Chain reaction transfer

The importance of chain transfer in the polymerization of TXN is illustrated by the fact that the molecular weights of polymers cannot be increased much above 105 although kinetic chain lengths frequently reach 106. 

Chain transfer may occur with any compound possessing reactive hydrogen atoms be these impurities or purposely added to the system. The most common impurity is water, a very active chain transfer agent 99 100 . Alcohols, carboxylic acids, acetals, some aromatic compounds (m-xylene, mesitylene, anthracene), and halogenated 

Chain transfer with water involves formation of the corresponding oxonium ion with the subsequent release of a proton which initiates a new chain  

Even in most rigorously purified systems, molecular weights corresponding to the length of the kinetic chains cannot be attained. Search for the cause of chain shortening led to the discovery of hydride transfer in TXN polymerization70). 

As the result of hydride transfer from the monomer to the macrocation, the polymer would acquire a —OCH3 end group and the l,3,5-trioxane-2-ylium cation formed would initiate a new chain  

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Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

Chain transfer reactions to monomer and/or solvent also occur and lower the kinetic chain length without affecting the rate of polymerization ... 

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked. These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching. 

By polymerising styrene in solution many problems associated with heat transfer and the physical movement of viscous masses are reduced, these advantages being offset by problems of solvent recovery and the possibility of chain transfer reactions. In 1955 Distrene Ltd started a plant at Barry in South Wales for the production of styrene by such a solution polymerisation process and some details have been made available. The essential details of this process are indicated by Figure 16.7. 

Upon thermal destruction of polyethylene the chain transfer reactions are predominant, but depolymerization proceeds to a much lesser extent. As a result, the products of destruction represent the polymeric chain fragments of different length, and monomeric ethylene is formed to the extent of 1-3% by mass of polyethylene. C—C bonds in polypropylene are less strong than in polyethylene because of the fact that each second carbon atom in the main chain is the tertiary one. 

The increase in the temperature reduces the viscosity of the polymerization medium which increases the termination reactions. This is attributed to an increase in chain transfer reactions higher than that of propagation reactions [16,51]. Consequently, the weight-average molecular weight of the formed polymer decreases. 

Chain transfer reactions involving the monomer could also result in unsaturation of the chain ends according to the following two reactions [Eqs. (3) and (4)]. 

This is a highly selective process and very good results on cellulose and starch grafting have been observed. Ceric ion initiated grafting is usually carried out at lower temperatures and, therefore, wastage of monomer in chain transfer reactions is minimal. 

Chain-transfer reactions take place during vinyl polymerization involving abstraction of an atom such as... 

However, when MAIs are thermolyzed in solution, the role of the cage effect has to be taken into account. The thermolytically formed macroradicals can, due to their size, diffuse only slowly apart from each other. Therefore, the number of combination events will be much higher for MAIs than for low-molecular weight AIBN derivatives. As was shown by Smith [16], the tendency toward radical combination depends significantly on the rigidity and the bulkiness of the chain. Species such as cyclohexyl or diphenylmethyl incorporated into the MAI s main chain lead to the almost quantitative combination of the radicals formed upon thermolysis. In addition, combination chain transfer reactions may... 

HPO group is sensitive to light, but stable to heat. Using this MAI, St was thermally polymerized at the first step, and then MMA was photopolymerized at the second step [12]. Block efficiency was 40-55% and the amount of PSt homopolymer decreased, while that of PMMA homopolymer increased, presumably due to chain transfer reaction. 

Chain transfer reaction during propagation gives homopolymers as well as block copolymers. Separation of the homopolymers is performed by extraction with suitable solvents. Homopolymer A together with a small amount of block copolymer rich in component A are extracted... 

A polymer molecule may have just a linear chain or one or more hranches protruding from the polymer hackhone. Branching results mainly from chain transfer reactions (see Chain Transfer Reactions later in this chapter) and affects the polymer s physical and mechanical properties. Branched polyethylene usually has a few long hranches and many more short hranches... 

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent ... 

Intermolecular chain transfer reactions may occur between two propagating polymer chains and result in the termination of one of the chains. Alternatively, these reactions take place by an intramolecular reaction by the coiling of a long chain. Intramolecular chain transfer normally results in short branches ... 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

A chain transfer reaction between the monomer and the growing polymer produces an unsaturated polymer. This occurs when the concentration of the monomer is high compared to the catalyst. Using ethylene as the monomer, the termination reaction has this representation ... 

In solution polymerization, an organic solvent dissolves the monomer. Solvents should have low chain transfer activity to minimize chain transfer reactions that produce low-molecular-weight polymers. The presence of a solvent makes heat and viscosity control easier than in bulk polymerization. Removal of the solvent may not be necessary in certain applications such as coatings and adhesives. 

Suspension polymerization produces polymers more pure than those from solution polymerization due to the absence of chain transfer reactions. As in a solution polymerization, the dispersing liquid helps control the reaction s heat. 

Since 1 is a monomer with low activity, copolymers 2 obtained at any stage of the copolymerization process, irrespective of the monomer ratio in the initial mixture, always contain a smaller amount of monomeric units of 1 than that in the corresponding monomer mixture. 1 being prone to enter the chain-transfer reaction, the increase of its content in the initial monomer mixture reduces substantially the reaction rate and decreases the molecular mass of the copolymers. It was found that copolymers 2 which contain 2—8% of monomeric units of 1 and are suitable for obtaining fibres must have a molecular mass between 45 000 and 50000. Such copolymers can be obtained with a AN 1 ratio in the initial mixture between 95 5 and 85 15. Concentrated solutions of copolymers, especially those with a molecular mass smaller than the above limit, are characterized by a very low stability which is a substantial shortcoming of these copolymers. 

It is, however, possible that the difference, to a certain extent, can be explained by the participation of the aldehyde groups in the chain-transfer reaction ... 

Reactivity ratios for the copolymerization of AN with 7 in methanol at 60 °C, proved to be equal to rx AN= 3,6 0,2 and r%n = 0 0,06, i.e., AN is a much more active component in this binary system. The low reactivity of the vinyl double bond in 7 is explained by the specific effect of the sulfonyl group on its polarity26. In addition to that, the radical formed from 7 does not seem to be stabilized by the sulfonyl group and readily takes part in the chain transfer reaction and chain termination. As a result of this, the rate of copolymerization reaction and the molecular mass of the copolymers decrease with increasing content of 7 in the initial mixture. 

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