Chain extension mechanism

It seems of importance to elucidate what the stresses are that should be applied to the melt in order to (1) ensure formation of a nematic phase in the melt, and (2) attain values of > fiCI so that the crystallization caused by melt extension should proceed by the chain-extension mechanism. It is also desirable to answer the question whether the formation of the nematic phase is an indispensable intermediate stage preceding formation of ECC51). 

Two mechanisms have been proposed in the literature to account for the interaction of polyester and magnesium oxide in the reactive monomer (e.g., styrene) medium. One is a chain extension mechanism and the second is the formation of a coordinate complex, also known as two-stage thickening mechanism [28]. The common starting point for these two mechanisms is the formation of basic and neutral salts with the polyester carboxylic acid (—COOH) end groups according to the following reactions ... 

In the chain extension mechanism, it is postulated that dicarboxylic add groups on the UP chains react with MgO to produce a very high molecular weight species (via condensation polymerization) and, thus, give rise to a large increase in... 

Chain Extansion Mechanism. The chain extension mechanism in the oxidative polymerization of 2,6-Me2P has been almost established (Fig. 3). It has... 

The chain extension mechanism is shown in Figure 8 (201). Diphenyl disulfide is oxidized to a cation radical, which reacts with diphenyl disulfide to give phenylbis(phenylthio)sulfonium cation, followed by electophilic attack of diphenyl sulfide on the cation. 

We at NASA reopened the question of learning to chain extend these fluoroether systems In consort with scientists at PCR Inc. The objective was to prepare long-chain difunctional polyperfluoroethers and investigate chain extension mechanisms, as well as to convert these materials to stable cross-linked polymers for sealant applications. The nitrile, acetylene, and isocyanate groups were considered. Each of these is capable of both trimerization reactions and dipolar cycloadditions. From this base line, one could then evoke both chain extension and cross-linking with a variety of reaction schemes. 

Finally, the possibility to connect chains of two different polymers by this chain extension mechanism can also be applied to make block copolymers. 

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. 

Macromonomers such as 66, 68 and 94 are themselves catalytic chain transfer agents (Section 6.2.3.4) and transfer to macromonomer is one mechanism for chain extension of the initially formed species. The adduct species in the case of monomeric radical adding dimer (100) may also react by chain transfer to give 101 which is inert under polymerization conditions (Scheme 6.25). Polymerizations to... 

The polymerization process for nylon 6 consists primarily of the three types of reaction illustrated in Fig. 23.6. Each of the reactions is reversible, tvith the equilibrium of the products being controlled primarily by the concentration of water in the reaction vessel. The reaction is initiated by the hydrolytic ring opening of caprolactam to form 6-aminohexanoic acid, as shown in Fig. 23.6 a). Chain extension of the type shotvn in Fig. 23.6 b) dominates when water is abundant (10 to 20%) in the reaction mixture. At lower water levels (2 to 5%) chains grow primarily by the mechanism shown in Fig. 23.6 c). In order to limit the average molecular... 

The mechanism of chain extension was studied by observing the OH and NH absorptions at 3460 cm-1 and 3340 cm-1 respectively. Since one hydroxy moiety is consumed for each chain extension while one urethane NH is formed, the ratio of OH/NH infrared absorptions will decrease as chain extension occurs. The results of this study are shown in Table I and Figure 1. 

For many polymers, as one moves from a good solvent to poorer solvent, the extent of close coiling increases resulting in a decreased viscosity because the more tightly coiled polymer chains now reside in fewer flow planes (Figure 7.6). A similar phenomena can occur as the temperature is decreased (above Tg) since the available energy for chain extension becomes less. For some polymers additional specific mechanisms are in action. 

Many researchers have investigated the use of amines and alcohols as initiators for the ROP of lactones. As a rule, amines and alcohols are not nucleophilic enough to be efficient initiators, and it is then mandatory to use catalysts to perform the polymerization successfully. Nevertheless, highly reactive p-lactones exhibit a particular behavior because their polymerization can be initiated by nucleophilic amines in the absence of any catalyst. As far as tertiary amines are concerned, the initiation step implies the formation of a zwitterion made up of an ammonium cation and a carboxylate anion, as shown in Fig. 20. Authors coined the name zwitterionic polymerization for this process [80]. Nevertheless, this polymerization is not really new because the mechanism is mainly anionic. Interestingly, Rticheldorf and coworkers did not exclude the possibility that, at least at some stage of the polymerization, chain extension takes place by step-growth polycondensation [81]. 

The first step of the chain extension reaction mechanism has been shown to b e etiolate formation, the by-product of which Is ethane gas. The nitrogen line should be attached to a large adapter to provide adequate venting of the gas. 

In studying the reaction mechanism for chain extension it is assumed, in line with the fundamental reaction of chain extension,... 

As oxidation processes were clarified, it was observed in other chain extension reactions that R02 radicals reacted with oxidation products hydroperoxides, alcohols, and ketones. The high reactivity of hydroperoxides and alcohols strongly influences the mechanism of oxidation processes. Chain rupture results from recombination of R02 radicals. 

Effects of Curing Agent Type. Epoxide-Cured Propellant. Carboxyl-terminated polybutadiene is a linear, difunctional molecule that requires the use of a polyfunctional crosslinker to achieve a gel. The crosslinkers used in most epoxide-cured propellants are summarized in Table IV and consist of Epon X-801, ERLA-0510, or Epotuf. DER-332, a high-purity diepoxide that exhibits a minimum of side reactions in the presence of the ammonium perchlorate oxidizer, can be used to provide chain extension for further modification of the mechanical properties. A typical study to adjust and optimize the crosslinker level and compensate for side reactions and achieve the best balance of uniaxial tensile properties for a CTPB propellant is shown in Table V. These results are characteristic of epoxide-cured propellants at this solids level and show the effects of curing agent type and plasticizer level on the mechanical properties of propellants. 

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