymerization temperature, which may vary within a wide range, 
depends principally on the nature of the initiator and is in general 
between 30.degree. and 170.degree. C., preferably between 50.degree. and 
150.degree. C. 
It is also possible to modify finished polymers, e.g. olefin polymers, 
styrene polymers or polymers of conjugated dienes, e.g. polybutadiene, by 
means of the compounds, containing thio bridges, of the general formula 
(I). For this purpose, the polymers are reacted, preferably in the 
presence of a solvent or diluent, with the compounds containing thio 
bridges, at temperatures at which there is a significant decomposition of 
the latter compounds, whereby the groups containing sulfur bridges are 
introduced into the polymers.

The Examples which follow illustrate the invention; parts and percentages 
are by weight. The stated molecular weights of the polymers are 
number-average molecular weights determined osmometrically. The compounds, 
containing thio bridges, which were employed, were prepared by the process 
described in Chemische Berichte 105 (1972), 3,892-3,904. 
EXAMPLES 1 to 7 
100 parts of styrene (freshly distilled under very pure nitrogen and 
degassed for 3 hours at 50.degree. C. under 0.1 mm Hg), are mixed with one 
part of initiator of the type indicated in Table 1 and heated for 5 hours 
at 100.degree. C. Table 1 shows the yields and molecular weights of the 
polymers obtained. 
COMATIVE EXAMPLES A to C 
The polymerization of Example 1 was repeated, but in the absence of an 
initiator (Comparative Example A), using lauroyl peroxide (Comparative 
Example B) and using azodiisobutyronitrile (Comparative Example C) as the 
initiator. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Example/ 
Comparative Yield Molecular 
Example Initiator parts weight 
______________________________________ 
##STR11## 25 6,000 
2 (CH.sub.3 S).sub.4 C 
26 5,700 
3 
##STR12## 33 6,100 
4 
##STR13## 35 4,600 
5 
##STR14## 42 6,100 
6 
##STR15## 66 7,100 
7 
##STR16## 75 8,100 
A without initiator 12 6,900 
B lauroyl peroxide 49 5,800 
C azodiisobutyronitrile 
75 6,400 
______________________________________ 
The polymers obtained as described in Examples 1 to 7 and Comparative 
Examples A to C were dissolved in toluene, precipitated with methanol and 
then dried (for 8 hours at 80.degree. C. under 2 mm Hg), and the analyses 
were carried out on the samples treated in this way. 
The polymers from Examples 1 to 7 contained sulfur, and the content of the 
latter remained constant even after two reprecipitations, indicating that 
the sulfur formed part of the molecule. 
The Comparative Examples A to C show that, surprisingly, the compounds 
containing thio bridges, when used as initiators, all give a higher yield 
than Comparative Experiment A, which is carried out without initiator, 
whilst the molecular weight of the product is virtually comparable; this 
means that the sulfur in the compounds containing thio bridges in this 
case exerts virtually no inhibiting or regulating action on the 
polymerization and, on the contrary, gives results which are comparable to 
those with conventional initiators (Comparative Experiments D and C). 
EXAMPLES 8 to 10 
If the procedure described in Example 7 is followed but the styrene is 
replaced by the monomers shown below, the results listed in Table 2 are 
obtained. 
TABLE 2 
______________________________________ 
Yield in Molecular 
Example Monomer parts weight 
______________________________________ 
7 Styrene 75 8,100 
8 n-Butyl acrylate 
82 9,400 
9 2-Ethylhexyl acrylate 
81 9,600 
10 Methyl methacrylate 
79 8,000 
______________________________________