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Polymer Polymer chemistrychemistry

Polymer Polymer chemistrychemistry

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1.5Microcosmic Structure of polymer

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1.5.1 SequenceThe properties of polymers are strongly influenced

by details of the chain structure.

These details include the overall chemical composition and the sequence of monomer units in the case of copolymers, the stereochemistry or tacticity of the chain, and the geometric isomerization in the case of diene-type polymers for which several synthesis routes may be possible.

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Often, it is possible to obtain polymers with new and desirable properties by linking two or three different monomers or repeating units during the polymerization. Polymers with two different repeating units in their chains are called copolymers. In the less frequent case when there are three chemically-different repeating units, the resulting polymer is termed a terpolymer.

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Commercially, the most important copolymers are derived from vinyl monomers such as styrene, ethylene, acrylonitrile, and vinyl chloride.

The exact sequence of monomer units along the chain can vary widely depending upon the relative reactivities of each monomer during the copolymerization process.

At the extremes, monomer placement may be totally random or may be perfectly alternating.

The actual sequence of monomer units is determined by the relative reactivities of the monomers.

Under special circumstances, it is possible to prepare copolymers that contain a long block of one monomer (A) followed by a block of the other monomer (B). These are called AB-block copolymers.

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Triblock copolymers have a central B block joined by A blocks at the end.

A commercially important ABA-triblock copolymer is polystyrene-block-polybutadiene- block -polystyrene (SBS), a thermoplastic elastomer.

In addition to these copolymer structures, graft copolymers can be prepared by polymerizing a monomer in the presence of a fully formed polymer of another monomer.

Graft copolymers are important as elastomeric (e.g., SBR) and high-impact polymers (e.g., high-impact polystyrene and ABS).

7Figure 1－ 4 .Possible structures of copolymers containing A and B repeating units.

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1.5.2TacticityIn addition to the type, number, and sequential arrangement o

f monomers along the chain, the spatial arrangement of substituent groups is also important in determining properties.

The possible steric configurations of an asymmetric vinyl-polymer chain can be best represented by drawing the chain in its extended-chain or planar zigzag conformation, as illustrated in Figure 1-5.

A conformation describes the geometrical arrangement of atoms in the polymer chain while configurationdenotes the stereochemical arrangement of atoms. Unlike the conformation, the configuration of a polymer chain cannot be altered without breaking chemical bonds.

For long, flexible polymer chains, the total number of conformations is nearly infinite. The extended-chain conformation for vinyl polymers is often the lowest-energy conformation.

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As illustrated in Figure 1-5, several different placements of the asymmetric substituent group, R, are possible.

(1)Isotactic

As examples, a substituent group may be a methyl group as in polypropylene, a chlorine atom as in poly(vinyl chloride), or a phenyl ring as in polystyrene. In one configuration, all the R groups may lie on the same side of the plane formed by the extended-chain backbone. Such polymers are termed isotactic.

(2)Syndiotac-tic

If the substituent groups regularly alternate from one side of the plane to the other, the polymer is termed syndiotac-tic.

(3) Atactic

Polymers with no preferred placement are atactic. More complicated arrangements of

substituent groups are possible In the case of 1 ， 2-disubstituted polymers ； however ， these are commercially less important and will not be discussed here.

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CH2 CH CH2

CH3CH CH2 CH

CH3

CH3

CH2 CH

CH3Atactic

Figure. Three forms of stereochemical configuration of an extended-chain vinyl polymer having a substituent group R other than hydrogen.

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1.5.3 Geometric IsomerismWhen there are unsaturated sites along a polymer chain, several different isomeric forms are possible.

As next figure illustrates, 1,3-butadiene (structure A)

can be polymerized to give 1,2-poly( 1,3-butadiene) (B)

or as either of two geometric isomers of 1,4-poly( 1,3-butadiene) (C and D).

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The numbers preceding the poly prefix designate the first and last carbon-atoms of the backbone repeating-unit.

1,2-Poly( 1,3-butadiene) has a vinyl-type structure, where the substituent group (ethene) contains an unsaturated site;

therefore, this geometric isomer can be atactic, syndiotactic, or isotactic.

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In the case of the commercially more important 1,4- poly(l,3-butadiene), all four carbons in the repeating unit lie along the chain.

Carbons 1 and 4 can lie either on the same side of the central double bond (i.e., cis-configuration, C) or on the opposite side (i.e., trans-configuration, D).

The structure of polybutadiene used in SBR rubber (i.e., a copolymer of styrene and butadiene) is principally the trans-1,4 isomer with some cis- 1,4- and 1,2-poly( 1,3-butadiene) content.

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butadiene

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1.6 Microcosmic shape

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1.6.1 linear polymers:

The simplest of all polymers, the linear polymers, is made up of one molecule after another, hooked together in a long chain.

This chain is called the backbone.

That means the structural units are connected one to another in linear sequence. Such a polymer may be represented by the type formula 

A'-A-A-A-··· -A" or A'(-A-)x-2-A"

 

 

 

 

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1.6.2 Nonlinear, or branched polymers:

Alternatively, the structural units of the polymer may be connected together in such a manner as to form nonlinear, or branched, structures of one sort or another.

The branching units may, of course, have a valency exceeding three.

Some, at least, of the structural units must then possess a valency greater than two.

A branched polymer chain has extra beginnings (branches!) along the chain and so it has lots of ends.

No matter where you start, you can't trace the entire polymer without backtracking.

 

 

 

branched

 

 

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1.6.3 Network polymer

Highly ramified molecular species can be formed through further propagation of the branched structure.

It may, in fact, interconnect with itself to yield a network structure. A planar network analogous to that of graphite might conceivably occur.

