Living polymerization systems

The use of single-site initiators for the polymerization of acrylates is attractive, since steric protection of the metal center should eliminate the unwanted side reactions described above, allowing living polymerization systems to be developed. Further, stereocontrol may be achievable by appropriate ligand selection. 

The advantages provided by the continuous reactor prompted us to explore CCTP in a reactor with two CSTRs connected in series [11]. This reaction scheme depicted in Scheme 6 provides a highly flexible process for production of a wide range of diblock OBC compositions. The block composition can easily be varied by changing the production rate in either reactor. The comonomer content of either block can also be independently tailored by varying the feed compositions because the process operates in two independent reactors. This CCTP scheme also produces multiple chains per catalyst, an advantage over stoichiometric living polymerization systems, but is necessarily stoichiometric in CSA. The reaction produces... 

In coi itrast, in living polymerization systems, the polymerization occurs without chain transfer or chain termination, giving greater control over polydispersity of the resultant polymers. Such polymerization systems allow the controlled synthesis of water-soluble polymers and enable precise control over the composition of block copolymers. 

Aluminum isopropoxide has been used for the preparation of block copolyesters [147, 148]. Tri-block poly(e-CL-b-DXO-e-CL) was prepared by the sequential addition of different monomers to a living polymerization system initiated with aluminum isopropoxide in THF or toluene solution [95]. An alternative route for the preparation of the tri-block copolymer was to react the diblock poly(e-CL-b-DXO) containing an -OH functionality at the chain end using a difunctional coupling agent such as isocyanate or acid chloride (Scheme 18). However, the molecular weights were low and full conversion of monomers was not achieved. 

Polymers with even narrower mass distributions, e.g. with PDI values close to 1, arise in living polymerization systems, in which no chain termination processes can occur at all, such that all chains remain bound to the metal centre from which they have started to grow at the same time. Living polymerizations, which offer useful opportunities, e.g. with regard to the production of block copolymers by exchange of one monomer for another, occur in anionic polymerizations of styrenes or butadienes such as are induced by simple lithium alkyls. For a-olefin polymerization catalysts of the type discussed above, living polymerizations are rare. These more elaborate catalysts can thus release a newly formed polymer chain within a time interval of typically less than one... 

Another consequence of living polymerization systems is that they can be used to synthesize block copolymers. Under these conditions, once the initial quantity of monomer in a given reaction is consumed, the resultant polymer chains are terminated with metal carbene end groups that are still active for aUcene metathesis. As long as these carbenes do not react rapidly with the acyclic aUcenes in the polymer chain, addition of a second monomer will result in the synthesis of a block copolymer. This reaction is illustrated in equation (13) for the synthesis of a polymer that consists of block of x repeat units of monomer A followed by a block of y repeat units of monomer B. 

Sc) True living polymerization system Formation of Cp 2Sc(C3H7) [31]... 

In a living polymerization system, Beste reports that the polymerization rate is directly related to the concentration of initiating species (6). As shown in Figure 4, it seems that polymerization rate of this ECH polymerization is directly related to the concentration of Initiator. No further study on the polymerization kinetics was conducted in the present study. 

This is the difference between a living polymerization system (chains grow until the reaction is terminated) and a nonliving system (continuous and competitive growth and termination). Phillips catalysts are nonliving systems, and in the above example an active Cr site produces about 20,000 chains during its 1-h lifetime. 

In a series of papers43 4S), the kinetics of anionic polymerization of ethylene oxide in conjunction with different catalysts were studied. These studies expand our understanding of the mechanism of living polymerization systems and provide new information on the processes of active center association. Herein, primarily, lies the specific nature of the heteroatomic systems, as compared with the vinyl monomers studied earlier 9 ... 

The polymerization of pivalolactone can be initiated by a variety of nucleophiles. For example, tributylphosphine (TBP) attacks the molecule exclusively at the beta position, causing ring opening and propagation via a carboxylate ion. The high stability of this growing macrozwitterion results in a living polymerization system. 

Although the system can essentially be described as a living polymerization system, termination reactions are possible. These occur especially at high temperatures under the conditions encountered on processing the plastic in an extruder. 

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