Mixing monomers

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

At the current stage of development, the multiple pump capabilities of the Mk.II unit permits full programming of all four liquid feeds. In addition to the roles illustrated by Figure 5b for two, three or four mixed monomers and initiators, where only the normal dual feeds are required, the extra two pumps may be used for controlled additions over a few minutes at the start and end of the process, as Illustrated in Figure 8. 

Nylon copolymers can be prepared by either heating the blend of different nylons together or by polymerising mixed monomers. 

According to Equation 9 polymers with close to theoretical molecular weight distributions could be prepared even at very high conversions provided [M] and [I] remain constant throughout the polymerization. This condition can be fulfilled by continuously adding a mixed monomer/inifer feed at a sufficiently low constant rate to a coinitiator charge, making certain that the rate of monomer/inifer addition and that of monomer/inifer consumption are equal over the course of the polymerization. 

Studies in the grafting of mixed monomers to cellulose have also been reported by Sakurada (113). Binary mixtures studied included butadiene with styrene or with acrylonitrile, and styrene with acrylonitrile. Remarkable increases in rate in the case of mixed monomer similar to those found by RAPSON were found in many cases. For example, about 10% of butadiene increased the grafting yield about ten fold. Similar results were found with the addition of acrylonitrile to butadiene and to styrene. Ternary mixtures of monomers were also investigated by both Rapson (109) and Sakurada (113). The large increases in rate with certain mixtures were interpreted by Sakurada as due to a particular balance of gd effects akin in many ways to popcorn polymerization. The effects were found also with polyvinyl alcohol but not with polyethylene where gel effects would perhaps be less prominent. 

The composition of the grafted side chain copolymer has also been determined by Sakurada (113) and found to be different from the normal copolymer formed with acrylonitrile and butadiene. With styrene the grafted copolymers were found to be richer in acrylonitrile than the normal copolymer. Similar differences were found by Resting (114) with methyl methacrylate and styrene grafted to cotton and by Odian et al. (115) with grafting mixed monomers to Teflon and to polyethylene. It is believed that one monomer may be preferentially sorbed or diffused faster than the other, leading to a different monomer ratio at the actual site of grafting. 

The copolymerization of trioxane with cyclic ethers or formals is accomplished with cationic initiators such as boron trifluoride dibutyl etherate. Polymerization by ring opening of the six-membered ring to form high molecular weight polymer does not commence immediately upon mixing monomer and initiator. Usually, an induction period is observed during which an equilibrium concentration of formaldehyde is produced. 

Another problem associated with the batch technique is poor reaction control (unsatisfactory stirring, temperature control, etc). To overcome the problems outlined above a semi-continuous polymerization technique has been introduced [27]. In this technique a mixed monomer/inifer feed is added at a sufficiently low constant rate to a well stirred, dilute BC13 charge. Due to stationary conditions maintained during the whole polymerization, well-defined telechelic products with symmetrical end groups and theoretical polydispersities could be obtained. The kinetics of the polymerization has been discussed and the DPn equation has been derived. In contrast to the batch technique, the DPn for the semi-continuous technique is simply given by the [monomer]/[inifer] ratio. Thus, very reactive or unreactive inifers, unsuitable for batch polymerization, can also be used in the semi-continuous process. 

IPN s and related materials) in fact) have a long history. For example) IPN s were first synthesized to produce smooth sheets of bulk polymerized homopolymers (11), IPN s were next used as solution polymerized ion exchange resins. (12) 13) Further development of IPN s included the syntheses oT interpenetrating elastomer networks (lEN s) and simultaneous interpenetrating networks (SIN s) (14). lEN s consist of a mixture of different emulsion polymerized elastomers which are both crosslinked after coagulation. SIN s are formed by the simultaneous polymerization of mixed monomers by two noninterfering reactions (3 ) 16). 

When a mixed monomer feed is used, the course of the copolymerization may be sensitive to the volume ratio between the monomer and water phases. 

The use of stop-flow techniques to observe the formation of carbenium ions in actual polymerising systems was introduced by Pepper et al. about ten years ago and is presently exploited by various research groups with increasingly fast equipment. These experiments consist essentially in mixing monomer and catalyst solutions in an appropriate flowing system coupled vrith a rapid detection apparatus which takes absorption spectra and can measure other physical parameters, such as the electrical conductivity of the reaction mixture. This technique is certainly the most appropriate for studying the rise and fate of ionic active species in cationic polymerisation and the few, but remarkable, results obtained so far will be reviewed in the various sections dealing vrith specific systems. 

Inagaki and Yasuda [3] investigated transient-stage polymer deposition by using mixed monomers, of which one component is N2. N2 is a non-polymer-forming reactive gas that does not form polymer by itself but copolymerizes with another monomer. In one type of experiment (method A), a steady-state flow of mixed monomer is established and maintained for 5 min without discharge, in which period the adsorption of organic monomer onto the substrate surface (quartz thickness monitor) and other surfaces occurs. 

The miscible monomers, ethenylbenzene (styrene) and diethenylben-zene (divinylbenzene, DVB), undergo a free radical induced copolymerization reaction initiated by a benzoyl peroxide catalyst. The exothermic reaction is carried out in an aqueous suspension whereby the mixed monomers are immiscibly dispersed as spherical droplets throughout the reacting medium resulting in discreet beads of copolymer being formed. Correct reaction conditions and the use of suspension stabilizers enable the particle size distribution of the... 

Hyperbranched polymers synthesized by ATRP using mixed monomers , structures that contain combinations of (meth)acrylates with a-haloesters [262], have also been reported. For example, 2-(2-bromopropionyloxy)ethyl methacrylate (BPEM) contains the methacrylate and bromopropionate groups which form tertiary and secondary radicals, respectively. Likewise, the monomer 2-(2-bromoisobutyryloxy)ethyl acrylate (BIEA) contains the secondary acrylate group with a tertiary 2-bromoisobutyrate fragment. With these monomers, highly branched macromolecules were obtained. In a similar way, macroinitiators were used to reduce the proportion of branched units [263]. They may be considered a segmented block copolymers. 

Scheme 30.17 One-pot synthesis of arborescent polystyrene by a semibatch process with mixed monomer additions. Source Reproduced with permission from Yuan Z, Gauthier M. Macromolecules 2063 2006 39 [114]. Copyright 2006 American Chemical Society.

The liquid monomers were stored with a trace of iodine (0.01-0.1%) to prevent polymerization. When such a monomer was used to prepare a polymer, the charge of monomer or mixed monomers was washed with a few drops of concentrated aqueous sodium thiosulfate to remove the iodine. Then a small amount of sodium sulfate was added and the monomer charge was decanted. 

Nylon copolymers can be obtained by heating a blend of two or more different nylons above the melting point so that amide interchange occurs. Initially, block copolymers are formed, but prolonged reaction leads to random copolymers. For example, a blend of nylon-6,6 and nylon-6,10 heated for 2 h gives a random copolymer (nylon-6,6-nylon-6,10) which is identical with a copolymer prepared directly from the mixed monomers. Other copolymers of this type are available commercially. 

Polymer networks synthesized by mixing monomers or linear polymers (prepolymers) of the monomers together with their respective cross-linking agents and catalysts in melt, solution, or dispersion, followed, usually immediately, by simultaneous polymerization by noninterfering modes, are called simultaneous interpenetrating networks (SIN). In the latter process, the individual monomers are polymerized by chain or stepwise polymerization, while reaction between the polymers is usually prevented due to different modes of polymerization. 

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