New Frontiers in Sciences, Engineering and the Arts: Vol. I Introduction to New Classifications of Polymeric Systems and New Concepts in Chemistry

By Sunny N.E. Omorodion

Introduction to New Classifications of Polymeric Systems and New Concepts in Chemistry is the first volume of New Frontiers in Sciences, Engineering and the Arts. It is the first, because of the importance of polymers in our lives and world. With life, steroids, DNAs RNAs, enzymes, most of the food we eat, and more are polymeric in character. Without the polymer industry, the largest industry in the whole worldelectrical and electronic industries, Automobiles industries, kitchen and household industries, packaging industries, oil and gas industries, sound and current industries, and more too countless to listcannot exist both in our physical and natural world, a complex world.

The greatest subject area in chemistry is polymer chemistry; for without it, one would not have been able to see what the atom is. It was only in recent years that some great scientists began to see that the electron does not exist outside the nucleus of an atom, but probably inside the nucleus. Indeed, where they exist is still unknown to them. What indeed are outside the nucleus are what some great scientists sometimes call radicals. What these radicals are have remained unknown to humanity. For example, how can radicals add during free-radical or radical polymerizations when they have not been identified just like the ions and charges have with, for example, their males and females. One was able to ask this very deep question based on ones background in polymer chemistry at secondary, bachelors, masters, and PhD levels. Otherwise, others can. Finding solutions to it led to the New Frontiers using universal data abundantly available.

With this newly added definition for an atom, one discovered that what all scientists, engineers, medical doctors, and experts in all disciplines have been doing have all been based on the use of universal data, 95 percent of which have wrong interpretationsa complete world of illusions. Is it not shocking to note that all chemists, whether polymeric or not, do not know what a monomer or compound is, or how chemical and polymeric reactions take place. Is it not shocking to note that the electrical and electronic engineers do not know how current and sound flows in some metals and liquids and in space. These and so much more are the origins of the New Frontiers, beginning with this first volume wherein new concepts seen in the first book, The Beginning of a New Dawn for Humanity emerged from.

• Language: English
• Publisher: AuthorHouse UK
• Release date: Apr 27, 2017
• ISBN: 9781524679125

Sunny N.E. Omorodion

Sunny N. E. Omorodion has been a teacher mostly all his life. He started his teaching career at the age of nineteen in a High school teaching students many of whom were older than him. After graduating from the University of Ibadan with a BSc (Hons) in Chemistry at the age of twenty three, he left for Canada after teaching in two High schools again, to acquire another bachelor (B Eng.) in Chemical Engineering at the University of Alberta, since his dream career was Chemical Engineering since he was a child. In the same university, he acquired two Bachelors in one year in Mathematics and Physics, since when a student at Ibadan he only needed one year and two years to complete degrees in Mathematics and Physics respectively. It was the civil war in the country that made him to study Chemistry at a time when Chemical Engineering did not exist as a discipline in any of the Nigerian Universities, something which can be said to be a blessing in a different way, since in the process he was fully exposed to Mathematics and Physics at the tertiary level, while graduating with only Chemistry. During the acquisition of four Bachelor degrees, he left for McMaster University in Ontario, Canada to acquire M Eng. and Ph. D degrees in an area which is a hybrid of Chemistry, Mathematics, Physics and Chemical Engineering-Polymer Engineering. He then worked in an industry (Polysar-Sania, Ontario, Canada) for about two years, before coming back to his teaching career at the University of Benin. After about twenty years of service as teacher/consultant, he left on sabbatical leave and leave of absence to teach at three universities- University of Regina, Saskatchewan, Canada, University of Windsor, Ontario, Canada, and University of Toledo, Ohio, USA. Introduction of three new courses at post-graduate level along with the teaching of other courses in Canada and USA made him one of the best professors in all the universities. Presently, he is now back to University of Benin to complete the cycle of one of the stages of life. Sunnys research interests include Chemical and Polymer reaction Engineering very different from what exists in Present-day Science and Engineering, Environmental Science and Engineering with respect to Pollution Prevention, Waste Management, Enzymatic Chemistry and Engineering, Energy Sources and Conservation, Unit Separations and Process Control of Industrial Systems. Based on The New Science, some research works which were thought not to be possible, have been made possible, such as oxidation of propane to propanol, polymerization of some monomers, which could not previously be polymerized to give useful products and so on. These are works which cannot be published without introducing The New Science in The New Frontiers universally. Sunny is a member of American Institute of Chemical Engineers since 1972, Canadian Society of Chemical Engineers since 1974, Chemical Society of Canada since 1972, American Chemical Society since 2002, American Association for the Advancement of Science since 1996, African Academy of Science since 1990, Nigerian Society of Chemical Engineers since 1988, Polymer Institute of Nigeria since 1990, The Association of Professional Engineers and Geoscientists of Saskatchewan, since 2002. Because he has been so involved in writing, spending at least fourteen hours everyday since Jan 1st 1992, he has not been an active member of these bodies and even rejected serving the State Gov. as Commissioner and other appointments. Sunny is a Fellow of some Professional bodies such as Institute of Industrial Administration since 2008, American and Cambridge Biographical centers and Professional bodies since 1997, and Strategic Institute for Natural Resources and Human Development since 2012. Sunny has won more than twenty International awards with respect to Who is Who from ABI, IBC, The Marquis and more. We are TENANTS not only in this WORLD, but also in our PHYSICAL BODY, for there is no DEATH of the BEING, BUT THE DEATH of the PHYSICAL BODY. The Author

