Diene polymers vinyl branching

Diene polymers contain occasional vinyl branches along the chain. How do you think these branches might arise ... 

We can create crosslinks during chain growth polymerization by copolymerizing dienes with vinyl monomers. When the two vinyl functions of the diene are incorporated into separate chains, a crosslink is formed. This process is shown in Fig. 2.18. When we use a low concentration of dienes, we produce a long chain branched polymer, while high concentrations of dienes create a highly crosslinked polymer network... 

A vinyl branch in a diene polymer is the result of an occasional 1,2 double bond addition to the polymer chain. Branching can also occur in cationic polymerization for the same reason. 

The polymer formed from a diene such as 1,3-butadiene contains vinyl branches. Propose an anionic polymerization mechanism to account for the formation of these branches. 

By designing the repeat unit into the parent diene (containing either an alkyl branch or functionality), only a single type of repeat unit is formed upon polymerization, giving pure polymer microstructures. To date, perfectly controlled ADMET ethylene copolymers have included ethylene-CO,34 ethylene-vinyl alcohol,35 ethylene-vinyl acetate,36 and ethylene-propylene.20 Figure 8.12... 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

As far as the mechanisms of branching and crosslinking are concerned, there appear to us to be certain weaknesses in those commonly accepted. With ethylenic monomers, there can be little doubt that if branching were to occur at all, it will arise from radical attack upon the polymer already formed. It would be immaterial whether this transfer takes place on backbone carbon atoms or via side chains, as is almost certainly true for, say, vinyl acetate. When dienes are present, it has been generally accepted that the residual double bonds are the main seat of reaction, thereby creating the immediate possibility of crosslinking. However, the internal residual double bonds—that is, those... 

Of great industrial interest are the copolymers of ethene and propene with a molar ratio of 1/0.5, up to 1/2. These EP-polymers show elastic properties and, together with 2-5 wt% of dienes as third monomers, they are used as elastomers (EPDM). Since they have no double bonds in the backbone of the polymer, they are less sensitive to oxidation reactions. As dienes, ethylidenenorbomene, 1,4-hexadiene, and dicyclopentadiene are used. In most technical processes for the production of EP and EPDM rubber in the past, soluble or highly disposed vanadium components are used [69]. Similar elastomers can be obtained with metallocene/MAO catalysts by a much higher activity which are less colored [70-72]. The regiospecificity of the metallocene catalysts toward propene leads exclusively to the formation of head-to-tail enchainments. The ethylidenenor-bornene polymerizes via vinyl polymerization of the cyclic double bond and the tendency to branching is low. The molecular weight distribution of about 2 is narrow [73]. 

More recently, ADMET was used to synthesize precision poly(ethylene-co-vinyl amine) with primary amine branches placed on every 9th, 15th, 19th, or 21st carbon along the PE backbone [81]. This was accomplished by synthesizing a symmetrical diene monomer that included a BOC-protected amine, which was thermally deprotected after polymerization. The thermal deprotection yielded a minimally soluble product, as did chemical approaches to deprotection, but the thermal approach resulted in less sample contamination. This insolubility hampered characterization efforts, but the final deprotected polymer was characterized by solid-state NMR, NMR, and TGA, all of which proved... 

---