 

 

 

 

 

 

 

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A regular three-dimensional space network analogous to diamond could be imagined.

The random nature of polymerization processes, however, renders unlikely the assemblage of units in a structure which conforms to a regular pattern.

The space network structures frequently encountered among nonlinear polymers are highly irregular labyrinths.

The structure may abound with "closed circuits" leading from a given junction via a sequence of linear chains and polyfunctional junctions back to the given junction, but these circuitous paths will be extremely varied and in general will involve many chains and many polyfunctional junctions.

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1.7 CHEMICAL STRUCTURE AND THERMAL TRANSITIONS

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Many important synthetic polymers such as polystyrene and poly(methyl methacrylate) consist of long, flexible chains of very high molecular weight. In many cases, individual chains are randomly coiled and intertwined with no molecular order or structure. Such a physical state is termed amorphous. Commercial-grade (atactic ) polystyrene and poly(methyl methacrylate) are examples of polymers that are amorphous in the solid state.

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Below a certain temperature called the glass-transition temperature(Tg), long-range, cooperative motions of individual chains cannot occur; however, short- range motions involving several contiguous groups along the chain backbone or substituent group are possible. Such motions are called secondary-relaxation processes and can occur at temperatures as low as 70 K.

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By comparison, glass-transition temperatures vary from 150 K for polymers with very flexible chains such as polydimethylsiloxane

 to well over 600 K for those with highly rigid aromatic backbones such as the high-modulus fiber, poly[2, 2' - (m-phenylene)- 5,5'-bibenzimidazole] (PBI) .

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Polymer chains with very regular structures ， such as linear polyethylene and isotactic polypropylene ，can be arranged in highly regular structures called crystallites. Each crystallite consists of rows of folded chains. Since sufficient thermal energy is needed to provide the necessary molecular mobility for the chain － folding process ， crystallization can occur only at temperatures above Tg.

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If the temperature is too high ， chain folds become unstable and high thermal energy disorders the crystallites － a crystalline － amorphous transition then occurs.

The temperature that marks this transition is called the crystalline –melting temperature or Tm.

Crystalline -melting temperatures can vary from 334 K for simple ， flexible-chain polyesters such as polycaprolactone.

to over 615 K for aromatic polyamides such as poly （ m － phenylene isophthalamide ）

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As an approximate rule of thumb, Tg, is one-half t

o two-thirds of Tm expressed in absolute temperature

(Kelvin).

The glass-transition and crystalline-melting temperature can be determined by a wide range of techniques including measurement of volume, specific heat (calorimetry), and mechanical properties, particularly modulus (e.g., dynamic mechanical analysis).

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In view of the diversity of properties displayed by different high polymers, some being viscous liquids, others rubbery, and still others very hard and tough, the question arises as to how these most apparent physical properties relate to structure and to composition.

It is to be noted at once that no one of these attributes should be considered as an invariant characteristic of polymers of a given structural unit.

Starting with virtually any structural unit, or units, polymers conceivably may be constructed which are oils, rubbers, or fibers, depending on the temperature and on the molecular chain length, or, more generally, on the manner in which the units are connected.

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Thus it is no more fitting to ascribe rubberlike character to the polymers obtained given monomers than to state that compounds containing certain elements are gaseous without specifying the molecular constitution, temperature, and pressure. On the other hand, the temperature at which the polymer becomes brittle is specific for the given structural unit(assuming that the molecular weight is high). Similarly, the melting point of a crystalline polymer relates directly to the symmetry and interaction forces of its repeating unit.

Other properties such as solubility, viscosity (above Tg , and

Tm), modulus of elasticity, and strength are highly dependent on

the polymer architecture, or pattern of the interconections between units. In other words ， these properties vary to a marked degree with changes in the molecular weight and in the degree of cross-linking.

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It is instructive to consider what steps may be taken to ascertain qualitatively the structural type and physical state of a given polymeric substance. To this end one should first of all determine whether or not the substance is soluble without decomposition in any solvent, and whether or not it will, on heating, soften to a liquid which displays measurable fluidity. A positive result from either test assures that the substance is of the linear, or non-network, type. Negative results do not necessarily prove the presence of a network structure: the melting point may exceed the decomposition temperature, in which case the solubility is likely to be negligible.

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A high degree of swelling (several-fold or more on a volume basis) in the best candidate for a good solvent would indicate a loose network structure such as occurs in vulcanized rubber. If the polymer qualifies either as a linear or as a loose network structure on the basis of these tests, the temperature at which it hardens (or softens) should be determined.

Whether crystallinity or embrittlement is involved at this temperature can usually be deduced from the transparency (opacity indicating crystallinity) or, with greater certainty, from X-ray diffraction.

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If the polymer is hard, insoluble, and infusible without decomposition, and if it refuses to swell greatly in any solvent, it may be assumed either that it is highly crystalline, with a melting point above its decomposition temperature, or that it possesses a closely interconnected network structure (e.g., as in a highly reacted glyceryl phthalate or a phenol-formaldehyde polymer).

Differentiation between these possibilities is feasible on the basis of X-ray diffraction.

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From the results of tests of this nature, the type of structure occurring in a given polymer usually can be deduced.

Following these qualitative observations, chemical composition and structure determinations would be logical next steps.

If the polymer is soluble, measurement of the average molecular weight and determination of the molecular weight distribution enter as first objectives in the quantitative elucidation of constitution.

If it is insoluble owing to a network structure, ordinary physicochemical methods obviously cannot be applied.

Certain information on the density of cross-linking within the network structure can be secured from equilibrium swelling measurements, however.