Monomeric copolymers

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

A substantial fraction of commercially prepared methacrylic polymers are copolymers. Monomeric acryUc or methacrylic esters are often copolymerized with one another and possibly several other monomers. Copolymerization greatiy increases the range of available polymer properties. The aH-acryhc polymers tend to be soft and tacky the aH-methacryhc polymers tend to be hard and brittie. By judicious adjustment of the amount of each type of monomer, polymers can be prepared at essentially any desired hardness or flexibiUty. Small amounts of specially functionalized monomers are often copolymerized with methacrylic monomers to modify or improve the properties of the polymer directiy or by providing sites for further reactions. Table 9 lists some of the more common functional monomers used for the preparation of methacrylic copolymers. 

Polycarbonates are prepared commercially by two processes Schotten-Baumaim reaction of phosgene (qv) and an aromatic diol in an amine-cataly2ed interfacial condensation reaction or via base-cataly2ed transesterification of a bisphenol with a monomeric carbonate. Important products are also based on polycarbonate in blends with other materials, copolymers, branched resins, flame-retardant compositions, foams (qv), and other materials (see Flame retardants). Polycarbonate is produced globally by several companies. Total manufacture is over 1 million tons aimuaHy. Polycarbonate is also the object of academic research studies, owing to its widespread utiUty and unusual properties. Interest in polycarbonates has steadily increased since 1984. Over 4500 pubflcations and over 9000 patents have appeared on polycarbonate. Japan has issued 5654 polycarbonate patents since 1984 Europe, 1348 United States, 777 Germany, 623 France, 30 and other countries, 231. 

Association Complexes. The unshared electron pairs of the ether oxygens, which give the polymer strong hydrogen bonding affinity, can also take part in association reactions with a variety of monomeric and polymeric electron acceptors (40,41). These include poly(acryhc acid), poly(methacryhc acid), copolymers of maleic and acryflc acids, tannic acid, naphthoHc and phenoHc compounds, as well as urea and thiourea (42—47). 

Photopolymerizable compositions based on monomeric acryflc or other ethylenicaHy unsaturated acid derivatives are becoming increasingly popular. When multiftmctional derivatives are employed, three-dimensional networks having high strength and abrasion resistance are possible on exposure to light. A typical composition may contain an ethoxylated trimethylolpropane triacrylate monomer, a perester phenacjhdene initiator (69), and an acryflc acid—alkyl methacrylate copolymer as binder. 

The most effective and widely used dispersants are low molecular weight anionic polymers. Dispersion technology has advanced to the point at which polymers are designed for specific classes of foulants or for a broad spectmm of materials. Acrylate-based polymers are widely used as dispersants. They have advanced from simple homopolymers of acryflc acid to more advanced copolymers and terpolymers. The performance characteristics of the acrylate polymers are a function of their molecular weight and stmcture, along with the types of monomeric units incorporated into the polymer backbone. 

Since 1 is a monomer with low activity, copolymers 2 obtained at any stage of the copolymerization process, irrespective of the monomer ratio in the initial mixture, always contain a smaller amount of monomeric units of 1 than that in the corresponding monomer mixture. 1 being prone to enter the chain-transfer reaction, the increase of its content in the initial monomer mixture reduces substantially the reaction rate and decreases the molecular mass of the copolymers. It was found that copolymers 2 which contain 2—8% of monomeric units of 1 and are suitable for obtaining fibres must have a molecular mass between 45 000 and 50000. Such copolymers can be obtained with a AN 1 ratio in the initial mixture between 95 5 and 85 15. Concentrated solutions of copolymers, especially those with a molecular mass smaller than the above limit, are characterized by a very low stability which is a substantial shortcoming of these copolymers. 

It was established that for the further modification of copolymers, as well as that of finished fibres, using the aromatic amino group, and, in particular, to achieve deep staining, a content up to 2% of monomeric units of 7 in the copolymer is sufficient. 

The optimum molecular mass (Mw) of these copolymers containing 1,5-2% of monomeric units of 7, amounts to 47000—55 000 which makes it possible to obtain concentrated (15—17%) solutions with sufficiently high stability and to spin fibres with high physicomechanical properties (P = 36—40 gf/tex, (4—4,4 g/denie),... 

Copolymers of AN with diene monomers and, in particular, with butadiene and isoprene, deserve special attention. These copolymers with a predominating content of monomeric units of dienes are known to have been produced in the form of rubbers for a long time and are finding a broad application in various branches of technology. 

However, no studies have been carried out until recently on the synthesis of AN copolymers containing only a small quantity of monomeric diene units which may have fibre forming properties. It is only in the last few years that several reports have appeared on the copolymerization of AN with butadiene in DMF28 and on the use of AN-butadiene copolymers to obtain fibres29. ... 

