Selective cyclopolymerization

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

Zirconocene complexes with ferrocenyl groups promote selective cyclopolymerization of 1,5-hexadiene to give a polymer with high content of the trans-unit (up to 98% trans selectivity) (Eq. 21) [100]. Sita reported living... 

Enantioselective Radical Polymerization. The 2,2 -azo(bis)isobutyronitrile/copper(n) triflate/chiral diamine ligand system was used as an asymmetric reverse atom transfer polymerization initiating system for the enantiomer-selective cyclopolymerization of (25,4S 2/ ,4/5-2,4-pentanediyl dimethacrylate. Results indicate that the asymmetric reverse ATRP initiating system was effective for the enantioselective radical cyclopolymerization, leading to optically active polymers. Three different chiral diamines were used as ligand including (—)-sparteine. 

Recently, an extremely selective cyclopolymerization has been developed by using specially designed difunctional perfluoromonomers such as perfluoroallyl vinyl ether (13). This monomer is readily derived from the carboxylated perfluorovinyl ether according to the scheme shown above, and characterized by the different reactivities of the two double bonds, that is, a vinylether type and an allyl type. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

As for olefins different from propene, molecular modeling studies have also been able to rationalize the dependence on metallocene symmetry of E-Z selectivity for 2-butene copolymerization as well as the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings, for cyclopolymerization of nonconjugated dienes. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

Optically active catalyst 1 can be obtained either by enantiomer-selective reaction of rac.-2 with optically active lithium (l,l -binaphthyl)-2,2 -diolate or by direct resolution by chiral HPLC. Optically active 21 and 22 in addition to 1 were successfully obtained by HPLC resolution and used for the polymerization of 1,5-hexadiene [60-62], Both catalysts gave an optically active polymer through cyclopolymerization. The optical activity and the content of tranj-structure in the main chain of the polymers obtained with 21 and 22 were comparable with those of the polymers synthesized with 1 [61,62],... 

Abstract Metathesis-based polymerizations of 1-alkynes and cyclopolymerizations of 1,6-heptadiynes using late transition metal catalysts are reviewed. Results obtained with both binary, ternary, and quaternary catalytic systems and well-defined molybdenum- and ruthenium-based catalysts are presented. Special consideration is given to advancements in catalyst design and mechanistic understanding that have been made in this area over the last few years advancements that have facilitated tailor-made syntheses of poly(ene)s. In addition, the first supported ruthenium-based cyclopolymerization-active systems are summarized. Finally, selected structure-dependent properties will be outlined where applicable. 

Diisocyanobenzene derivatives yield helical polymers via a cyclopolymerization mechanism by the polymerization with Pd and Ni complexes. Optically active polymers were initially obtained by the method illustrated in Figure 8.139 143 Monomer 66 was reacted with an optically active Pd complex to form diastereomeric pentamers 67, which were separated into (+)- and (—)-forms by HPLC. The polymerization of 68 using the separated 69 led to a one-handed helical polymer.139 The polymerization of 68 using the initiators having chiral binaphthyl groups, 69—71, also produced optically active polymers.142 The helix-sense selectivity in the polymerization using 69... 

Another approach to the study of the cyclic units formed in these cyclopolymerizations is through the study of the radical cyclization reaction of selected model compounds. Thus, extensive studies have shown that the five-membered ring is predominant, while new evidence indicates that radical stability exerts a marked influence on the ring size. These conflicting aspects of cyclopolymerization have been discussed in detail by Butler(L7) he has pointed out that considerably more investigations will be necessary before definite conclusions can be drawn with respect to the ratio of five- to six-membered rings in the many cyclopolymers already synthesized, and a satisfactory explanation for these extensive variations from one system to another is available. 

Thus, we investigated in detail the factors influencing intramolecular addition modes in the radical cyclopolymerization of some unconjugated dienes, including diallyl and dimethallyl dicarboxy-lates, acrylic and methacrylic anhydrides, and allyl -substituted acrylates and discussed the selectivity of reaction mode of intramolecular hh or ht addition in terms of the thermodynamical standpoint. The present article gives a summary of our recent work including published papers(18-23). 

In conclusion, the structure of poly(divinyl formal) was shown to vary with the polymerization conditions (Table I). It is expected that highly stereoregular polymers are obtainable by selecting proper polymerization comditlons. These results will be described elsewhere, together with the kinetic analysis of the cyclopolymerization process. 

