Styrene/vinylcyclohexane

Other polymers that have been subjected to studies of thermal decomposition mechanisms include synthetic rubbers [24], polyarylether ketone [25], three polyarylates based on bisphenol-A, l,l-dichloro-2,2-bis(4-hydroxyphenyl) ethylene and 4,4"-dihydroxy-3-ethoxy-benzylidenoacetophone [26], polylactide [27], polyethylene composites [28], polyvinylcyclohexane and styrene-vinylcyclohexane... 

The presumed 14 electron Ti(C5Me5)(2,4-C7Hn), Ti(Ph2CH)2, and Ti(Ph2C-SiMc3)2 complexes (Section IIIA), as well as a variety of other open and half-open titanocenes, have been reported to yield, upon activation with MAO and/or poorly coordinating borate ions, catalysts for the polymerization of ethylene/a-olefin mixtures, styrene, vinylcyclohexane, vinylcyclohexene, butadiene, and related spedes." The polystyrene was isolated predominately in the syndiotactic form. 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

In 1985, Warwel and Winkelmiiller reported a series of catalyst systems for the CM of either styrene or 4-vinylcyclohexane with unfunctionalized olefins (Scheme 9). Using heterogeneous catalyst systems of RceOy/ AI2O3, among others, the authors demonstrated that both a substrate s electronic and steric properties govern CM product selectivity. Unfortunately, as the stereoselectivities of these reactions were not reported, the effect of a secondary allylic carbon on olefin stereoselectivity was not determined. Nevertheless, the non-statistical product distribution obtained in these reactions constitutes the first example of a product selective CM reaction. 

Although the isospecific polymerisation of styrene monomers has much less steric demands for the Ziegler-Natta catalysts than that of x-oldins, it proceeds with much lower propagation rate constants by comparison with the polymerisation of x-olefins for example, on a molar basis, styrene is ca 100 times less reactive than propylene in the polymerisation [30,33], Also, compare the relatively slow polymerisation of styrene and other vinylaromatic monomers with the relatively fast polymerisation of vinylcyclohexane [20,31,34-36]. 

Fully saturated SBC polymers have also been investigated. Vinylcyclohex-ane-ethylene/propylene-vinylcyclohexane triblock copolymers have been prepared by complete hydrogenation of SIS polymers using a supported palladium catalyst [53]. Under the appropriate conditions, hydrogenation of the styrene blocks can also be accomplished using Ziegler-type catalysts [54]. 

A few attempts have been made to copolymerize vinylcyclohexane with other monomers using coordination-type polymerization catalysts. The successful copolymerization of styrene and vinylcyclohexane [50] with a Zeigler-type catalyst has been reported. The copolymerization of the closely related monomer 4-vinylcyclohexene with ethylene using a Zr-based metallocene and methy-lalumoxane activator has been described [51]. 

Fig. 3.5. Pyrogram of a mixture of natural, butadiene-styrene and butadiene rubbers. 1 = Beginning of experiment (pyrolysis) 2 = butadiene 3 = isoprene 4 = vinylcyclohexane 5= styrene 6 = dipentene. From ref. 69.

Fig. 3.10. Characteristic peak-area ratio versus pyrolysis temperature for Curie-point cell. 1 and 3 = mixtures of homopolymers (polybutadiene and polystyrene) 2 and 4 = statistical copolymer of styrene and butadiene (Europrene 1500). 1 and 2 = ratio of peak areas of styrene and vinylcyclohexane 3 and 4 = ratio of peak areas of styrene and butadiene. From ref. 108.

Naturally, the more complex the composition of the substances to be pyrolysed, the more characteristics are needed for identification. For example, in identifying isoprene rubbers (NK, SKN-3, SKIL, Natsyn, Coral, Cariflex IR), the characteristic pyrolysis products are isoprene and dipentene, whereas with butadiene rubbers (SKB, SKD, Budene, Diene NF, Buna CB, Asadene NF, Cariflex BR, Ameripol CB) they are butadiene and vinylcyclohexane. With copolymer rubbers, the number of characteristic products necessary for identification increases to three, viz., butadiene, vinylcyclohexene and styrene are used for butadiene -styrene rubbers (SKS-10, SKS-30, Buna S. Europrene-1500, Solprene) and butadiene, vinylcyclohexene and methylstyrene are used for butadiene-methylstyrene rubbers (SKMS-10, SKMS-30) [139, 140]. Fig. 3.12 [139, 140] shows as an example pyrograms of individual general-purpose rubbers and a four-component mixture of rubbers. The shaded peaks correspond to those components in the pyrolysis products which are used for identification. The ratio of the pyrolysis products changes depending on the composition of the copolymer and the structure of the polymer. 

