Polymerization and Conjugation

FIGURE 13.4 Biotic pathways for polymerization of (a) 2,4-dichlorophenol and (b) 2,4,6-trinitrotoluene. 

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It should be remembered that the iodine value gives no reliable information about the unsaturation of polymerized and conjugated oils60 61 the diagrams discussed above... 

Rosin is polymerized by acid treatment at elevated temperatures, whereby double bonds are eliminated by dimerization. Dimerized rosin (10) has a higher molecular weight and a correspondingly higher softening point. It is more stable because fewer double bonds remain and conjugation is reduced. 

The discovery by Ziegler that ethylene and propylene can be polymerized with transition-metal salts reduced with trialkyl aluminum gave impetus to investigations of the polymerization of conjugated dienes (7—9). In 1955, synthetic polyisoprene (90—97% tij -l,4) was prepared using two new catalysts. A transition-metal catalyst was developed at B. E. Goodrich (10) and an alkaU metal catalyst was developed at the Ekestone Tke Rubber Co. (11). Both catalysts were used to prepare tij -l,4-polyisoprene on a commercial scale (9—19). 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Ideal electrochemical polymerization was considered to give ideal linear and conjugated polymeric chains. The real situation is that films electrogenerated from the basic monomers are insoluble and infusible. Only polyaniline films are partially soluble in some solvents. 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Several experimental facts have been rationalized in terms of different n-allyl insertion mechanisms, depending on the nature of the catalytic systems and the diene monomer, mainly by the extensive work of Porri and co-workers, as reviewed in Refs. 181 and 182, and Taube and co-workers, as reviewed in Ref. 183. A widely accepted scheme for cis-1,4 and 1,2 polymerizations of conjugated dienes is reported in Scheme 1.5. In particular, it has been... 

Scheme 7.5 Monomer coordinated and monomer free intermediates of a widely accepted mechanism for d.v-l, 4 and 1,2 polymerizations of conjugated dienes.

M. Matsumi, K. Naka, and Y. Chujo, Extension of ir-conjugation length via the vacant p-orbital of the boron atom. Synthesis of novel electron deficient ir-conjugated systems by hydroboration polymerization and their blue light emitting, J. Am. Chem. Soc., 120 5112-5113, 1998. 

The acid-catalyzed conversions of hydrocarbons have been extensively studied and are widely reported in chemical literature. Many important petroleum and petrochemical processes involve catalysis by acids. In contrast to this, the use of bases to catalyze hydrocarbon conversions has received little attention except for polymerization of conjugated dienes and styrene to high polymers. 

Multiple conjugate additions anionic polymerization and superglue... 

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