Monomer molecules, growing chains

These are generated by the reaction between the primary radicals and the monomer molecules. Growing-chain radicals continue to add monomer molecules sequentially this reaction is known as propagation. Reaction between a... 

Kinetic gelation simulations seek to follow the reaction kinetics of monomers and growing chains in space and time using lattice models [43]. In one example, Bowen and Peppas [155] considered homopolymerization of tetrafunctional monomers, decay of initiator molecules, and motion of monomers in the lattice network. Extensive kinetic simulations such as this can provide information on how the structure of the gel and the conversion of monomer change during the course of gelation. Application of this type of model to polyacrylamide gels and comparison to experimental data has not been reported. 

This new free radical adds to the double bond of another monomer molecule, growing the polymer chain. The polymerization process ends as the unpaired electrons of two free radicals combine to form a single bond ... 

The prediction of the molecular weight distribution for a step-growth polymerization assumes that both the probability of the reaction and the reaction rate of two functional groups are independent of the sizes of the involved molecules (monomers or growing chains), as explained by Rudin [4]. 

Two growing chains can combine to form one long polymer molecule (termination by combination) or a chain radical abstracts a proton from a neighboring group in another chain end, resulting in two nongrowing chains (termination by disproportionation). A third mechanism is chain transfer, where an active chain transfers its radical to a monomer molecule. The chain stops growing and the monomer molecule acts as the start of a new chain. 

Sometimes, polyesterificaion of a monomer (Pj) is carried out in the presence of monofunctional compounds such as cetyl alcohol, ethyl benzoate, or ethyl terephthalate. Let us denote these by MF]. As the polymerization is carried out, the monomer molecules grow in size to give P . Large-chain monofunctional polymers MF are formed when P and MFj interact. The overall polymerization can be expressed as follows ... 

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). 

The radicals then initiate chain growth by adding an unsaturated monomer molecule, M, to form growing chain... 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

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 ... 

We shall consider now the various degrees of order which characterize polymeric molecules. The addition of a monomeric unit to a growing chain may take place in more than one way. In the case of a vinyl or vinylidine monomer, i.e., CH2—CHA or CH2—CAB, head-to-head or head-to-tail addition may occur. In most cases the head-to-tail addition has a vastly greater probability than the head-to-head or tail-to-tail addition, and thus the latter is responsible only for small imperfections in the chain structure. Studies of head-to-tail and head-to-head additions were vigorously pursued in the 30 s and 40 s, and a good account of this work is available, for example, in Flory s recent monograph.15... 

This radical attacks another monomer molecule, and the chain grows longer ... 

Chain polymerisation Monomer concentration decreases steadily with time. High molar mass polymer is formed at once and the molar mass of such early molecules hardly changes at all as reaction proceeds. Long reaction times give higher yields but do not affect molar mass. The reaction mixture contains only monomer, high molar mass polymer, and a low concentration of growing chains. 

In a batch reactor, the relative monomer concentrations will change with time because the two monomers react at different rates. For polymerizations with a short chain life, the change in monomer concentration results in a copolymer composition distribution where polymer molecules formed early in the batch will have a different composition from molecules formed late in the batch. For living polymers, the drift in monomer composition causes a corresponding change down the growing chain. This phenomenon can be used advantageously to produce tapered block copolymers. 

In addition polymerizations which proceed exclusively by addition of monomer to the end of a growing chain, the mobility of the monomer, being affected only to a comparatively small degree by surrounding polymer molecules, should be adequate to maintain an equilibrium concentration of monomers in the vicinity of active centers. Even in... 

The immediate result of the intervention of the chain transfer process indicated in the first step is the termination of a growing chain and the reactivation of a polymer molecule, which then adds monomer to gener-... 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

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