Living polymerization copolymer formation

As with all living polymerizations, the formation of bloek eopolymers is possible if the active chain end of one polymer block can initiate the polymerization of a second monomer. This may mean that when block copolymers are prepared, the sequence of monomer addition may be critical. In this respect, the CRP techniques are no different from the other ionic reactions, but they do have one specific advantage. Because many of the CRP processes do not reach 100% conversion, it is best to separate and purify the polymer adduct formed in the first step, which can then be stored and used as a macroirfitiator for the growth of the second block, with no loss of activity. The use of such a macroinitiator helps to control the subsequent reaction more effectively and produces products with very low polydispersities, because for the macroinitiator, diffusion and reactivity are decreased, thereby mininuzing radical-radical coupling. 

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

As illustrated in Fig. 24, the addition of ethylene during the living polymerization of propylene resulted in rapid increases in both yield and Mn of the polymers. After the rapid increases which required several minutes, yield and lVln increased by a slower rate, identical with that of the propylene homopolymerization. The propylene content in the resulting polymers attained a minimum value several minutes after the addition of ethylene. These results indicate that the second stage of the polymerization with ethylene was complete within several minutes to afford a diblock copolymer, followed by the third stage of propylene homopolymerization leading to the formation of a triblock copolymer. The 13C NMR spectra of the diblock copolymers showed that the second block was composed of an ethylene-propylene random copolymer sequence. 

Using this technology, the preparation of the fifth tier, monoaldehyde dendrimer was reported.[102] Subsequently, living poly(methyl methacrylate) (PMMA, prepared by group transfer polymerization) was treated with the site-specific cascades. Polydispersi-ties determined for the copolymer were similar to those recorded for the living polymer when smaller dendrimers were used, whereas the use of larger dendrimers for copolymer formation leads to a dependence of polydispersity on the dendrimer. 

A truly living polymerization allows for the preparation of block copolymers without the formation of the respective homopolymers. Reports are available on experiments in which Nd catalysis was applied to synthesize both diene/diene as well as diene/non-diene block copolymers. 

The first report is available from Shen et al. who studied the preparation of BR/IR block copolymers by sequential polymerization of BD and IP [92]. Shen et al. found that the polymerization of the second monomer batch resulted in an increase of solution viscosity by 100%. The viscosity increase was considered as strong evidence in favor of block copolymer formation. Further evidence came from stress strain measurements in which the respective BD/IP block copolymers were compared with blends of BR and IR (at the same molar masses). It was found that the block copolymer exhibited higher elongation at break and higher tensile strength. Unfortunately, Mn data were not provided. Therefore, these results are not fully relevant regarding requirement No. 5 for a living polymerization. 

As mentioned above, the ability to have living polymerizations offered the potential to make block copolymers. In the preparation of a block copolymer the sequence of addition can be important to ensure that the second monomer is capable of adding to the living end. An example is the formation of a polystyrene—polymethyl methacrylate block copolymer.38 In this case polystyrene is polymerized first, followed by addition of the methyl methacrylate. The block copolymer could not be formed if methyl methacrylate were polymerized first, as styrene will not add... 

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

Sequential Polymerization. The sigmoidal reaction curves, which indicate a tendency for the molecular weight to increase during the course of the reaction, and other considerations led to the suggestion that the polymerizing chains had long lifetimes (9), similar to the chains in a living polymerization. If this is the case, sequential addition of different monomers would lead to block copolymer formation. To check this hypothesis, PMDS and HMDS were polymerized sequentially with suflScient time between additions for the first monomer to be consumed. In one experiment, PMDS was the first monomer, and in another experiment, HMDS was the first. In... 

The dormant polymer is living in the sense that it grows until the monomer is depleted, and that it can grow on after additional monomer feed as in an ionic living polymerization.3 The final degree of polymerization is determined by the initial concentrations of the monomer and of the radical precursor Ro Y, and the formation of block copolymers is possible. 

The concept of living polymerizations started in 1956 when Szwarc found that in the anionic polymerizations of styrene (St) the polymer chains grew until all the monomer was consumed [9], and that the chains continued growing when another batch of monomer was added. The addition of another monomer resulted in the formation of block copolymers. These polymerizations proceeded without termination or chain transfer occurring in the system. Prior to this work, the conditions used for the polymerizations had not been stringent enough to keep the active species alive and allow observation of this type of behavior. The polymer molecular weights were predictable based on the ratio of... 

Non-living polymerization techniques can be combined with CRP methods to produce block copolymers. The first example of transforming a hydroxy functionality into an ATRP initiator was demonstrated by Gaynor and Matyjaszewski [223], who converted a polysulfone,prepared through the condensation polymerization of 4,4-difluorosulfone with an excess of bisphenol A, to an ATRP initiator by reaction with 2-bromopropionyl bromide for subsequent controlled polymerization reactions (cf. Scheme 26). The transformation proved to be quantitative and the macroinitiator (Mn=4030,Mw/Mn=1.5) was used for formation of triblock copolymers with St (Mn=10,700,Mw/Mn=l.l) or nBA (Mn=15,300,Mw/Mn=1.2) as shown in Fig. 31 [223]. DSC analysis provided evidence of the presence of two distinct blocks with Tg=153-159 °C (polysulfone) and 104 °C (pSt) or -41 °C (pBA). 

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