On the other hand, the other direction of addition (yd-addition) produces a monosubstituted alkylidene that is sterically comparable to the initiating alkylidene, making this the desired direction of addition. Formation of a six-membered ring in a reaction involving a more reactive terminal alkylidene would transform 43b into 44b. The NMR spectrum of poly-6 reveals two carbonyl carbon resonances. Therefore it was speculated that poly-6 consist of a random distribution of five- and six-membered rings formed through what is nominally tail-to-tail and head-to-head cyclopolymerization of the two acetylenic bonds in the same monomer (Scheme 1). Other uncertainties in catalyst systems of the type are the rate at which alkylidene rotamers interconvert, the extent to which the reactivities of the two alkylidene rotamers differ, and the degree of selective formation of one rotamer when a triple bond reacts with a Mo=C bond. ... 

Whereas poly(a-olefins) have only two microstructures of maximum order (isotactic, syndiotactic). cyclopolymers ° have /bur microstructures due to the rings present in their main chain which can be either cis or trans in configuration (Scheme 17). While the key issues concerning selectivity in the polymerization of a-olefins are regioselectivity (head-to-tail monomer incorporation) and enantioface selectivity (tacticity). cyclopolymerization of a.co-diolefins has added concerns. First, since the monomer has two olefins, either cyclization or cross-linking of the... 

Molybdenum imido alkylidene complexes also have been employed for a variety of other catalytic reactions of interest to the organic or polymer chemist, among them selective cross-couplings of olefins, [99] polymerization of terminal alkynes, [100-102] step-growth polymerization of dienes, [103,104] and cyclopolymerization of 1,6-heptadiynes. [105-107]... 

Belokon and co-workers (50,51) attributed their template effects (see "Distance Accuracy of Two Functional Groups") to the existence of cyclopolymerization, since polymers of low crosslinking also showed a good selectivity. What takes place is an intrachain reaction rather than an interchain one. Only in one case have we observed a similar behavior (23,49,52). In our case a cyclo-polymerization was proved by copolymerization of 3.4-0-isopropylidene-D-mannitol 1, 2,5,6-bis-O-[(4-vinylphenyl)... 

Enantiomer selection has been found for a dimethacrylate cyclopolymerization under living free radical polymerization condition using CuBr complexed with (-)-Sp. ... 

Chain Transfer to Organometallic Compounds. Hydroxy-terminated poly(methylene-l,3-cyclopentane) has been prepared by selective chain transfer to aluminum in the cyclopolymerization of 1,5-hexadiene with the sterically... 

Recently, Choo and Way mouth performed the copolymerization of ethylene with 1,5-HD using various metallocene catalysts (12, 13, 14, Figure 19.2). 1,5-HD cyclopolymerized exclusively to give MCP units in the copolymers, with only traces of uncyclized 1,2-inserted 1,5-HD. The diaste-reoselectivity of the cyclocopolymerization favored the formation of 1,3-cyclopentane rings for metallocenes (74% trans for 12, 81% trans for 13, and 66% trans for 14). For metallocenes 12 and 14, the ethylene/1,5-HD copolymerization yielded copolymers with similar comonomer compositions and sequence distributions to those observed for ethylene/1-hexene copolymerization with these catalysts. On the other hand, the copolymers derived from metallocene 13 showed very different compositions and sequence distributions. At comparable comonomer feed ratios, the poly(ethylene-c -l,5-HD)s were enriched in the 1,5-HD comonomer and deficient in ethylene as compared to the analogous polymers prepared from ethylene and 1-hexene. The copolymerization behavior of 13 provided support for a dual-site alternating mechanism for 1,5-HD incorporation, wherein one coordination site of the active catalyst center is highly selective for the initial 1,2-inserion of 1,5-HD and the other site is selective for cyclization. 

In addition to simple a-olefins such propylene, Sita and coworkers showed that lla-c/ [PhNMe2H][B(C6F5)4] were active for the cyclopolymerization of 1,5-hexadiene at —10°C (Jayaratne et al, 2000). The polymers produced possessed >98% MCP units, and exhibited narrow polydispersities MJM = 1.03-1.09). The selectivity of ring closure was ubiquitously trans, the stereoselectivity increasing with increasing steric bulk of the amidinate ligand (lla % trans = 6A llc % trans = S2). 

Figure 23 The metallocene-mediated cyclopolymerization of 1,5-hexadiene can be (a) trans selective or (b) cis selective, depending on the catalyst structure, (c) The cyclopolymerization of a,a>-dienes occurs efficiently for 1,5-hexadiene, 1,6-hepfadiene, and 1,7-ocfadiene.

Polymerization of divinyl compounds (e.g., divinylbenzene) under usual acidic conditions results in cross-hnked insoluble polymers. By selecting reaction conditions, however, it is possible to obtain non-cross-linked, soluble polymers in which cyclic repeating units are incorporated into the main chain. One of these processes is the so-called cyclopolymerization (Eq. (9)) that involves an intramolecular cycliza-tion of the propagating species. Abundant examples of this, both radical and ionic, are known... 

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