Fig. 3.12. Pyrograms of some commonly used rubbers and their mixtures. (A) Pyrogram of a mixture of rubbers isoprene (SKI), butadiene (SKD), butadiene-styrene (SKS), butadiene-methylstyrene (SKSM). (B) SKS. (C) SKD. (D) SKI. 1 = Butadiene 2 = isoprene 3 = vinylcyclohexane 4 = dipen-tene 5 = styrene 6 = methylstyrene. From refs. 139 and 140.

In polymer production the water content in the starting monomer and in solvents is of great importance with respect to product quality. The proposed method was used to determine water in some diene and olefin monomers (isoprene, styrene, heptene, vinylcyclohexane, etc.), which are normally inert with respect to lithium aluminium hydride solutions. The method was also apphed to the determination of water in ethers and cyclic esters. 

For instance, the reaction of EtaSiH and 2 equiv. of p-methoxystyrene in toluene with 1.0 mol% of 16a afforded at 100°C within 6 h the dehydrogenative silylation product ( )-l-(p-methoxystyryl)-2-(triethyl-silyl)ethylene in 95% yield. The reaction is of high selectivity that neither (Z)-isomers, nor branched dehydrogenative silylation products were seen. Less hydridic silanes, such as triphenylsilane, were less efficient than for instance EtsSiH. Other substituted styrenes such as p-methyl, p-chloro-, and p-fluorostyrene also afforded the corresponding tran -vinylsilanes in high yields and selectivities (up to 98%). In the case of aliphatic alkenes, such as -octene, allyltriethoxysilane, vinylcyclohexane, and ethylene, dehydrogenative silylations were still preferred, but showed less E/Z selectivity. Cyclic olefins, such as cyclooctene, furnished low conversions under the same reaction crmditions. The results are summarized in Scheme 19. 

Polymerization of propylene with these catalysts yields polymers with very little ciystallinity. Higher a-oletins yield completely amorphous polymers. Formation of partially crystalline polymers was reported for vinylcyclohexane, allylcyclohexane, and 4-phenyl-1-butene. Styrene does not polymerize at all. Isoprene forms a trans-lA polymer. 

In a complementary paper by Class and Chu, model resins - polystyrene and poly(vinylcyclohexane) - in combination with natural and styrene-butadiene rubbers, were used to study effects of resin structure, molecular weight and concentration on viscoelastic properties of pressure-sensitive adhesives resulting from these combinations. 

Ammendola, P Tancredi, T. Zambelh, A. Isotactic polymerization of styrene and vinylcyclohexane in the presence of a C-enriched Ziegler-Natta catalyst Regioselectivity and enantioselectivity of the insertion into metal-methyl bonds. Macromolecules 1986,19,307-310. 

Note that based on earlier experimental studies from Sharpless group, it was concluded that some n-n stacking may bring important consequences for tile reaction enantioselectivity when unsaturated substrates are used. For example, whereas dihydroxylation of styrene afforded the product witii 97% ee and vinylcyclohexane afforded the product with 84% ee. The value 84% for nonsaturated substrate can be explained in terms of weak C(sp )-H... ji hydrogen-bonding interactions between a substrate and a catalyst. Similar C(sp )-H...n hydrogen-bonding interactions between the... 

A comparison of these scales with Taft s induction constants a and Taft s steric constants for different alkyl groups leads to the conclusion that both electronic and steric factors influence the doublebond activity in stereospecific catalysis but that steric factors seem to be more important. An especially significant decrease in the olefin activity was found for olefins branched vicinally to the double bond (3-methyl-pentene-1,3-methylbutene-l, vinylcyclohexane, styrene). This is protebly connected with the space limitations for monomer coordination in the stereospecific active sites (IS, 16). 

The examination of rj t2 values for hundreds of different comonomers polymerized by different mechanisms (2) reveals that in the overwhelming majority of cases these r, r2 values are close to or less than 1 very few examples of ionic processes were found with rjr2>l (J69). For this reason the appearance of a significant number of cases with rir2>t can be regarded as characteristic of complex catalysis. The mentioned tendency is especially pronounced when the comonomers have alkyl groups of different size (ethylene—4-methylpentene-l, pro-pyIene-butene-1, propylene-styrene, propylene-4-methylpentene-l, pro-pylene-vinylcyclohexane). On the other hand, when the alkyl groups are of similar bulkiness (4-methylpentene-l-vinylcyclohexane, 4-methyl-pentene-l-3-methylbutene-l, vinylcyclohexane-styrene), the copolymers obtained are mainly random or have a tendency to alternation. 

Zaikin, V.G., Mardanov, R.G. etal. (1990) Pyrolysis-gas chromatographic/mass spectrometric behaviour of polyvinylcy-clohexane and vinylcyclohexane-styrene copolymers. /. Anal Appl Pyrolysis, 17, 291. 

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