SEQUENTIAL POLYMERISATION

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

Irreversible termination of growing macromolecules during the final stages of ATRP are particularly disadvantageous if the synthesis of block co-polymers by sequential polymerisation is attempted. Due to different solubilities of catalyst, monomer and polymer in the ionic liquid phase, a larger amount of active molecules may be observed in the presence of an ionic liquid. 

Preparation by Sequential Polymerisation. Two-polymer composite latex particles may be prepared using either emulsion or dispersion polymerisation techniques. A dispersion (latex) of particles of a first polymer may be prepared in the usual manner after complete conversion of monomer to polymer, a different monomer or monomer mixture is added and polymerised to provide the second polymer. 

Sequential polymerisation may be operated by producing particles of the first polymer by either emulsion or dispersion polymerisation and by adding the monomer for the second polymer together with free-radical initiator at a slow and controlled rate such that the rate of addition and the rate of polymerisation are equal. The amount of free monomer remains at a low level throughout the process. This monomer-starved process (5) has been used with an aqueous continuous phase in many studies. Samples may be taken at regular intervals to... 

The synthesis of methacrylate-containing polymers has been developed to include new monomers. Stearyl side chains were obtained by sequential polymerisation of S and StMA under low-temperature polar conditions with DPE and LiCl present to yield PS-PStMA diblock copolymers (30 < Mn/(kg/mol) < 464 1.04 < Mw/M <1.18) [17]. 

The requirements for a polymerisation to be truly living are that the propagating chain ends must not terminate during polymerisation. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerisation is the only mechanism that yields truly living styrene... 

The styrene-diene triblocks, the main subject of this section, are made by sequential anionic polymerisation (see Chapter 2). In a typical system cc-butyl-lithium is used to initiate styrene polymerisation in a solvent such as cyclohexane. This is a specific reaction of the type... 

Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed hy Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymerisation (see Chapter 2). 

TEMPO combines with the radical chain and keeps the concentration of the growing radical chain low, such that the recombination of radicals is suppressed. This type of radical polymerisation is called Atom Transfer Radical Polymerisation (ATRP). It has the properties of a living polymerisation, as the molecular weight increases steadily with time and one can make block polymers this way by adding different monomers sequentially. 

Mix 1 ml of the enzyme preparation with the same volume of aniline solution in the PTFE cell immediately prior to electropolymerisation. Aniline containing acetylcholinesterase can be polymerised onto sonicated polydiaminobenzene-coated SPEs by sequentially cycling for 10 min between —200 and +800 mV versus Ag/AgCl at 50mV s-1. As the aniline polymerises at the exposed microelectrode elements, the polymer forms mushroom-like protrusions that extend outwards from the electrode surface and within which the acetylcholinesterase becomes entrapped. After polymerisation, the electrodes must be immediately submerged in pH 7.4 phosphate buffer at 4°C to prevent enzyme denaturation and stored at 4°C prior to use. 

Polyfunctional mercaptans can be used to impart some control to free-radical polymerisation. For methacrylate monomers, it has been demonstrated that polymers with polydispersities as low as 1.25 can be made. Unlike conventional radical polymerisations, this proceeds with an increase in molecular weight with time. The reason for this is that the thiol groups react sequentially as polymerisation proceeds. The polymerisation is statistically more controlled. 

Since tandem reactions first appeared they have been given many diverse names such as cascade, consecutive, and coupled processes, domino reactions, interrupted polymerisation, one-pot, sequential and serial reactions, and tactical combinations. Some authors have tried to classify... 

The non-covalent imprinting process is perhaps best considered in terms of the sequential nature of the individual steps involved, which are summarised in Figure 6.8. The key to MIP formation is a series of regiospecific non-covalent interactions between the template and functional monomers to form a pre-polymerisation complex in the liquid phase. 

Part of the mannoprotein structure, as found in the yeast Saccharomyces cereviseae [80] contains a series of unbranched oligosaccharide chains of degree of polymerisation (d.p.) 1-5, with a(l-2) and a(l-3) linked Man residues, that are a-glycosidically linked to the OH of serine or threonine in the polypeptide, and which can be released by base catalysed P-elimination. Methods of determination of structures of oligosaccharides have included n.m.r. and mass spectrometry, deacylation with hydrazine, sequential glycosidase hydrolysis, partial enzymic and acidic hydrolysis and acetolysis, poiarimetry and methylation analysis. 

