Sequential copolymers, controlled

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

Possibilities offered by controlled initiation have been exploited for the preparation of new sequential copolymers and mac-romers and are discussed below. 

A large number of sequential copolymers comprising many combinations of glassy and elastomeric segments have been prepared (1). The block and graft copol3nners prepared by controlled initiation are remarkably well defined and many are free of homopolymers. Sequential copol3nners obtained by controlled cationic initiation have been recently comprehensively surveyed (1,8). 

The preparation of well-defined sequential copolymers by an anionic mechanism has been explored and utilized commercially for some time now. Initially, the cationic methods received less attention until it was demonstrated by Kennedy that a large variety of block copolymers can be formed. The key to Kennedy s early work is tight control over the polymerization reaction. The initiation and propagation events must be fundamentally similar, although not identical ... 

Socka, M., Duda, A., Adamus, A., Wach, R, Ulanski, P., 2016. Lactide/trimethylene carbonate triblock copolymers controlled sequential polymerization and properties. Polymer 87, 50-63. 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

A second route is termed sequential anionic polymerization. More recently, also controlled radical techniques can be applied successfully for the sequential preparation of block copolymers but still with a less narrow molar mass distribution of the segments and the final product. In both cases, one starts with the polymerization of monomer A. After it is finished, monomer B is added and after this monomer is polymerized completely again monomer A is fed into the reaction mixture. This procedure is applied for the production of styrene/buta-diene/styrene and styrene/isoprene/styrene triblock copolymers on industrial scale. It can also be used for the preparation of multiblock copolymers. 

So far the discussion was focused on copolymers derived from a mixture of styrene and a diene. In view of the "living" nature of organolithium polymerization, it is also possible to synthesize block polymers in which the sequence and length of the blocks are controlled by incremental (or sequential) addition of monomersr This general method of preparing block polymers is readily adaptable to commercial production, and, indeed, a number of block copolymers are manufactured this way. Those that have received the most attention in recent years are the diene-styrene two-phase... 

When the propagating species are long-lived it is possible to make block copolymers of controlled chain length by sequential addition of monomers, for example with ClC=CC4H9/ClC=CCi4H29686 693 and with acetylene/norbornene396,628. 

The synthetic procedure for the synthesis of the inverse starblock copolymers is given in Scheme 25. Diblock arms (I) having the living end at the PS chain end were prepared by anionic polymerization with sequential addition of monomers. In order to accelerate the crossover reaction from the PILi to the PSLi chain end a small quantity of THF was added prior the addition of styrene. The living diblock (I) solution was added dropwise to a stoichiometric amount of SiCl4 until two arms are linked to the silane. This step was monitored by SEC and is similar to a titration process. The end point of the titration was determined by the appearance of a small quantity ( 1%) of trimer in the SEC trace. The diblock (I) was selected over the diblock (II) due to the increased steric hindrance of the styryl anion over the isoprenyl anion, which makes easier the control of the incorporation of only two arms into the silane. 

Diblock, triblock, and multiblock copolymers are typically prepared by sequential monomer addition to polymerization systems in which the chain-breaking reactions are sufficiently suppressed. Polymer properties can thereby be varied by manipulating the constituent blocks compatibilities, hydrophilicities/hydrophobicities, and hardness/softness. New and/ or novel topologies can also be prepared by controlled processes, including cyclic polymers and/or copolymers, comb-like macromolecules, and star polymers. The synthetic range of cationic vinyl polymerizations will be discussed in detail in Chapter 5. 

More recently, Kennedy reported another initiating system that controls styrene polymerization with an added nucleophile 2,2,4-trimeth-ylpentyl chloride (TMP-Cl)/TiCl4 with N,N-dimethylacetamide (DMA) in CH3Cl/methylcyclohexane (4 6 v/v) mixture at -80° C [165]. The use of another additive, 2,6-di-ferf-butylpyr idine (proton trap), is described as beneficial. The molecular weight and MWD are controlled in this system, but the role of the added DMA is still ambiguous [166]. This system with the aliphatic (erf-chloride was designed to extend to the synthesis of isobutene-styrene block copolymers via sequential cationic polymerization (Chapter 5). 

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