As it has been shown lately, insertion of a small quantity (5—15% of the copolymer weight) of ISP monomeric units into the PAN macfomolecules results in an appreciable decrease of stiffness and in an increase of flexibility of the chain, which makes it possible to improve considerably the fatigue properties of usual PAN fibres30. In addition to that, by inserting a comparatively large amount (25—30%) of flexible ISP monomeric units into the copolymer one can decrease substantially the yield temperature of PAN, which makes it possible to spin fibres from thermoplastic state31. ... 

The insertion of ISP monomeric units also results in significant changes in the thermomechanical properties of the copolymers. The glass-transition temperature... 

As it can be seen from the above data, by introducing 4—15% of monomeric units of ISP into the macromolecules of the AN copolymer, the elastic properties of PAN fibres and, especially, their resistance to abrasion and double bends can considerably be improved. 

Copolymers of AN with ISP, containing more than 25% of monomeric units of ISP with a molecular mass of 50000 to 60000, obtained in emulsion at pH 3, in distinction to PAN, are capable of passing into the state of viscous flow without destruction and cyclization and are processed into fibres at 180—220 °C. When copolymers of higher molecular mass are used it is necessary to raise the temperature of processing. This leads to an intensive crosslinking and to cyclization, due to which it becomes impossible to obtain fibres from them. 

Fibres based on AN copolymers containing 4—10% of monomeric units of JO42 and obtained by wet spinning from solutions in DMF have a much better (2-8 times) resistance to multiple deformations than PAN fibres and have a higher light-fastness than PAN fibres. They are, however, inferior to the latter with respect to abrasive resistance and thermal stability. 

The presence of the 3-chloro-2-butenyl groups in the macromolecules of these AN copolymers, the structure of these groups being similar to that of the monomeric... 

Reactivity ratios for the copolymerization of AN and DM WS in DMSO were found to be rj =0,53 and r2=0,036, and in water r1=0,56 and r2=0,25. The higher reactivity of DM VPS in the copolymerization with AN in aqueous medium, as compared with its reactivity in DMSO, can be explained by a higher degree of dissociation of DMVPS in aqueous medium. This fact also produces a considerable effect on the character of the distribution of monomeric units within the copolymers, which manifests itself in the change of their solubility in water. Copolymers containing 30% of monomeric units AN obtained from a 90 10 mixture of AN and DMVPS in DMSO, irrespective of the level of conversion, are completely soluble in water, whereas copolymers of the same composition, but obtained in aqueous medium with a yield 40%, are insoluble in water. 

Just as an example, the X-ray diffraction patterns of compression moulded samples of PVDF, poly(vinylfluoride), and of some VDF-VF copolymers of different compositions are shown in Fig. 17 [90]. The degrees of crystallinity of the copolymer samples (40-50%) are high and analogous to those of the homopolymer samples. This indicates a nearly perfect isomorphism between the VF and VDF monomeric units [90, 96], The diffraction patterns and the crystal structures of the copolymers are similar to those of PVF, which are in turn similar to the X-ray pattern and crystalline structure of the P form of PVDF. On the contrary, the X-ray pattern of a PVDF sample crystallized under the same conditions (Fig. 17 a) is completely different, that is typical of the non-piezoelectric a form [90]. 

For instance, also the homopolymers PVF and PVDF have been described to crystallize in separate crystals in their blends [99] (Though constituted by isomorphous monomeric units which can cocrystallize in the copolymers in the whole range of composition, as seen in Sect. 4.1). Moreover, at least for the studied conditions, the polymorphic behavior of PVDF is not altered by the presence of PVF [99]. 

In the same scheme, moreover, it is evident that, besides phosphazene homopolymers, the substitution of the chlorines with two (or more) different substituents leads to the preparation of substituent phosphazene copolymers [263] containing different homosubstituted and heterosubstituted monomeric units. Moreover, the cationic polymerization of phosphoranimines [215-217] produces polymers with hving reactive ends (vide supra) from which the preparation of chain phosphazene copolymers (block copolymers) [220,223,225, 229,232-235,239, 240] formed by different polymeric backbones linked together in a unique macromolecule could be obtained. 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

The structural versatility of pseudopoly (amino acids) can be increased further by considering dipeptides as monomeric starting materials as well. In this case polymerizations can be designed that involve one of the amino acid side chains and the C terminus, one of the amino acid side chains and the N terminus, or both of the amino acid side chains as reactive groups. The use of dipeptides as monomers in the manner described above results in the formation of copolymers in which amide bonds and nonamide linkages strictly alternate (Fig. 3). It is noteworthy that these polymers have both an amino function and a carboxylic acid function as pendant chains. This feature should facilitate the attachment of drug molecules or crosslinkers,... 

