Methyl-1,3-pentadienes, polymerization

Only a definite scope of monomers can be polymerized in a given host. This indicates that inclusion polymerization displays a sterically different boundary condition from other polymerizations. That is, an increase of one methylene unit of one monomer can induce an inhibition of the polymerization in a given channel (Fig. 5a,b). Moreover, the relative sizes of the channels change the polymerizabilities of the identical monomers. Even though a monomer does not polymerize in a small channel, the same monomer polymerizes in a larger channel, (Fig. 5b,c). For example, 4-methyl-1,3-pentadiene polymerized in the smaller channels of deoxycholic acid. 

FIGURE 17.14 End groups detected in poly (4-methyl-1,3 pentadiene) polymerized with CpTiCl3/MAO in the presence of A1( CH3)3 2,1-insertion (A) l,4-insertion (B) (P = polymer chain). 

SCHEME 17.1 Schematic representation of the monomer insertion process in 4-methyl-1,3-pentadiene polymerization that leads to the two different polymer end groups A and B (L = ancillary ligand). 

Longo, R Grisi, R Proto, A. ZambelU, A. Chemoselectivity in 4-methyl-l,3-pentadiene polymerization in the presence of homogeneous Ti-based catalysts. Macromol. Rapid Commun. 1997, 18, 193-190. 

One of the most important results of inclusion polymerization is the synthesis of optically active polymers from nonchiral compounds. Asymmetric polymerization of /ra 5-pentadiene in PHTP has been reported. The optical purity of the polypentadiene is about 7%. DCA and ACA, as natural hosts, induce a greater asymmetric polymerization. The cis and rra/i5-2-methyl-pentadiene gave the highest asymmetric polymerization values [88]. The optical rotatory power disappeared with temperatures higher than 70°C, indicating that this process is reversible and results from a conformational transformation. 

The 1,4 polymerization of trans-1.3-pentadiene has been studied by Natta, Porri and coworkers. Their results also show that the cis polymerization occurs with catalysts of ionicities in the middle region, while the trans structures come in the more ionic regions. The methyl group at the end of the diene systems shifts the transition points... 

The 1.4-cis polymerization of 1.3-pentadiene offers a second type of steric control. The methyl group of the new monomer can be sterically oriented by the methyl groups at the end of the growing polymer chain. Isotactic cis polymers can be obtained by a planar six membered ring... 

When polymerizing 1.3-pentadiene, the optical activity of the isotactic cis-1.4-polymer obtained in the presence of catalysts prepared from (—)-titanium tetramenthoxide and aluminum triethyl, is far higher than that of the analogous polymers obtained with catalysts prepared from titanium tetrabutylate and (+)-tris-[(S)-2-methyl-butyl]-aIumin-um (05). 

Diene Polymers Polymerization of a 1,3-diene yields a polymer having true asymmetric centers in the main chain and ozonolysis of the polymer gives a chiral diacid compound (12) whose analysis of optical purity discloses the extent of chiral induction in the polymerization (Scheme 11.2) [12,35-39], The polymerization of methyl and butyl sorbates methyl and butyl styrylacrylates and methyl, ethyl, butyl, and /-butyl 1,3-butadiene-1-carboxylates using (+)-2-methylbutyllithium, butyllithium/(-)-menthyl ethyl ether, butyllithium/menthoxy-Na, butyllithium/bomeoxy-Na, butyllithium/Ti((-)-menthoxy)4, and butyllithium/bomyl ethyl ether initiators [35-37] and that of 1,3-pentadiene in the presence of... 

I, 3-diene polymerization. Monomer molecules are included in chiral channels in the matrix crystals, and the polymerization takes place in chiral environment. The y-ray irradiation polymerization of trans- 1,3-pentadiene included in 13 gives an optically active isotactic polymer with a trans-structure. The polymerization of (Z)-2-methyl-1,3-butadiene using 15 as a matrix leads to a polymer having an optical purity of the main-chain chiral centers of 36% [47]. 

Inomata 211) studied the H-NMR spectra of poly(penta-1,3-diene) and concluded that with hexane as polymerization medium the polymers were about 49% cis-1,4 and 40% trans-1,4 enchained. The polymer derived from the cis monomer had 12% of 1,2-units which were exclusively trans that from the trans monomer had some 10 % of 1,2-units, two thirds of which were trans. Aubert et al.2I6) made a more extensive study of pentadiene polymers using both1H and 13C-NMR spectroscopy and modified the cis and trans-1,4 methyl resonance assignments made by Inomata 2U). 

This group covers polymeric peroxides of indeterminate structure rather than polyfunctional macromolecules of known structure. These usually arise from autoxidation of susceptible monomers and are of very limited stability or explosive. Polymeric peroxide species described as hazardous include those derived from butadiene (highly explosive) isoprene, dimethylbutadiene (both strongly explosive) 1,5-p-menthadiene, 1,3-cyclohexadiene (both explode at 110°C) methyl methacrylate, vinyl acetate, styrene (all explode above 40°C) diethyl ether (extremely explosive even below 100°C ) and 1,1-diphenylethylene, cyclo-pentadiene (both explode on heating). 

