Chains growing time

A number of papers and patents describe polymerization processes to poly(tetramethylene ether) glycols having a narrow molecular weight distribution = 1.2—1.4). In principle, this can be achieved by having all chains grow quickly at one time, either by high temperature initiation (33)... 

Ionic polymerizations are almost exclusively solution processes. To produce monodisperse polymers or block copolymers, they must be mn batchwise, so that all chains grow for the same length of time under identical conditions. 

The number of isotactic chains growing at the same time, was assumed as being equal to the number of conventional active centers 1 X 10 mol./ mol. of a-TiCla. The results obtained are plotted in Fig. 36. 

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

Sometimes experimenters are tempted to determine the number of chains formed during polyerization and assume each site makes one chain, but a site terminates and reinitiates chains continuously, making this approach invalid except at very short reaction times. Quick-kill experiments in this laboratory (69) tend to confirm Hogan s number (77), but to actually see the first chain growing with time, the polymerization must be artificially slowed by using noncommercial conditions, and the results are not very reproducible. 

Of these, reaction (a) although rapid would soon disappear with chain growth because the epoxide would be displaced from the catalyst complex by the chain ether groups. Reaction (b) should contribute to the rate for some time since hydroxyl groups have similar basicities to ethers, but it is probably not a rapid reaction and its importance should decrease steadily as the chains grow longer it probably makes its maximum contribution at di- or triethylene glycol. 

Other polymerization mechanisms can produce distributions of molecular weights that are different from the most probable distribution. Much narrower distributions are obtained if a specified number of chains is initiated at the same time, and these chains grow exclusively by the addition of monomer to the reactive end. Then... 

Conventional, butyllithium-initiated polymerization faces a major drawback. Owing to the living nature of anionic polymerization, all chains grow at the same time. As a consequence, turnover rates are exceedingly fast at high monomer concentrations, and removal of the heat of polymerization is difficult. Hence anionic polymerization has been restricted to dilute solutions and low temperatures. Under these conditions, polystyrene cannot be produced economically. However, with certain Lewis acids such as dibutylmagnesium this problem could be solved [18,19]. 

This carbon radical reacts with more monomer, so that Step [3] occurs repeatedly, and the polymer chain grows. Each time a carbon radical adds to a double bond, a new C-C bond and a new carbon radical are formed. 

The peptide chain grows in length by the addition of amino acid residues from the amino-terminal end (D5). The assembly time of one chain is estimated at 15-30 seconds at 37°C. Termination of the chain assembly is probably due to the presence of the mRNA codon UAA (or the less likely codons UAG and UGA) which is read as end chain and causes the chain to be detached from the assembly line. 

Thus, in an uncatalysed reaction the polymer chain grows with the square root of the time of reaction, a direct consequence of third-order kinetics. Consequently, in any practical application, the use of an external catalyst is necessary in order to achieve high-molar-mass polymer in the shortest possible time. 

It was observed experimentally that if the EO addition rate is high (in fact a high ethoxylation pressure), cloudy polyols and low primary hydroxyl content are formed [51]. The explanation of this behaviour is that if the rate of EO addition is too high, the equilibrium of alcohol - alcoholate does not have enough time to get established and the polyethylene oxide chains grow on a limited number of hydroxyl groups. As an immediate... 

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