Monomer addition technique

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. 

To prove that under these conditions, the IB polymerization is living, a monofunctional analogue of 1,2-p-methoxyphenyl-2-methoxypropane, was used to study the kinetics by incremental monomer addition technique. Results of this study indicated Hving polymerization with slow initiation [61,62]. 

In Experiment 12, a small amount of a buffering salt is used. By use of a mixture of monomers, some of w.iich have a high boiling point, a monomer system is used that can be maintained at a reasonably high reaction temperature. By use of a gradual monomer addition technique, the exothermic reaction is controlled. 

Block polymers were prepared by organolithium-initiated polymerization in cyclohexane solution by using the sequential monomer addition technique (3). Polymers were both of the linear-SBS and radial -branched (SB) type. Blends were prepared in cyclohexane solution, either before or after coupling the initially linear SBLi precursor. Coupling agents investigated were ethyl acetate (for linear coupling), epoxi-aized soybean oil (ESO), and SiCh. 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

The DB of branching can be modified by special synthetic approaches, as demonstrated by the NMR quantification of subunits. Copolymerization - for example, the addition of bifunctional monomers AB - resulted in an increase in linear units and, therefore, in a decrease of the DB [109-112]. An enhancement of DB was realized, for example, by employing a slow monomer addition technique [113], the polymerization of prefabricated dendron macromonomers [56, 114], and by a stepwise addition of the monomer mixture for the (A2 + B3) approach [92]. Whereas dendritic and terminal units are essential for a dendritic structure, in the case of hb polymers the content of the linear units can vary greatly. To date, few examples of AB2 hb polymerizations have been reported were the linear unit is a chemically labile structure that either breaks down to the initial educts, or reacts immediately with a further terminal unit to form the stabile dendritic unit. Thus, a hb polymer containing only T and D units is formed, with 100% DB [35,115-119]. 

Procedure 2-14 is an example of the polymerization of methacrylic acid initiated by ammonium persulfate in a single batch operation, while Procedure 2-15 makes use of the gradual monomer addition technique. This latter approach potentially permits the preparation of poly(meth-acrylic acid) solutions of concentration levels greater than the usual 20-25% level. 

The predominant approach toward the synthesis of olefin-based BCPs has focused on development of living coordination polymerization systems. Unfortunately, one feature that makes coordination polymerization catalysts so efficient for production of RCPs also limits their use for synthesis of conventional BCPs. These catalysts are susceptible to several chain termination and transfer mechanisms and typically produce many chains during polymerization. Therefore, a sequential monomer addition scheme produces a physical polymer blend with a conventional catalyst (Scheme 1). However, by designing systems that suppress these termination processes, advanced catalysts have been used to make BCPs via sequential monomer addition techniques (Scheme 1). These systems have produced many new BCPs with interesting structures. Unfortunately, the fundamental features that enable precision synthesis also make the processes very inefficient and thus of limited commercial appeal. Conventional catalysts produce hundreds to thousands of chains per metal center, but these living systems produce only one. For these materials to be competitive with other large-volume TPEs, more efficient protocols for BCP synthesis must be developed. 

Synthetic pol5oners with pendent carbohydrate moieties, which are referred to as glycopolymers, have potential applications in biomedical techniques [32]. Fukuda et al. reported the synthesis of diblock copolymer LB-9 by the sequential monomer addition technique. The first step was the polymerization of St in bulk with a CuBr/4,4 -di-n-heptyl-2,2 -bipyridine (2dHbpy) complex and 1-phenylethyl bromide as initiator at 110°C. After more than 90% conversion of St, a fresh feed of 3-0-methacrylol-l,2 5,6-di-0-isopropyldiene-D-glucofuranose with CuBr/2dHbpy complex dissolved in veratrole was added in vacuum to the precursor PSt-Br, and then polymerization was performed at 80 °C, forming diblock copolymer LB-9. When LB-9 was treated with formic acid, amphiphilic copolymer LB-10 was obtained [32]. 

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