Radicals growing

Each of these reactions is characterized by a propagation constant which is labeled by a two-digit subscript The first number identifies the terminal repeat unit in the growing radical, and the second identifies the adding monomer. The rate laws governing these four reactions are... 

Any discussion based on reactivity ratios is kinetic in origin and therefore reflects the mechanism or, more specifically, the transition state of a reaction The transition state for the addition of a vinyl monomer to a growing radical involves the formation of a partial bond between the two species, with a corre sponding reduction of the double-bond character of the vinyl group in the monomer ... 

For a growing radical chain that has monomer 1 at its radical end, its rate constant for combination with monomer 1 is designated and with monomer 2, Similady, for a chain with monomer 2 at its growing end, the rate constant for combination with monomer 2 is / 22 with monomer 1, The reactivity ratios may be calculated from Price-Alfrey and e values, which are given in Table 8 for the more important acryUc esters (87). The sequence distributions of numerous acryUc copolymers have been determined experimentally utilizing nmr techniques (88,89). Several review articles discuss copolymerization (84,85). 

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Although reactivity ratios indicate that VP is the more reactive monomer, reaction conditions such as solvent polarity, initiator type, percent conversion, and molecular weight of the growing radical can alter these ratios (138). Therefore, depending on polymerization conditions, copolymers produced by one manufacturer may not be identical to those of another, especially if the end use appHcation of the resin is sensitive to monomer sequence distribution and MWD. 

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. 

In mutual termination the rate of reaction is determined by the concentration of growing radicals and since two radicals are involved in each termination the reaction is second order. 

R may be a radical formed by the decomposition of an initiator or a growing radical chain. Similarly, grafting by the chain-transfer mechanism occurs when the branched part consists of another monomer. Since cellulose is a poor transfer agent [8], the efficiency of grafting is quite poor. Incorporation of—SH groups into cellulose enhances the probability of chain transfer. This can be achieved as follows ... 

Stereospecificity of this reaction reaches 15 1 for telomer T3. Telomer T3 is a crystalline product, this allowed the authors to use X-ray diffraction analysis for studying stereochemistry. Stereoselectivity observed in the formation of T3 shows that both addition step and the step of halogen transfer to the growing radical proceed stereoselectively in this case. 

A superficially related dependence of on the medium has been observed by Norrish and Smith working with methyl methacrylate, and by Burnett and Melville with vinyl acetate. Rates in poor solvents are high, and determination of by the rotating sector method reveals what appears to be a decrease in kt in the poor solvents. This apparent decrease in kt accounts for the increased rate of polymerization. Actually, precipitation of the polymer seems to be responsible for the effect. The growing radicals become imbedded in precipitated droplets, presumably of very small size. The termination reaction is suppressed owing to isolation of the chain radical in one droplet from that in another. This gel effect is fairly common in systems yield-... 

The highly important deduction that one-half of the polymer particles will contain a single growing radical and the other half will contain none at any given time was first set forth by Smith and Ewart. They immediately pointed out that the rate of polymerization per cubic centimeter of water should then be given by... 

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

It has been suggested that the formation of a coordination complex between comonomers and growing radical species reduces the reactivity of the TBSM. .. MA complex towards growing MA and TBSM radicals. 

From this kinetic scheme, two characteristic parameters can be defined, r and r2 r is a measure of the ratio between the rate of addition of monomer 1 to the rate of addition of monomer 2 on a growing radical terminated by monomer 1, while r2 is the ratio between the rate of addition of monomer 2 to the rate of addition of monomer 1 on a growing radical terminated by monomer 2. 

Nitroxyl spin adducts (A ) react with the growing radical chain leading to the formation of labile end groups (Scheme 2.208). 

However, when = r2 = 0, each growing radical has a preference to add to another monomer unit yielding a perfectly alternating copolymer of the type ... 

TEMPO combines with the radical chain and keeps the concentration of the growing radical chain low, such that the recombination of radicals is suppressed. This type of radical polymerisation is called Atom Transfer Radical Polymerisation (ATRP). It has the properties of a living polymerisation, as the molecular weight increases steadily with time and one can make block polymers this way by adding different monomers sequentially. 

The rotating sector method requires the introduction of the parameter is, defined as the average lifetime of a growing radical under steady-state conditions. The radical lifetime is given by the steady-state radical concentration divided by its steady-state rate of disappearance ... 

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