Polymer chains: initiation, growth

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

In mbber production, the thiol acts as a chain transfer agent, in which it functions as a hydrogen atom donor to one mbber chain, effectively finishing chain growth for that polymer chain. The sulfur-based radical then either terminates with another radical species or initiates another chain. The thiol is used up in this process. The length of the mbber polymer chain is a function of the thiol concentration. The higher the concentration, the shorter the mbber chain and the softer the mbber. An array of thiols have subsequendy been utilized in the production of many different polymers. Some of these apphcations are as foUow ... 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

The peroxide 179 dissociates in the presence of a monomer giving rise to alkoxyl (CO-) and borinate (BO-) radicals, but the latter are believed to be too stable to initiate polymerization. It should be mentioned that the molecular weight continuously increases throughout the process implying the pseudo-living mechanism for chain growth. After the completion of the process borane residue is completely oxidized into diol <2004MM6260>. Thus, the 8-boraindane molecule not only initiates the polymerization, but also is a precursor to two functionalities in the polymer chain. 

To elaborate a theory of interphase copolymerization at an oil-water boundary the necessity arises to consider initially the growth of an individual polymer chain near the surface separating the organic and water phases. By the model introduced in paper [74], molecules of only one of the monomers are presumed to be solved inside either of these two phases. A theoretical examination of the formation of macromolecules turns out here to be substantially simpler, since their chemical structure under such an approximation is the same as that of a traditional block copolymer. 

In some cases termination may be brought about by transfer reactions . In this type of reactions though the growth of one Polymer chain is stopped due to formation of dead polymer as in coupling or disproportionation reaction, yet there is a simultaneous generation of a new free radical that is capable of initiating a fresh Polymer chain growth. 

The formation of the biopolymer dextran is a complex process, where the fructose is known to inhibit the polymer chain growth. Consequently, the separation of the fructose from the reaction zone results in a higher molecular mass of the dextran even at high initial sucrose concentration [173]. In a first investigation, a fixed-bed chromatographic reactor was filled with calcium charged polystyrene. Fructose was retarded on this matrix while the dextran was prevented... 

Zhao and Brittain [280-282] reported the LCSIP of styrene on planar silicon wafers using surface modifications of 2-(4-(ll-triethoxysilylundecyl)phenyl-2-methoxy-propane or 2-(4-trichlorosilylphenyl)-2-methoxy-d3-propane respectively. Growth of PS brushes from these SAMs has been successfully achieved factors that influence PS thickness included solvent polarity, additives and TiC concentration. Sequential polymerization by monomer addition to the same silicate substrate bearing the Hving polymer chains resulted in thicker PS films. FTIR-ATR studies using a deuterated initiator indicated that the initiator efficiency is low, and the... 

Returning to ion-pair zirconocene catalysts, the initiation of the polymerisation process requires the displacement of the anion so that the alkene can be coordinated. The mobility of the anion is therefore an important factor and has become the focus of a number of detailed investigations. The original mechanistic scheme of alkene insertion and polymer chain growth (Scheme 8.4) implied dissociation of the anion and formation of a 14-electron cationic intermediate, which then reacted... 

Lastly, there are graft copolymers that result when a polymer chain of one monomer is grafted on to an existing polymer backbone by creation of a free radical site along the backbone that initiates growth of a polymer chain. 

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