The fiber-optic TR-FTIR technique has also been used to investigate the role of additives such as proton traps and electron pair donors (EDs) in carbocationic polymerisation. The role of additives is not clear and is actively debated. " In the sequential block copolymerization of IB and Sty it was shown that that in order to obtain efficient crossover from the living IB, the use of additives (both electron pair donors such as N,N-dimethylacetamide (DMA) and proton traps like 2,6-di-tert-butylpyridine (DrBP), or diphenylethylene is necessary. TR-FTlR monitoring revealed that when DMA was added from the beginning of the IB polymerization phase, the band characteristic of the C=C stretch in IB at 1655 cm appeared as a split signal, as shown in Figure 12a. 

Polyaniline/poly(N-vinyl carbazole), polypyrrole/poly(N-vinyl carbazole) and polypyrrole/polyaniline films (or reverse order of each pair in the films) were synthesised on platinum foil electrodes by sequential electrolysis. Confocal Raman microprobe spectroscopy was used to characterise both the solution sides and the electrode sides of the films. Two types of film were observed, depending on the conditions. Either the second polymer was incorporated into the initially coated layer or a doublelayer film with a well-defined interface was formed. Electrolysis of pyrrole and aniline monomer mixtures gave films rich in pyrrole when the pyrrole aniline molar ratios were greater than 0.12. However, polymerisation of N-vinyl carbazole and pyrrole monomer mixtures gave only polypyrrole over a wide range of molar ratios. 19 refs. 

The polymerisation process consists of a series of sequential stages with the main ones being initiation, propagation, and material chain termination. In order to produce a quality product it is necessary to provide optimum conditions for corresponding stages. These conditions include operation temperatures, pressure, concentrations of ingredients, and a sufficient degree of field uniformity for all characteristics. At the... 

The process by which polymers are formed from their respective monomers by the sequential addition of monomers, without loss of any by-product, is known as addition or chain growth polymerisation. The polymerisation proceeds through three distinct steps ... 

IPNs are formed sequentially by more than one IPN in a process, whereas simultaneous IPNs are formed simultaneously. However, semi-IPNs are formed by the polymerisation of a monomer in the presence of a polymer. All the types of IPN listed earlier are found in vegetable oil-based polymers. These IPNs have some advantages over polymer blends or cross-linked polymers. 

An interpenetrating polymer network (IPN) is defined as a combination of two crosslinked polymers, at least one of which has been synthesised [98] and/or crosslinked in the immediate presence of the other. From the topological point of view, IPNs are closely related to pol)nner blends and to block, graft and crosslinked copolymers. From the synthesis point of view, IPNs can be classified, broadly, into two general types (a) sequential IPNs where a polymer network is formed which is then swollen by the monomer, plus a crosslinking agent and an activator, which is then polymerised in situ to form the second network and (b) simultaneous IPNs (SIPN) where the components necessary to form both networks are mixed and polymerised, at the same time, by non-competing mechanisms. If one of the two polymers is linear (uncrosslinked), a semi-IPN results. A homo-IPN results if both the network polymers are identical in chemical composition [98]. 

Synthesis of block copolymers with well-defined structure has received considerable attention, as their properties are potentially of great interest (see Section 4.1). Until recently, the possibilities were limited to the use of either sequential addition of monomers in living anionic polymerisation systems, or coupling of polymers possessing reactive ends, e.g. telechelic polymers. Advances in radical controlled polymerisation have opened new perspectives. 

Butyl acrylate-methyl methacrylate copolymer latices with a core-shell structure were prepared by a sequential emulsion polymerisation technique. SEM and transmission electron microscopy studies undertaken on the polymer dispersions, powders obtained by spray drying and latices prepared by redispersing the powders in water revealed the influence of polymerisation parameters on the micromorphology of the starting latices, and correlations between the dimensional and micromorphological characteristics of the starting latices, the powders and the redispersed latices. 8 refs. 

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