Conjugated diene, 1,2-addition polymers

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

The increased ionic freedom between the propagating polymer ion and its gegen ion occurs concurrently with increased space separation between the two ion species. The studies of Schuerch and co-workers and of Yoshino and co-workers (98) with deuterated acrylates and by Natta and co-workers (99) with sorbic esters show that this increased separation allows trans addition to mono olefins and 1,4 trans addition to conjugated dienes before complete loss of isotactic steric control at the end of the chain. The increased freedom between the propagating ion and the less closely associated gegen ion appears to result in a distortion of the cyclic transition state which permits backside attack at the beta position of the incoming acrylate monomer and 1,4 attack on the incoming sorbate monomer. 

Many of the polymers formed from conjugated dienes are elastic and are used to manufacture synthetic rubbers. The raw polymers usually are tacky and of little direct use, except as adhesives and cements. They are transformed into materials with greater elasticity and strength by vulcanization, in which the polymer is heated with sulfur and various other substances called accelerators, with the result that the polymer chains become cross-linked to one another by carbon-sulfur and carbon-carbon bonds. Some of the cross-linking appears to occur by addition to the double bonds, but the amount of sulfur added generally is insufficient to saturate the polymer. With large proportions of sulfur, hard rubberis formed such as is used in storage-battery cases. 

The conjugated dienes can polymerize in four modes cis 1,4-, trans 1,4-, 1,2- and 3,4-, the latter pair being equivalent in the absence of appropriate substitution. Early workers relied entirely upon IR spectroscopy to analyze the concatenation in their polymers. There are a number of problems associated with the technique correct assignment of peaks, the additivity and the inherent insensitivity arising from the smallness of the extinction coefficients of double bonds bearing more than one substituent (such as arises from 1,4-enchainment). In consequence, the reliability of much of the early work is uncertain the advant of NMR spectrometers has,... 

The polyinsertion reaction of conjugated dienes can proceed in three modes which yield three different isomers 1,2-polymers, 3,4-polymers and 1,4-polymers [297]. The situation is more complicated as the 1,4-isomers either exhibit as-1,4 or trans-, 4 configuration. In addition, the 1,2-/3,4-polymers can have an atactic, isotactic or syndiotactic structure (Scheme 1 in Sect. 1.2) [623]. The various moieties are either randomly distributed along the polymer chain or are aligned block wise. 

These are the most important. The two double bonds mutually activate each other conjugation is essentially not destroyed by addition to the growing chain end. Therefore the conjugated dienes are difunctional monomers. They are polymerized by a relatively simple mechanism. Of all the polymers generated in living tissues, we have so far been able to imitate most closely natural rubber, poIy-cis-l,4-isoprene. Butadiene, isoprene and chloroprene are the dienes most often employed in macro-molecular chemistry. 

Note that styrene and conjugated dienes can be copolymerized to yield statistical or block copolymers. The latter process, which involves additions of one monomer to a living polymer of the other monomer, is described in the following section. 

A wide range of polymers are obtained from the addition of conjugate diene monomers, notably 1,3-butadiene, isoprene, and chloroprene ... 

The heterogeneous polymerization of a conjugated diene or any other monomer can be visualized as involving two distinct steps. In the first step, the monomer is adsorbed on the catalyst surface and/or forms an active monomer-catalyst complex. The second step involves addition of the adsorbed and/or complexed monomer unit to the polymer chain. 

Alkyllithium initiators yield stereoregular polymers of conjugated dienes if the polymerization is carried out in hydrocarbon solvents. Addition of tetrahydrofuran or other more polar solvents changes the microstructure of the polymers that are produced... 

Conjugated dienes are polymerized by n-BuCal (in admixture with n-Bu2Ca) by a typical anionic addition mechanism at 20°-50°C. Initiation by addition is slow compared to the subsequent polymerization, and the microstructure of the polymer is solvent dependent. Polybutadiene with predominantly trans-1,4 links (48-72%) is obtained in hydrocarbons or Et20, but 1,2-polymerization is promoted in THE or by addition of hexamethylphosphoramide (HMPA). Polymerization of butadiene by PhjCMX (M = Ca, Sr, Ba X = Cl, Br) at — 10°C in THE yields polymer with increasing... 

---