Block copolymer formation

In a third type of block copolymer formation. Scheme (3), the initiator s azo group is decomposed in the presence of monomer A in a first step. The polymer formed contains active sites different from azo functions. These sites may, after a necessary activation step, start the polymerization of the second monomer B. Actually, route (3) of block copolymer formation is a vice versa version of type (1). It has been shown in a number of examples that one starting bifunctional azo compound can be used for block copolymer synthesis following either path. Reactions of type (3) are tackled in detail in Section III of this chapter. 

As was explained, block copolymer formation by azo initiators always involves at least one radically polymeri-... 

The catalysts described in Table XII cannot be used to make tailored-block copolymers because of reaction (19). The latter continues in the absence of monomer resulting in detachment of chains from the transition metal centers forming hydride (XX). Introducing a second monomer would lead to realkylation of the chain centers giving a homopolymef of the second monomer. Hence mixtures of homopolymers would be obtained with little block-copolymer formation. 

For example, we have described that nearly monodisperse PEs can be formed by 2/ MAO (1 min polymerization, atmospheric pressure 25 °C Mn 52,000, MJMn 1.12 50 °C Mn 65,000, MJMn 1.17) and 38 (Fig. 25)/MAO (1 min polymerization, atmospheric pressure W25n°C M 8000, M /M 1.05 50 °C M 9000, M IM 1.08) [28, 68, 69]. Additionally, Coates and coworkers subsequently reported that Ti-FI catalysts 34 (Fig. 22) and 39 (Fig. 25) can form nearly monodisperse PEs under controlled conditions [70]. With these Ti-FI catalysts, however, synthesizing high molecular weight and narrow molecular weight distribution PEs is generally difficult (e.g., 5 min polymerization, atmospheric pressure, 50 °C 2Mn 132,000, MJMn 1.83 38Mn 24,000, MJMn 1.46) [28, 68]. Moreover, normally, these catalysts cannot be applied to block copolymer formation. 

Fig. 26. Polymerization of acrylonitrile by polylmethyl methacrylate) mastication. Schematic representation of block copolymer formation and...

Transformation of epoxy resins, which are viscous liquids or thermoplastic solids, into network polymers is a result of interaction with alkali or acid substances by means of to polyaddition and ionic polymerization mechanisms.10 A resin solidified by to the polyaddition mechanism, is a block copolymer consisting of alternating blocks of resin and a hardener or curing agent. A resin solidified by the ionic mechanism is a homopolymer. Molecules of both resin and hardener contain more than one active group. That is why block copolymer formation is a result of multiple reactions between an epoxy resin and a curing agent.11... 

When the product of monomer relative reactivity ratios is approximately one r x r2 = 1), the last inserted monomeric unit in the chain does not influence the next monomer incorporation and Bernoullian statistics govern the formation of a random copolymer. When this product tends to zero (r xr2 = 0), there is some influence from the last inserted monomeric unit (when first-order Markovian statistics operate), or from the penultimate inserted monomeric units (when second-order Markovian statistics operate), and an alternating copolymer formation is favoured in this case. Finally, when the product of the reactivity ratios is greater than one (r x r2 > 1), there is a tendency for the comonomers to form long segments and block copolymer formation predominates (or even homopolymer formation can take place) [448],... 

Many attempts have been made to synthesise ethylene/propylene block copolymers, referred to as polyallomers, with isospecific Ziegler-Natta catalysts. However, true block copolymers can hardly be synthesised. This is due to the short life of the growing polymer chains [68,241]. Therefore, only in a few cases, when the copolymers are synthesised by adding two comonomers sequentially and under very specific conditions in order to reduce chain transfer reactions, does unambiguous evidence for true block copolymer formation with isospecific catalysts exist [457]. 

The applicability of organolanthanide metallocenes as polymerisation catalysts can also be seen from the results of the block copolymerisation of ethylene and methyl methacrylate. The persistence of the lanthanide-alkyl bond has been utilised to prepare ethylene copolymers with polar poly(methyl methacrylate) blocks. For this purpose, ethylene is introduced as the first monomer into the polymerisation system with the samarocene catalyst, and then methyl methacrylate is polymerised, which leads to block copolymer formation [532-534] ... 

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. 

There is an equilibrium between living chain ends which are attached to Nd and dormant chain ends which can be attached to Al, Mg and Zn. Very likely these chain ends exhibit differences in reactivity towards modification agents and polar monomers. This assumption will possibly result in the interpretation of inconsistent results observed in end-group functionalization and block copolymer formation, e.g. with polar comonomers such as e-caprolactone. 

Marie et al. [49] also studied the in-situ block copolymer formation via reactive blending of functionalized homopolymers. In their work, blends were characterized by SEM, DSC and dynamic mechanical spectroscopy (DMS). It should be noted that their blends (PA-6/PDMS and PS/PDMS) were composed totally using functionalized homopolymers. The different reactions under investigation were amine(NH2)/anhydride(An), amine(NH2)/epoxy(E) and carboxylic acid(COOH)/epoxy(E) (Fig. 5). 

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... 

Reaction mechanism of PTFE polyamide block copolymer formation by transa-midation... 

The tribological investigation of the chemically bonded PTFE-PA materials shows that besides the low friction coefficients the high wear resistance is the most important property of such products. Examinations with several different PTFE polyamide materials certify this predicate. This phenomenon could be explained by the chemical coupling of the PTFE distributed in PA. The compati-bilisation of PTFE by block copolymer formation causes a better connection to the PA matrix as compared to physical blends. 

Graft copolymerization has some of the features of block-copolymer formation except that the radical centres are not at the end of the chain but in-chain. The polymer to which the graft is attached is the backbone polymer. Cowie (1989a) has recognized three different ways in which graft copolymers may be formed ... 

The rate of block copolymer formation in a poor solvent is inversely related to the difference between the solubility parameters of the macroradical and the monomer used to form the block. Thus, when these solubility parameter values are similar—as with the addition of styrene and maleic anhydride to the styrene-maleic anhydride macroradical—a 100% weight increase is observed in a few hours. 

Under similar conditions, but with the addition of racemic 3,7-dimethyl-l-octene in a two-step version after homopolymerization of (5)-3-methyl-l-pentene has started, a polymer consistent with block copolymer formation of the 5-enantiomers is obtained73 ... 

On the basis of these observations, block copolymer formation was claimed. It was reported, however, that some grafting also occurs when DXL is polymerized in the presence of preformed polypMOSt. Thus, when dead polypMOSt was added to the solution of living polyDXL, up to 70% of polyDXL was grafted by aromatic alkylation onto polypMOSt ... 

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