Unsaturated chain ends from initiation

The polymer chains initiated at the zeolite surface by a cationic mechanism would be held in omtact with the surface by intermolecular interactions. Under these conditicms, the propi tion step would also i obably be on the zeolite sur-hice. Termination would be likely to occur by dissociation from the zeolite surface producing an unsaturated chain end. The activated zeolite site would also be reformed. This mechanism is somewhat similar to that used by Benson and co-workers for polymerization by add day catalysts. 

The termination reactions were believed to result from hydride-ion transfers [262], Later, however, polyethylene, formed with these catalysts, was found to contain approximately equal numbers of saturated and unsaturated chain ends. This contradicts the above concept. It may also mean that the initiation involves 71-complexes that disproportionate to yield coordination of the metal to -CH=CH2 and to -CH2-CH3 groups [262]. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Another type of cationic initiation procedure starting from an unsaturated compound has been proposed 49). 3-Phenylallyl bromide is reacted in THF solution with AgSbF6 whereby a poly-THF macromonomer bearing a styryl residue at the chain end is formed ... 

The reasons why complexes 8 and 9 are active in ATRP are presently nnclear. These complexes possess indeed two 18-electron rathenium centres and, as snch, shonld be nnable to activate the carbon-halogen bond of the initiator or of the growing polymer chain end. On the other hand, the fact that an indnction period was fonnd for the ATRP of MMA indicates that the mtheninm-vinylidene complexes have to be activated prior to the ATRP process. There are in principle several plansible explanations for the formation of a coordinatively unsatnrated 16-electron rathenium species from the rathenium-vinylidene complexes 8 or 9 either the splitting of the bimetallic scaffold into two different unsaturated rathenium intermediates (Path A, Scheme 5), the opening of a p.-chloro bridge (Path B), or the release of the vinylidene ligand (Path C, Scheme 5). 

From unsaturated polyesters with carboxylic end groups at both chain ends, after neutralization they are efficient emulsifiers for lipophilic monomers [95], and with styrene as comonomer microgels can be prepared with rather uniform diameter [96]. By using lipophilic initiators, such as 2,2 -azobis(isobutyronitrile) (AiBN), in the microemulsion copolymerization, diffusion of monomers is too slow compared with the reaction rate. Therefore, copolymerization is confined to the coherent, lipophilic phase [97,98] and very small microgel particles with a rather uniform size result. Research work by Funke and cited by Funke [76] indicates the usefulness of microemulsion copolymerization to convert unsaturated polyesters into microgels. 

With ethylene oxide, the active alkoxide chain end can abstract a hydrogen from the chain, leading to terminal unsaturation and a new chain-initiation site. 

ESBR and SSBR are made from two different addition polymerisation techniques one radical and one ionic. ESBR polymerisation is based on free radicals that attack the unsaturation of the monomers, causing addition of monomer units to the end of the polymer chain, whereas the basis for SSBR is by use of ionic initiators (qv). 

The monomers used in chain polymerisations are unsaturated, sometimes referred to as vinyl monomers. In order to carry out such polymerisations a small trace of an initiator material is required. These substances readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation from around or just beyond the blue end of the spectrum. The two most commonly used free radical initiators for these reactions are benzoyl peroxide and azobisisobutyronitrile (usually abbreviated to AIBN). They react as indicated in Reactions 2.1 and 2.2. 

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