Ability to analyze unreacted monomers was dependent on detector selectivity. The UV detector was operated at 254 nm for analysis of AN/S latex solutions. Styrene is a strong UV abosrber at this wavelength while acrylonitrile has no measurable absorbance at 254 nm. Thus, the UV detector was entirely selective to monomeric styrene. The refractometer detector was sensitive to both acrylonitrile and styrene when each was present in the desired copolymer proportions (70/30). However,... 

Graft copolymers and other polymers are prepared in a way that is common in polymerization techniques [1894]. For example, they are made by providing a foamed, aqueous solution of water-soluble monomeric material, initiating polymerization by adding an initiator, exothermically polymerizing... 

Polymer products synthesized in laboratories and in industry represent a set of individual chemical compounds whose number is practically infinite. Macro-molecules of such products can differ in their degree of polymerization, tactici-ty, number of branchings and the lengths that connect their polymer chains, as well as in other characteristics which describe the configuration of the macromolecule. In the case of copolymers their macromolecules are known to also vary in composition and the character of the alternation of monomeric units of different types. As a rule, it is impossible to provide an exhaustive quantitative description of such a polymer system, i.e. to indicate concentrations of all individual compounds with a particular chemical (primary) structure. However, for many practical purposes it is often enough to define a polymer specimen only in terms of partial distributions of molecules for some of their main characteristics (such as, for instance, molecular weight or composition) avoiding completely a... 

The simplest, from the viewpoint of topological structure, are the linear polymers. Depending on the number m of the types of monomeric units they differentiate homopolymers (m=1) and copolymers (m>2). In the most trivial case molecules in a homopolymer are merely identified by the number l of monomeric units involved, whereas the composition of a copolymer macromolecule is defined by vector 1 with components equal to the numbers of mono-... 

In terms of a statistical approach to every macromolecule of a copolymer specimen there can be put in correspondence a certain realization of a stochastic process of conventional movement along a copolymer chain. This movement can be conveniently thought of as a sequence of random transitions from a unit of the chain to the neighboring one. Here the probability of finding a monomeric unit of a particular type at every step is predetermined by the stochastic process which describes the particular polymer specimen. To consider the set of trajec-... 

For example, the enthalpy of mixing of copolymer specimen AH per mole of monomeric units can be expressed... 

Statistical characteristics of the second type define the microstructure of copolymer chains. The best known characteristics in this category are the fractions P [/k) (probabilities) of sequences Uk involving k monomeric units. The simplest among them are the dyads U2, the complete set of which, for example, for a binary copolymer is composed of four pairs of monomeric units M2M, M2M2. The number of the types of k-ad in chains of m-component copolymers grows exponentially as mk so that with practical purposes in mind it is generally enough to restrict the consideration to sequences Uk] with moderate values of k. Their calculation turns out to be rather useful... 

One more quantitative way to characterize the chemical structure of copolymers is based on the consideration of chemical correlation functions (correlators) [2]. The simplest of these, Ya k), describes the joint probability of finding two randomly chosen monomeric units divided along the macromolecule by an arbitrary sequence Uk ... 

For a number of copolymers, whose kinetics of formation is described by nonideal models, the statistics of alternation of monomeric units in macromolecules cannot be characterized by a Markov chain however, it may be reduced to the extended Markov chain provided that units apart from their chemical nature... 

When calculating the average copolymer composition and the probabilities P Uk] of the sequences of monomeric units it is possible to set Ta=0 in the expressions in (7), that is to neglect the finiteness of the size of the macromolecules. In this case the absorbing Markov chain (7) is replaced by the ergodic Markov chain with transition matrix Q whose elements ... 

This is the simplest of the models where violation of the Flory principle is permitted. The assumption behind this model stipulates that the reactivity of a polymer radical is predetermined by the type of bothjts ultimate and penultimate units [23]. Here, the pairs of terminal units MaM act, along with monomers M, as kinetically independent elements, so that there are m3 constants of the rate of elementary reactions of chain propagation ka ]r The stochastic process of conventional movement along macromolecules formed at fixed x will be Markovian, provided that monomeric units are differentiated by the type of preceding unit. In this case the number of transient states Sa of the extended Markov chain is m2 in accordance with the number of pairs of monomeric units. No special problems presents writing down the elements of the matrix of the transitions Q of such a chain [ 1,10,34,39] and deriving by means of the mathematical apparatus of the Markov chains the expressions for the instantaneous statistical characteristics of copolymers. By way of illustration this matrix will be presented for the case of binary copolymerization ... 

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