Polyfunctional initiators

FIGURE 14 Different skeletal structures of PCL and its copolymers derived from the polymerization of e-caprolactone using mono- and polyfunctional initiators. 

A challenging goal in this field, particularly from the synthetic point of view, is the development of general AB polymerization methods that achieve control over DB and narrow MWDs. Experimental results and theoretical studies mentioned above suggest that the SCV(C)P from surfaces, which are functionahzed with monolayers of initiators, permit a controlled polymerization, resulting structural characteristics (molecular weight averages, DB) of hyperbranched polymers. In particular, it is expected that the use of polyfunctional initiators with a different number of initiator functionahty, copolymerization, and slow monomer addition techniques lead to control the molecular parameters. 

New Telechelic Polymers and Sequential Copolymers by Polyfunctional Initiator-Transfer Agents (Inifers) End Reactive Polyisobutylenes by Semicontinuous Polymerization... 

There are limitations for all types of LRP. The occurrence of irreversible bimolecular termination of propagating radicals becomes considerable under certain conditions high monomer conversion, polyfunctional initiators, high initiator concentration, and high targeted molecular weight (about >100,000). 

Coupling of GTP living polymers with halide-terminating agents to form star polymers has been achieved [Hertler, 1996 Webster and Sogah, 1989]. Star polymers are also synthesized by using polyfunctional initiators or by copolymerization with dimethacrylate monomers. 

J.P. Kennedy and R.A. Smith, New telechelic polymers and sequential copolymers by polyfunctional initiator-transfer agents (inifers). II. Synthesis and characterization of a, a>-di(ferf-chloro)polyisobutylenes,. Polym. Sci., Part A Polym. Chem., 18(5) 1523-1537,1980. 

Table 1. Kinetic characteristics of polyfunctional initiators (kid is the dissociation constant)...

Radicals trapped at a solid surface or in a matrix lattice (network) can terminate with untrapped radicals or, though only rarely, by mutual collision, when they can migrate over the surface or in the lattice. When a suitable polymeric polyfunctional initiator is used for initiation, free radicals cannot occur in the system. The growing chains are attached to the polymer surface by covalent bonds. Consequently they cannot migrate, and termination hardly occurs. When the residual, and in any case very small possibility of... 

Coombes and Katchalski [29] have considered a slightly more complex version of this mechanism in which a second propagation coefficient operates above a critical degree of polymerization. Katchalski et al. [30] calculated the molecular weight distribution obtained in a system following scheme (12) but also including a bimolecular termination step. Various authors have analysed more complex systems in which the initiator is a polymeric species. Thus Gold [31] has shown that initiation by a poly a-amino acid with a Poisson distribution leads to a polymeric product with an over-all Poisson distribution, and Katchalski et al. [32] demonstrated that in multichain polymers synthesized from polyfunctional initiators Poisson distributions also arise. 

Then, the polyfunctional initiators are classified into three parts ... 

Alternatively, PEO stars were synthesized using cumylpotassium to polymerize DVB and thus prepare the polyfunctional initiator.20 The products exhibited... 

The polyfunctional initiator method provides the possibility to prepare end-functionalized stars by deactivating the living branches by suitable electrophilic terminating agents. Polystyrene (PS) and PEO stars having end hydroxyl groups were prepared by this method.1519... 

The incorporated -OH groups were transformed to alkoxides using an organometallic base (cumylpotassium) and a suitable cryptand to avoid the precipitation of the polyfunctional initiator. Addition of ethylene oxide produces the dumbbell polymers, as the following reactions indicate (Scheme 99). 

A novel polyfunctional initiator has been prepared by the reaction of a copolymer of styrene and methyl methacrylate with polytetrafluoroethylene (PTFE) radicals generated photochemically from the monomer and manganese carbonyl. These radicals react with aromatic rings by addition and substitution so that the product copolymer from this reaction carries short PTFE chains with Mn(CO)6 end groups of, for example, structures (12). At 100 °C, scission of the CFj— Mn(CO)s bonds occurs with formation of active radicals, and the copolymer behaves as a polyfunctional macroinitiator. On heating this material with styrene or JV-vinyl-2-pyrrolidone a network structure and a graft copolymer respectively are formed. 

Sodium salt of a-methyl styrene tetramer) Three-way growth — polyfunctional initiator ... 

Another approach to introduce branching is by the use of polyfunctional initiators. These initiators are based upon the reaction of an... 

POLYFUNCTIONAL INITIATOR WITH VARIOUS THERMAL STABILITIES ... 

Vinyl polymerizaton living branched polymer polyfunctional initiator RA/ rate of initiation equals rate of propagation, no... 

The arm-first methods are efficient at synthesizing well-defined star-shaped macromolecules. Difficulty arises, however, in the functionalization of the outer chain ends, which is only possible through the use of functional initiators to generate the precursor chains [58]. Core-first methods were developed extensively in polar solvents to access star-shaped macromolecules exhibiting functional groups at the outer chain ends. Once such species are obtained, they can serve as valuable intermediates in the elaboration of a large scope of macro-molecular architectures. Attempts to prepare polyfunctional initiators have been described by Nagasawa and co-workers [62], who s)mthesized core-first star... 

Several factors influence the functionality and the solubility of the plurifunc-tional initiator, including the solvent and counter-ion. In polar solvents, the solubility of the polyfunctional initiator is greater, the ionic associations are minimized, and the rate of initiation is greater compared to nonpolar solvents. Divinylbenzene (DVB) was chosen as the bifunctional compound for the preparation of a plurifunctional initiator in nonpolar solvents [65]. Motivation for the development of the core-first method for the polymerization of oxi-ranes in polar solvents was driven by the need to rend available functional poly(ethylene oxide) star-shaped macromolecules. The initiator systems developed by Burchard are based on lithium as a counterion, which is why it could not be applied to the preparation of multifunctional PEOs under mild conditions. 

As mentioned earlier these star-shaped polymers obtained from polyfunctional initiators are characterized by the presence of active sites located at the outer end of the branches. Thus a large variety of structures can be prepared from these functional star-shaped polymers. Some examples are given in Scheme 7. One should be aware that the distribution in functionalities is large and that advance control of that functionality is difficult. 

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