Chain-transfer and termination

While termination of chain growth in cationic polymerization may take place in various ways, many of the termination reactions are, in fact, chain transfer reactions in which the termination of growth of a propagating chain is accompanied by the generation of a new propagating species. 

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A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

The production rate is 2—4 t/h, depending on the feed rate, monomer concentration in the feed, and conversion. The conversion of isobutylene and isoprene typically ranges from 75—95% and 45—85%, respectively, depending on the grade of butyl mbber being produced. The composition and mol wt of the polymer formed depend on the concentration of the monomers in the reactor Hquid phase and the amount of chain transfer and terminating species present. The Hquid-phase composition is a function of the feed composition and the extent of monomer conversion. In practice, the principal operating variable is the flow rate of the initiator/coinitiator solution to the reactor residence time is normally 30—60 minutes. 

One chain-end is typically unsaturated due to chain transfer and termination mechanisms. Mol wts can range from several hundred to several million. There is no long-chain branching unless special synthesis methods ate employed. The mol wt distribution is commonly the most probable,... 

The polymerization of 2-methyl-2-oxazoline is a clean reaction, which is not disturbed by chain transfer and termination. In this polymerization, the propagating species having the structure of an oxazolinium salt is not fragile, which is conveniently utilized for syntheses of block copolymers and end-reactive polymers [28],... 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

Amination. The synthesis of polymers with primary amine end-group functionality has been a challenge because the primary amine group can undergo rapid chain transfer and termination reactions with car-banionic chain ends (14). Schulz and Halasa (15) used a phenyllith-ium initiator with a bis(trimethylsilyl)-protected amine group to prepare amine-terminated polydienes. Nakahama and coworkers (16,17)... 

F. Infiuence of the Temperature on the Single-Chain Transfer and Termination Processes. 40... 

The chain transfer and termination processes have been studied by the following methods ... 

The terms of the numerator indicate the rates of chain transfer and termination processes depending, respectively, on the concentration of the growing chains, on the partial pressure of olefin, on the concentration of alkylaluminum, and on the amount of titanium compounds. 

Sr = sum of the rates of chains transfer and termination processes Ct = number of growing chains at time t... 

A measure of the relative importance of these processes may be obtained by dividing the number of depolymerization steps occurring by the number of chain transfer and termination steps. This parameter is the zip length and gives a measure of the tendency of the polymer to break down into a monomer. If the zip length is low, there will be little monomer in the volatile products with an increased probability for a higher char yield. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

In an alkyne polymerisation system with Ziegler-Natta catalysts, chain transfer and termination reactions, similar to those postulated for olefin polymerisation, have been suggested to take place [25]. A possible chain transfer reaction is the formation of the Ti H species from the propagating chain end by /h hydride elimination ... 

GPC has become especially useful in polymerization reaction engineering since it permits comparatively rapid and precise determination of MWD. As shown in Figure 5, Xw obtainable from the MWD in a batch reactor can be used to determine the initiation rate constant as well as kp2/kt assuming the pseudo-steady state, the absence of chain transfer, and termination by combination. Similar relationships with CSTR s are available. 

DP is the average number of monomer (or repeat) units per polymer chain and so is directly related to molecular weight (or viscosity). This relationship shows that we must have control over variables that have a significant effect on propagation, chain transfer, and termination to achieve the desired polymer properties. What are these variables They are the same as those we have discussed throughout this chapter temperature, reactant monomer concentrations, concentrations of chain transfer agents or other impurities that affect polymerization, initiator or catalyst concentration, residence time, etc. 

In contrast to chain transfer to solvent which would be prevalent from the initial stages of a polymerization due to the solvent s high concentration, chain transfer to polymer often does not compete noticeably with propagation until the end of the polymerization when monomer is depleted. In addition, chain transfer and termination reactions generally have higher activation energies than propagation, and therefore can be... 

Since the 1960s [20], much of the research in polymer synthesis has been directed at establishing living conditions for chain polymerizations. The only requirements for a polymerization to be considered living are that no chain-breaking reactions occur during the polymerization. That is, the rate constants of both chain transfer and termination should be equal to zero (Atr = 0, k, = 0). 

In practice, linear semilogarithmic kinetic plots and linear dependencies of molecular weight on monomer conversion require only that the rate constants of chain transfer and termination are much less than that of propagation klr kp, k,< kp). This is therefore the practical requirement for the synthesis of well-defined polymers, such that complete monomer conversion can be reached and the chain ends can be functionalized quantitatively. However, because chain-breaking reactions are actually present, we prefer to call such systems controlled polymerizations rather than living polymerizations. 

Along with block copolymers, polymers with terminal functions, or end-functionalized polymers, are another typical class of well-designed polymers that living polymerizations can provide. On the basis of the absence of chain transfer and termination, when coupled with the quantitative and selective initiation from a well-defined initiator, living polymerizations offer two basic methods to prepare end-functionalized polymers, as Scheme 4 illustrates for cationic processes ... 

Observed molar mass of poly(/3-pinene) are low (M from 600 to 2000), due to important chain transfer and termination processes, either spontaneous or involving alkylation of the aromatic solvent. Transfer might also involve a sterically hindered nonpropagating camphenium ion... 

Another key point to note is that chain transfer and termination by radical combination leads to radicals and molecules with more carbon atoms than the feed (propane). Subsequent involvement of these moieties in the radical chain propagation leads to larger molecules. In practice this maimer of radical cracking of ethane and propane cracking leads to some C4, C5 and Ce+ products forming pyrolysis gasoline. 

A special case arises when both the rate of chain initiation is greater than the rate of chain propagation and when chain transfer and termination rates are much smaller than... 

Chain transfer and termination reactions leave their imprint primarily on the resulting structure of the end groups. Unless this structure is known any speculation about these two processes will be premature. The chemical nature of the end groups has, however, been studied only in few systems. 

The basic steps in ftee-radical polymerizstion are initiation, propagation, chain transfer, and termination. 

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