Block copolymer polymerization with

The synthesis of block copolymers by macromonotner RAFT polymeriza tion has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thioearbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block formation is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared.476 638 The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

A half-metallocene iron iodide carbonyl complex Fe(Cp)I(CO)2 was found to induce the living radical polymerization of methyl acrylate and f-bulyl acrylate with an iodide initiator (CH3)2C(C02Et)I and Al(Oi- Pr)3 to provide controlled molecular weights and rather low molecular weight distributions (Mw/Mn < 1.2) [79]. The living character of the polymerization was further tested with the synthesis of the PMA-fc-PS and PtBuA-fi-PS block copolymers. The procedure efficiently provided the desired block copolymers, albeit with low molecular weights. 

In previous sections, much emphasis has been put on block copolymer micelles with a spherical morphology. It was shown in Sect. 5 that the characteristic sizes of both the spherical core and corona of block copolymer micelles can be precisely adjusted by essentially controlling the chemical nature and the degree of polymerization of the constituent blocks. For several applications of block copolymers micelles including, e.g., micellar templating... 

Shibasiki Y, Sanada H, Yokoi M, Sanda F, Endo T (2000) Activated monomer cationic polymerization of lactones and the application to well-defined block copolymer synthesis with seven-membered cyclic carbonate. Macromolecules 33 4316-4320... 

Both methods require that the polymerization of the first monomer not be carried to completion, usually 90% conversion is the maximum conversion, because the extent of normal bimolecular termination increases as the monomer concentration decreases. This would result in loss of polymer chains with halogen end groups and a corresponding loss of the ability to propagate when the second monomer is added. The final product would he a block copolymer contaminated with homopolymer A. Similarly, the isolated macroinitiator method requires isolation of RA X prior to complete conversion so that there is a minimum loss of functional groups for initiation. Loss of functionality is also minimized by adjusting the choice and amount of the components of the reaction system (activator, deactivator, ligand, solvent) and other reaction conditions (concentration, temperature) to minimize normal termination. 

One of the most useful features of living polymerizations, which proceed in the absence of chain transfer to monomer and irreversible termination, is the ability to prepare block copolymers. Compared with living anionic polymerization the development of living cationic polymerization is rather re-... 

Quirk and Seung used Bu3NaMg for initiating the copolymerization of oxirane with styrene. They obtained block copolymers while with poly(sty-ryl)lithium or 1,1-diphenylhexyllithium, oxirane did not polymerize at all... 

Along with block copolymers, polymers with terminal functions, or end-functionalized polymers, are another typical class of well-designed polymers that living polymerizations can provide. On the basis of the absence of chain transfer and termination, when coupled with the quantitative and selective initiation from a well-defined initiator, living polymerizations offer two basic methods to prepare end-functionalized polymers, as Scheme 4 illustrates for cationic processes ... 

Block copolymers were synthesized with St, MA, and MMA using the homogeneous CuBr/dNbpy catalyst system. The molecular weight of the macroinitiator (Mn=15,400,Mw/Mn=1.39) increased with the formation of the second block (Mn=24,900 to 35,000) and the molecular weight distribution remained narrow (Mw/Mn>1.5) [274]. Difunctional macroinitiators were also prepared. Once the THF polymerization was complete, the polymer was reacted with sodium 2-bromopropionate to produce the ATRP initiator, which was then used for the preparation of block copolymers, again with St, MMA, and MA. For both St and MA, the polymerization leading to the preparation of the second block was incomplete however, it proceeded smoothly for MMA. In both systems, block formation was confirmed through DSC analysis [274]. 

A number of block copolymers prepared with Ziegler-Natta catalysts have been reported however, in most cases the compositions may include significant amounts of homopolymer. The Ziegler-Natta method appears to be inferior to anionic polymerization for synthesizing carefully tailored block copolymers. Nevertheless, bock copolymers of ethylene and propylene (Eastman Kodak s Pofyallomers) have been commercialized. Unlike the elastomeric random copolymers of ethylene and propylene, these are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene. They also contain homopolymers in addition to block copolymers. 

Several commercial processes using difunctional initiators based on soluble organohthium compounds have been developed. These compounds can polymerize at both ends. Difunctional initiators are useful in the cases of ABA block copolymers where B can initiate A but A cannot initiate B. These difunctional initiators are useful in the preparation of SBS. The elastomeric butadiene block is polymerized with hexane as a solvent. The added styrene monomer is also soluble in hexane. This method is also useful in preparing triblocks with hydrocarbon middle blocks and polar end blocks such as poly(methacrylonitrile-h-isoprene-h-methacrylonitrile). 

Figure 12.3 Thermai gravimetric analyses of PEO-PBT multi-block copolymers. Reprinted with permission from Advanced Functional Materials, Tailor-made polymeric membranes based on segmented block copolymers for CO2 separation, by A. Car, C. Stropnik, W.

Styrenic block copolymer blends with polyolefins are among the most widely used blends and they vary in hardness from low Shore A on the softer side to >60 D on the harder side and are used in a wide variety of applications. EPDM blends with polyolefin that are made by compounding or by direct reactor polymerization have also found several applications in the automotive industry. At low concentrations ( 20% or less) EPDM and EPR have been used as an impact modifier for polyamides [16]. 

For the block copolymers obtained with PBLG-Xi as macro-initiator (Table 1/entry 1 and 2) a shoulder in the SEC chromatogram (Figure 3) was observed. This shoulder is supposed to be due to PS-homopolymer resulting from the styrene autopolymerization. The investigation of the polymerizations kinetics via GC (see Figure 4) shows a... 

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