1,2-Poly butadienes cyclization

The type of alkylaluminum compound has only a secondary influence on structure in the polymerization of diolefins in contrast to its strong effect on the structure of polypropylene. An exception is AlEtCb, which, apparently in connection with its cationogenic character, with j3-TiCl3 induces polymerization to trans-1,4 polybutadiene and, even without transition metal compound, leads to formation of cyclized polyisoprene. Incidentally, this indicates that poly-butadiene is much more stable towards cyclization than polyisoprene. 

Poly butadiene rapidly becomes crosslinked when irradiated in vacuo at 253.7 nm [54]. A decrease amounting to 80% of the original unsaturation is observed by infrared spectroscopy. Since no new unsaturation has been detected this decrease has been accounted for by cyclization. However, the absence of absorption at 1020 cm-1 implies that formation of cyclopropyl groups does not occur. Formation of a ladder polymer is also unlikely since all attempts to accomplish the free radical post-polymerization of 1,2-poly butadiene have been unsuccessful namely,... 

Figure 2-7. Ordered networks. 0, Adamantane as an example of a cage polymer (type 0) I, Cyclized and dehydrogenated l,2-poly(butadiene) as an example of a ladder or double-strand polymer (type 1) 2, graphite as a layer or parquet polymer (type 2). ( ) Carbon-to-hydrogen bonds (— and =) carbon-to-carbon bonds.

In the first step a polymer chain with regularly ordered substituents is produced, and in the second step the substituents themselves are polymerized. An example of this is the polymerization of butadiene to 1,2-poly(butadiene), which is then cyclized by polymerization of the vinyl groups ... 

Ma et al.177 synthesized a,co-dilithium poly(styrene-b-butadiene-b-styrene)s (PS-b-PBd-b-PS) by using l,3-bis(l-phenylethylenyl)benzene activated with 2 mol of s-BuLi as initiator for the sequential polymerization of butadiene and styrene in the presence of s-BuOLi in benzene. The cyclization reaction was performed under high dilution in cyclohexane with either dichlorodimethylsilane or MDPPE (Scheme 87). The cyclic copolymer was isolated by fractional precipitation. The only indication of the formation of this architecture was the lower intrinsic viscosity. 

Cationic cyclization of unsaturated elastomers such as poly(c -l,4-isoprene), poly(3,4-isoprene), poly(l,2-butadiene), and poly( 1,4-butadiene) usually leads to the formation of cyclized resinous products of no commercial value. An extensive review on the subject has been published by Schults et al. (1983). Cyclization of unsaturated elastomers, such as polyisoprene, can be carried... 

Other aldehydic compounds, such as glyoxal and chloral, also react in a similar way with polyisoprenes and unsaturated rubbers (e.g., poly(cfs-l,4-isoprenes), poly( fs-l,4-butadiene), and copolymers of isobutylene and iso-prene). The use of strong acids, or Lewis acids, causes complications, as the acids themselves, under suitable conditions, catalyze cyclization and cis-trans isomerization, and these reactions may occur simultaneously with the addition reactions. 

Halogenation and hydrohalogenation of elastomers have been reported extensively in the literature [26]. The main problems with these reactions are the cyclization and chain scission that occur parallel to the halogenation reaction. These introduce difficult problems in the characterization of the resulting products. Despite these problems, several products have been prepared and commercialized. Chlorination of poly(l, 4-butadiene) to prepare a product similar to poly(vinyl chloride) has been reported by several workers [27]. This process had extensive side reactions and chain degradation. The chlorination of butyl rubber and conjugated diene-butyl rubbers gives end products that are used in the tire industry as inner liners for air retention. 

Conj ugated Ladder Polymers. Since the 1930s double-stranded, ladder-type polymers have been prepared in a multistep process with limited success of cyclization (191,192). Other routes have also been explored such as those for poly(acrylonitrile) (193,194), poly(l,2-butadiene), poly(3,4-isoprene) (195), or poly(butadiyne)s (196). These materials were found to be poorly soluble and unworkable, with a considerable number of defects in the structure (incomplete cyclization, cross-linking, radical sites). The first successful synthesis of a ladder polymer with a completely defined structure was accomplished in 1991 by Sherf and Mullen (197). The first step was the AA/BB-t5q)e polycondensation of an aromatic diboronic acid with a substituted 2,5-dibromo-l,4-dibenzoylbenzene to give a single-stranded precursor PPP-type polymer, followed by cyclization to the ladder structure (Fig. 8). Several other examples exist that have resulted in ladder-type structures. These include angular polyacene (198,199), Diels-Alder polyaddition of AB-type diene-dienophiles (200), AA/BB-type Diels-Alder polyaddition of a bisdiene and a bisdienophile (201), thienylene imits (202),... 

Other polymer-analogous precursors employed include polyacrylonitrile (oxidative or nonoxidative cyclization [3-5]), poly( 1,2-butadiene) or poly(3,4-isoprene) [6,7], and polyalkinylacetylenes [8-11] prepared by the polymerization of butadiynes. The unsaturated side... 

Poly(l,2-butadiene) (>90% 1,2-structure) prep ired by reaction of butadiene with a complex of butyl lithium and tetramethylethylene diamine was a tacky substance [tjiOi <0.5) 23, 24). Treatment of this material in xylene at 120° with sulfuric acid as catalyst 5uelded a cream-colored powder (j inj, = 0.18), the infrared (IR) and nuclear magnetic resonance (NMR) spectra of which indicated that cyclization had occurred. Aromatization of the cydized material was attempted using chloranil as oxidant, XV -> XVI. 

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