Early studies on the homopolymerization of E-l,3-pentadiene yielded polymers with a high cis-1,4-content and an isotactic structure, whereas E-2-methyl-l,3-pentadiene resulted in a polymer with a mixed czs-1,4/transit-structure [487-492]. Investigations on the polymerization of E-1,3-pentadiene with the system NdN/TIBA/DEAC partially support these findings as a poly(l,3-pentadiene) with a cis- 1,4-threo-disyndiotactic structure was obtained [492]. A somewhat lower cis- 1,4-content of 70% was obtained when the polymerization of E-l,3-pentadiene was catalyzed by (CF3COO)2NdCl/TEA [493,494]. When 2,3-Dimethyl-1,3-butadiene is polymerized with the catalyst NdN/TIBA/EtAlC the resulting poly(2,3-dimethyl-butadiene) predominantly contains cis-1,4-units [495,496]. 

E-3-Methyl-l,3-pentadiene was polymerized using NdO/TIBA/DEAC and isotactic polymers with a high crystallinity were obtained. The polymers essentially consisted of cis-1,4 units (> 80%). Isotactic czs-l,4-poly(3-methyl-1,3-pentadiene) co-exists in two morphological structures [497]. 

A study on the homo- and copolymerization of a variety of dienes such as 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, E-l,3-pentadiene, E-l,3-hexadiene, E-l,3-heptadiene, E-l,3-octadiene, E,E-2,4-hexadiene, E-2-methyl-l,3-pentadiene, 1,3-cyclohexadiene mainly focused on mechanistic aspects [139]. It was shown that 1,4-disubstituted butadienes yield frans-1,4-polymers, whereas 2,3-disubstituted butadienes mainly resulted in cis- 1,4-polymers. Polymers obtained by the polymerization of 1,3-disubstituted butadienes showed a mixed trans-1,4/cis-1,4 structure (60/40). The microstructures of the investigated polymers are summarized in Table 26. 

In later studies on the homopolymerization of E-l,3-pentadiene with NdO/ TIBA/DEAC crystalline polymers with cis- 1,4-contents in the range 84-99% and a high isotacticity were obtained. It was found that the cis- 1,4-content increases when the polymerization temperature is decreased from room temperature to -30°C. The polymerization of E-2-methyl-l,3-pentadiene resulted in polymers which almost exclusively comprised cis- 1,4-units and no dependence of the cis- 1,4-content on polymerization temperature was observed. The obtained poly(2-methyl-l,3-pentadiene) was composed of various polymer fractions with different stereo regularities [165,166]. 

For the preparation of poly(isoprene), the monomer 2-methyl-1,3-buta-diene (= isoprene = IP) is required as feedstock. This monomer can be obtained by various condensation methods that utilize four principles to create the C5 skeleton. In the more modern process IP is obtained from the C5 cracking fraction which contains various double-bond containing hydrocarbons with 5 C-atoms (e.g. among other C5-compounds the fraction contains cyclopentadiene, various pentadienes and pentenes) [478]. The preparation of pure IP by either of these two routes is cost intensive. By the direct and selective polymerization of IP in the crude C5 cracking fraction the cost intensive isolation of pure IP is avoided. Thereby production costs for IR are considerably reduced [264,265]. The selective polymerization of IP in the crude C5 cracking fraction is achieved by the application of a NdP-based catalyst system. The latest patent of Michehn claims a process in which dehydrogenation of the C5 cut is applied prior to polymerization. In this way an IP-enriched C5-fraction is obtained which does not contain a high quantity of disubstituted alkynes, terminal alkynes and cyclopentadiene. The unpurified C5-fraction is used as the feedstock for polymerization [591,592]. 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

As found for the polymerization of styrene, CpTiCl3/MAO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3-dienes (Table 25) [218], Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene and 2,3-dimethylbutadiene yield polymers with different... 

According to Blake and Hole , methyl ketene decomposes into CO2 and pentadiene-2,3 as well as into CO and butene-2 subsequent polymerization and decomposition processes produce other products, especially at higher temperatures. The results indicate that the overall orders of both CO2 and CO formation are 1.5 and each reaction path is inhibited by isobutene. Blake and Hole suggested tentative chain mechanisms to account for the observed product formation and for the kinetics of the decomposition. Initiation and termination was assumed to occur at the surface of the vessel. 

In an attempt to prepare polycyclopentadiene which would be stable in toluene solution, the polymer was hydrogenated over a platinum oxide catalyst in a Parr bomb immediately after the completion of the polymerization reaction. Infrared analysis indicated the presence of residual unsaturation and the polymer became insolubilized on standing. An attempted copolymerization of cyclo-pentadiene with propylene gave a product whose infrared spectrum indicated the presence of C-methyl groups but which was still insoluble in toluene. No attempt was made to determine whether copolymerization had occurred. 

