Ideal living polymers

Ideally, living polymers should retain their activities forever provided that they are not subjected to a killing action. However, in any real system this is not the case. Some feasible slow side reactions which annihilate the growing ends are unavoidable, and these set an upper limit to the durability of living polymers. Nevertheless, if the rates of these reactions are sufficiently slow to permit successful completion of a desired task, the system may correctly be classified as composed of truly living polymers. 

All these data demonstrate the formation of almost ideal living polymers in the IBVE polymerization initiated with HI/I2 in n-hexane solvent. The MWD of the polymers is nearly monodisperse (M /M < 1.1), in support of our expectation of fast initiation by this initiating system. Thus, a nearly ideal living system has been... 

It will be explained in Section 3.4 that an ideal living polymer has a very narrow molecular weight distribution that is described by the Poisson distribution ... 

The GPC traces in Fig. 24 reveal a broad molecular weight distribution, MJMn = 4.42, for the dual reactor blend sample. On the other hand, the diblock OBC displays an overall MJMn of 1.67. The narrowing of the distribution indicates that the polymerization has CCTP characteristics. The theoretical molecular weight distribution from an ideal living polymerization in a series of two CSTR reactors is given by the following equation, where/j and/2 are the mass fractions of polymer comprising the two blocks [11] ... 

The polymerization of MA with 7 was carried out in the presence of 13, i.e., 7 and 13 were used as two-component iniferters [175]. When an identical amount of 13 to 7 was added to the system, the polymerization proceeded according to a mechanism close to the ideal living radical polymerization mechanism. Similar results were also obtained for the polymerization of VAc. These results indicate that the chain end of the polymer was formed by the competition of primary radical termination and/or chain transfer to bimolecular termination, and that it could be controlled by the addition of 13. 

Eq. (17) predicts that, when Pn is 100, the polydispersity is equal to 1.01, so that the polymer is virtually monodisperse. However, such ideal monodisperse polymers have scarcely been synthesized. The lowest values of polydispersity (Mw/Mn = 1.05-1.10) have been attained in homogeneous anionic polymerization 43). Gold 41) calculated the polydispersity of a polymer in the living polymerization with a slow initiation reaction and showed that the value of Pw/Pn increases slightly to a maximum (1.33) with an increase in polymerization time, followed by a decrease toward 1.00. Other factors affecting the molecular weight distribution of living polymer have been discussed in several papers 5S 60). 

Monomer added at the end of polymerizations linked to chains already present and molecular weights were determined solely by the monomer/initiator ratio. However, dispersity ratios, MJMn, do not approach the values (1.02-1.05) attained in truly ideal living polymerizations. The molecular weight distributions of polymer formed when monomer is added slowly to initiator solutions were even broader suggesting that initiation is slower than propagation. 

Johnston and Pepper conclude that phosphines initiate near ideal living polymerizations. However, when the authors turned to amine initiators they found that, although macrozwitterions were formed, the polymerization kinetics were very different. At comparable reagent concentrations room temperature rates were at least one thousand times slower, but paradoxically increased as temperature was reduced. Arrhenius plots indicated that by -100°C amine and phosphine polymerization rates would be equal. Polymer molecular weights were much higher than would have been expected had initiation been complete, and were uninfluenced by polymerization conditions. It is believed that molecular weights are determined by traces of weak acid transfer agents present in the monomer. 

The nature of free-radical polymerization has traditionally hindered attempts to produce an ideal living free radical polymerization technique. It is very difficult to prevent chain transfer and termination reactions in free-radical polymerizations and although several methods have afforded polymers with very low polydispersities < 1.1), these approaches are often referred... 

The concept of flash chemistry can be applied to polymer synthesis. Cationic polymerization can be conducted in a highly controlled manner by virtue of the inherent advantage of extremely fast micromixing and fast heat transfer. An excellent level of molecular weight control and molecular-weight distribution control can be attained without deceleration caused by equilibrium between active species and dormant species. The polymerization is complete within a second or so. The microflow system-controlled cationic polymerization seems to be close to ideal living polymerization within a short residence time. 

The polymerization of MMA by alkyllithium has been studied in greater detail than that by any other initiator. For bulky alkyllithiums such as 1,1-diphenylhexyllithium, the polymerization in THF proceeds in an ideal manner to give a living polymer ... 

Ideal living polymerization is a most desired process since it offers a vide range of opportunities for obtaining polymers with well-defined structure and molecular weights, and in many cases, narrow molecular weight distributions. However, by analyzing the existing... 

Under appropriate conditions, mostly using alkyl halides, triflates, or tosylates as initiator, the CROP of 2-oxazolines proceeds via a living mechanism [84, 86]. In such an ideal living polymerization, all polymer chains are initiated at the same time by nucleophilic attack of the imino ether onto an electrophilic initiator. Similar to the previously discussed cationic polymerizations, the CROP... 

Figure 13.1 Idealized living polymerization one monomer unit adds to the end of each polymer chain during each reaction step. Each black dot is a single monomer molecule and a string of black dots is a polymer chain.

A. -C. Shi, M. K. Georges, H. K. Mahabadi, Kinetics of controlled living free radical polymerization 1. Ideal case, Polym. React. Eng. 1999, 7, 283-300. 

The MWD of polymers prepared by living anionic polymerization is a good example [65]. According to the early theoretical work of Flory, an ideal living anionic polymerization is expected to yield a polymer with Poisson distribution of chain lengths [66] ... 

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