Ideal Living Polymerization

Chain transfer to methacrylate and similar maeromonomers has been discussed in Section 6.2.3.4. The first papers on the use of this process to achieve some of the characteristics of living polymerization appeared in 1995.380 The structure of macromonomer RAFT agents (163) is shown in Figure 9.3. An idealized reaction scheme for the case of a MMA terminated macromonomer is shown in Scheme 9.36. 

Most reviews on living radical polymerization mention the application of these methods in the synthesis of end-lunctional polymers. In that ideally all chain ends are retained, and no new chains are formed (Section 9.1.2), living polymerization processes are particularly suited to the synthesis of end-functional polymers. Living radical processes are no exception in this regard. We distinguish two main processes for the synthesis of end-functional polymers. 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

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] ... 

This type of polymerization process is called living polymerization because the polymer molecules grow forever until aU monomer is consumed. Ideally, there is no step that terminates growth. One type of this polymer is called ionic because the active species... 

Assuming perfect living polymerization in which all polymer chains are induced to initiate the simultaneous propagation of chains without any termination and chain transfer reactions, in such idealized scenarios, the number-average molecular weight, Mn, can be estimated using... 

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). 

Another real active species was isolated form the reaction of [Cp 2SmH]2 with two equivalents of MMA monomer (methyl methacrylate) [289]. Or-ganolanthanide complexes of type Cp2LnR (R = H, alkyl) are not only effective precatalysts in the polymerization of nonpolar monomers such as ethylene, but also initiate the ideal living polymerization of MMA [289-291]. 

Increase of Molar Mass on Sequential Monomer Addition In an ideally living polymerization, molar mass increases on the sequential addition of two or more monomer batches. In detail, Mn should proportionally increase with increasing ratios of mm/mncI as pointed out for criterion No. 3 Linear dependence of Mn on monomer/catalyst-ratio at constant monomer conversion . Usually the storage temperature and the length of shelf life prior to the addition of the second monomer batch are not further defined. 

In conclusion, styrene can be considered as an ideal monomer in photo-chemically induced polymerization and can be used successfully in quasi living polymerizations. 

The term living polymerization was originally used to describe a chain polymerization in which chain-breaking reactions are absent [I], In such an ideal system, after initiation is completed, growing or propagating chains would only propagate and would not participate in transferor termination. Each chain should infinitely retain its ability to react with monomer. 

Chains with monodisperse molecular weight distribution (Mw/Mn = 1.00) can occur in idealized conditions when all polymerizing centers initiate instantaneously and chain termination is absent. In these cases the catalyst is actually an initiator. These living polymerizations are quite rare among transition metal catalysts. More often, random chain termination leads to many chains formed per metal atom. A Schulz-Flory most probable distribution of polyalkene molecular weights (Mw/Mn = 2.00) is the result. In cases when more than one type of active site is present, bimodal or multimodal distributions of molecular weights result (Mw/Mn > 2.00). 

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. 

In ideal living polymerization, the following points are essential requirements ... 

In Section 9.4.1 we discussed ideal living polymerization. Let us examine the livingness of the microflow system-controlled cationic polymerization here. 

---