Bulk polymerization of frons-2-methyl-l,3-pentadiene lead only to i,4-trans addition polymer, however it allows randomization of the trans structure, leading to an atactic polymer. The polymerization of the clathrate of fr(Ms-2-methyl-l,3-pentadiene yielded an isotactic l, 4-trans addition polymer. The polymer formed from the bulk had a molecular weight of 20,000 (240 monomer units), and that formed from the clathrate had a molecular weight of lOOO (l2 monomer units). Similar results were obtained for other dienes, and the results are summarized in Table 4. it can be concluded that polymerization of dienes in the clathrate lead exclusively to a i,4-trans addition polymer, except in the case of 1,3-cyclohexadiene. For this monomer, although the polymer is formed entirely by 1,4-addition, the polymer formed is essentially atactic. In bulk polymerization, the polymerization proceeds in most cases through 1,4-addition (both trans and cis), but in the case of butadiene and 1,3-cyclohexadiene 1,2-additions were also observed. Actually, in the case of the bulk y-induced polymerization of 1,3-cyclohexadiene the 1,2-addition process was favoured over the 1,4-addition process by a ratio of 4 3. 

The system Cp2TiCl2/MAO is suggested to be less active than CpTiCl3/MAO and Cp2TiCl/MAO in the polymerization of 1,3-butadiene, 4-methyl-l,3-pentadiene, and styrene to give predominantly cis-, 4-polybutadiene, 1,2-syndiotactic poly(4-methyl-l,3-pentadiene), and syndiotactic polystyrene.1214... 

Non-conjugated dienes have also been co-polymerized with ethylene. Examples are 1,4-pentadiene,559 1,5-hexadiene,560-563 1,7-octadiene,562-564 and 7-methyl-l,6-octadiene.562... 

The catalytic activity of the titanocene derivatives increases in the order Cp2TiC2dienes also polymerize in the presence of CpTiCl3/MAO catalyst with the selectivity depending on the monomer structure. The polymerization of (Z)-l,3-pentadiene forms cis-... 

Zirconocene complexes show much lower activity as the catalyst of 1,3-butadiene polymerization than the titanocene catalysts [15]. The catalyst composed of rac-[CH2(3-terf-butyl-l-indenyl)2]ZrCl2 and MAO promotes the 1,4-polymerization of 1,3-butadiene or (Z)-1,3-pentadiene and 1,2-polymerization of (E)-1,3-pentadiene and 4-methyl-1,3-pentadiene (Eq. 4) [16]. The bulky tert-butyl group of the ligand is essential for smooth polymeriza-... 

A topochemical condition for polymerization is the proper approach of successive monomers at the growing chain-end within the channels. In this respect, conjugated dienes like butadiene, isoprene, etc. possessing reactive atoms in terminal positions, are very suited to inclusion polymerization. However, even bulkier monomers such as substituted styrenes or methyl methacrylate can polymerize if the space available inside the channels permits a favorable orientation and/or conformation of the monomer. The most studied examples are butadiene, vinyl chloride, bromide and fluoride, and acrylonitrile in urea 2,3-dimethylbutadiene and 2,3-dichlorobutadiene in thiourea butadiene, isoprene, cis- and trans-pentadiene, trans-2-methylpentadiene, ethylene and propylene in PHTP butadiene, cis- and trans-pentadiene, cis- and trans-2-methylpentadiene in DCA and ACA butadiene, vinyl chloride, 4-bro-mostyrene, divinylbenzene, acrylonitrile and methyl methacrylate in TPP. 

Product stereochemistry is controlled during transition complex generation. Two monomers differing only in the position of the methyl group, isoprene and 1,3-pentadiene, are polymerized in different ways with EtjAl—Ti(OR)4. This is ascribed to the induction effect of the methyl groups, leading to changes in the electron density on the monomer carbons in the transition complex [119]. Isoprene is added by the 3,4 mode ( catalyst)... 

Some work has also been reported on the polymerizations of various 1-substituted and 1,4- and 2,4-disuhstituted 1,3-hutadienes. Many more different stereoregular stmctures are possible for these monomers (Sec. 8-le). The result is that very few of the completely stereoregular polymers have been obtained in high yield coupled with high stereoregularity. For example, ( )-2-methyl-l,3-pentadiene (LIII) was polymerized by neodymium(III) octanoate to a polymer consisting of 98-99% cis 1,4-structure but different fractions differed in isotac-ticity [Cabassi et al., 1988]. Similar results were found for other monomers [Pasquon et al., 1989 Porri and Giarrusoso, 1989 Takasu et al., 2001]. 

Hydroboration of a.,oi-dienes. Cyclic hydroboration of ,cu-dienes can be effected with this reagent. The initial product is often polymeric, but six- to eight-membered cyclic B-chloroboracycloalkanes can be obtained after depolymerization by distillation in good yields. The B-chloroboracycIoaikanes (2) and (3) from 1,4-pentadiene (1) were transformed into the cyclic ketones (6) and (7) by reaction with a,a -dichloromethyl methyl ether (5, 200 203) followed by oxidation. ... 

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