Internal double bond polymerization

Free-radical polymerization, like coordination polymerization discussed in Chapter 2, involves the sequential addition of vinyl monomer(s) to an active center. For FRP the active centers are free radicals. The increase in chain length is very rapid an individual chain is initiated, grows to high MW and is terminated in a few seconds or less. After termination, the high-MW polymer chain does not react further (barring side reactions such as chain transfer to polymer or terminal/internal double bond polymerization) and is considered dead . Dead chains have a residence time of minutes or hours in the reactor, such that the final polymer product is an intimate mixture of chains formed under time and/or spatially varying conditions. 

Catalysts Temperature of polymerization (°C) Density of polymer Mn Number of methyls per 1000 C atoms Number of vinyls per 1000 C atoms Trans, internal double bonds per 1000 C atoms Number of methyls per chain Number of terminal double bonds per chain... 

A chromotrophic acid spot test for formaldehyde (23) was also negative for the polymer ozonolysis solution, while it was positive for a control solution containing formaldehyde equivalent to that expected in the experimental solution if one per cent of the double bonds were vinyl, i.e., polymerization via the internal double bond. 

The reduced reactivity of 5-methy1-1-hexene is consistent with the expected steric effect due to methyl substitution at the 5-carbon position. Apparently, the internal double bond in 5-methyl-l,4-hexadiene assists in its complexation at the active site(s) of the catalyst prior to its polymerization and thereby the "local concentration" of this monomer is higher than the feed concentration during copolymerization with 1-hexene. This view is consistent with the observation that the overall rates of polymerization, under the same conditions, are much lower for the system containing 5-methyl-1,4-hexadiene. 

The free-radical crosslinking polymerization can be regarded as a special example of specific diffusion control, in which the tendency to microgel formation and decrease of apparent reactivity of Internal double bonds depends on the size of the mlcrogel which in turn depends on the molecular weight of the primary chain. Polymerization of diallyl monomers exhibits much less of these features (W) because the degree of polymerization of their primary chains is extremely low due to degradative chain transfer. 

The same conclusions can be reached for the cis isomer. As already mentioned, isomerization of this isomer occurs in the polymerization medium. If we admit that only the trans isomer formed through isomerization polymerizes, we fall under the case examined above. If we admit also that the cis isomer can polymerize to cis-1,4 units, we must assume that also in this case the coordination occurs only through the vinyl group and not through the two double bonds, in the cis conformation. The cis isomer, in fact, cannot assume the cis conformation for steric reasons. On the other hand, coordination by the internal double bond, both for the cis and trans isomers, appears less probable (for steric reasons) than the coordination by the vinyl group. 

When the catalyst is not fully regioselective, chain release by a /3-H transfer after a secondary insertion with formation of internal double bonds is often observed. This has been reported for ethylene/a-olefin co-poly-mers, PP, and other polyolefins, as well as for 1-hexene polymerization with dialkoxide catalysts. The reaction is shown in Scheme 14 for the case of propylene, where kinetic studies have shown it to be a bimolecular process, following the rate law s/J/ -h=s / -h[sZr][m].217,257 [sZr] refers to the concentration of active Zr centers bearing a growing chain having a secondary propylene unit linked to the metal. 

The selectivity should, however, decrease with increasing temperature which might explain the decrease in the content of internal double bonds with in easing polymerization temperature. 

The minimum in degradation rate found for subsaturation PVC obtained around 55°C becomes less obvious if the monomer concentration at the reaction site is used as variable instead of the relative monomer pressure, P/PQ. The observed behavior is mainly due to the influence of the polymerization conditions on the formation of thermally labile chlorine, i.e. tertiary chlorine and internal allylic chlorine. Tertiary chlorine is associated with ethyl, butyl and long chain branches. The labile structures are formed after different inter-and intramolecular transfer reactions. Generally, the content increases with decreasing monomer concentration and increasing temperature in accordance with the proposed mechanisms. The content of internal double bonds instead decreases with increasing temperatures. 

On the other hand, we reexamined in detail the ring size of the cyclic structural units of poly-AA s by means of IR, 1H-NMR, and C-NMR spectroscopy these analytical procedures were applied to the structural analysis of poly-AA, the poly(acrylic acid) derived from hydrolysis of the poly-AA, and the poly(methyl acrylate) obtained by subsequent esterification of the poly(acryl-ic acid) in comparison with the corresponding model polymers of five- or six-membered ring structure. Then, we investigated in detail the effects of polymerization conditions on the ring size of poly-AA s, i.e., on the intramolecular addition modes in the cyclopolymerization of AA since five- or six-membered ring anhydride structure can be formed via intramolecular hh or ht addition of the uncyclized radical to the internal double bond(22,23). 

A detailed study of hydrogenation of several alkenes and polybutadiene was undertaken using the catalysts [RhCl(HEXNa)2]2 and [RhCl(OCTNa)2]2 (HEXNa and OCTNa, Structures 8 and 9) [52] with or without an added solvent (toluene). With both catalysts the terminal alkenes were hydrogenated much faster than the internal ones, and this was also reflected in the preferential hydrogenation of the pendant vinyl units (products of 1,2-addition) in polybutadiene versus the internal double bonds (from 1,4-polymerization) (Eq. 31). Internal double bonds in 2-pen-tene- and 3-pentenenitriles were hydrogenated unusually fast compared with simple alkenes such as 1-octene, with no concomitant reduction of the nitrile group. 

Sulfur dioxide does not homopolymerize, but on reaction with olefins it yields copolymers.Terminal olefins react more readily than those with an internal double bond. The presence of various substituents affects the rate of polymerization. Conjugated dienes copolymerize with sulfur dioxide to give linear polymers containing residual double bonds. 

Ability of internal double bonds of forming macromolecule to further transformations initiated by cationic initiators is the characteristic particularity of cationic polymerization. This character leads to reduction of polydienes unsaturation due to the formation of brunched and cyclic structures even at low monomer conversions [175]. Total unsaturation of resulting polypiperylene usually is about 60-90% from theoretical [243]. One supposes [244] that these chain units consist of monocycle units of type I-III ... 

PREPARATIVE TECHNIQUE H-H polystyrene has never been obtained directly from styrene monomer. It is synthesized by the selective hydrogenation of l,4-poly(2,3-diphenyl-1,3-butadiene) (PDPB) using potassium/ethanol. PDPB is prepared by the free radical polymerization of 2,3-diphenyl-l,3-butadiene to give a 45% cis, 55% trans structure. H-H PS is then given in the Scune ratio of erythro and threo linkages after the chemical reduction of the internal double bond of the PDPB. ... 

Figure 10.2 Dehydrochlorination rate versus tertiary chlorine atoms + internal double bonds [72] fractionated commercial suspension PVC, polymerization temperature 55°C [73,74], OPVC polymerized at reduced monomer pressure, polymerization temperature 55 C, monomer pressure between 59-92% of the saturation pressure of vinyl chloride at 55 °C [73,74], bulk and suspension polymerization by lUPAC Sub-Group on Defects in the Molecular Structure of PVC and their relation to thermal stability. [Modified from [72].]...

---