Book preview

References

1. F. Rodriguez, Principles of Polymer systems, McGraw-Hill Book Company, New York, 1970.

2. F.W. Billmeyer, Textbook of Polymer Science, Interscience Publisher, a division of John Wiley & Sons, Inc., New York, 1964.

3. R.B. Seymour, Introduction to Polymer Chemistry, International Student Edition, McGraw-Hill Book Company, New York, 1971.

4. C.R. Noller, Textbook of Organic Chemistry, W.B. Saunders Company, London, 1996.

5. Course Notes – Parts II and III, Polymer Reaction Engineering, An Intensive Short Course on Polymer Production Technology, McMaster University, Hamilton, Ontario, Canada, 1976.

6. S. N. E. Omorodion, The beginning of a New Dawn for Humanity- Introduction to the world of micro- and macro- molecular chemistry., in Press.

Problems

1.1. List the problems which have hindered the development of polymer engineering for so many years, despite the fact that it is linked to the largest based industry. What is polymer engineering?

1.2. Distinguish between a polymer, wax, grease, and a chemical compound. For polymers, list the different types of bonds and electrostatic forces that exist in the system. Does hydrogen bonding exist? If not, explain.

1.3. Define homo- and co-polymers. Provide the classification of polymers on the basis of repeating units. Describe the different types of repeating units that exist in polymeric systems.

1.4. What are active centers? How can they be generated? From the first column in Table 1.1 how many types of actives centers exist?

1.5. Based on the forms of usage of polymers, define and identify the major polymer processing industries existing worldwide.

1.6. What are the distinctive features of elastomeric polymers compared to plastic polymers? What is a plasticizer? How many types of plastics can be distinguished when heated?

1.7. Though it may be too early at this point in time to provide detailed explanation, do you have any idea why polyvinyl chloride, polyethylene and polystyrene cannot be used for producing elastomeric polymers? What is distinct about polymer additives compared to other forms of usages of polymers?

1.8. Based on polar and ionic characters, how many types of polymers exist? Distinguish between the different types of polymers. Vinyl acetate is polymerized to produce poly (vinyl alcohol). How is this made possible? Though it is too early at this point in time, can you explain why poly (vinyl alcohol) cannot readily be obtained from vinyl alcohol?

SECTION A

Characteristics and Structural Classification of Polymers

Polymers can be classified from so many points of view. One has already seen some ways by which they can be classified – homo- and co-polymers, synthetic and natural, carbon-chain and hetero-chain, industrial types, that is, elastomers, plastics, fibers, coatings and adhesives and foams, repeating unit types, etc. These are classifications directed to laying the foundations for a new polymer engineering student. From time to time, one will have course to refer to other disciplines as they apply. However, one will now address our attention to classifications relevant to polymer reaction engineering.

There are different types of structural arrangements on micro- and macro- scale bases. These structures are obtained under different polymerization conditions using different types of monomers, catalysts and different types of polymerization techniques. There are also different types of polymerization methods employed in producing these polymers. Thus, it will therefore be of interest for us to know first the different types of structural arrangements that exist. This is what this section with only one chapter entails.

Chapter 2

CLASSIFICATION OF POLYMERS BY STRUTURAL ARRANGEMENTS

2.0 Introduction

Structural arrangement of polymers can be viewed from the micro-scale and macro-scale points of view. When viewed from the micro-scale basis, one is particularly concerned with the chain compositions, stereo-regularity, molecular weight averages and distributions of the polymer, frequency of branching and the polymer stability. These are micro properties which do not depend on the physical state of the polymer. It must be noted that with polymers, molecular weight as exists with simple compounds is meaningless unless one can obtain a polymer with one single type of chain of the same length! Hence the reference to molecular weight AVERAGES and DISTRIBUTIONS.

When viewed from the macro scale basis, one is concerned about the physical state of the polymer as well as the micro properties of the polymer. Macro properties include properties such as particle size distribution and average porosity, bulk density, transition temperatures, flexibility of polymer chains, chemical properties, and electrical properties and so on. All these properties depend on polymer reactor techniques, designs and operations. One therefore begins by looking at the structural arrangement of polymers on a micro-scale basis.

2.1 Structural Arrangement of Polymers on micro Structural basis

On the micro sale basis of structural arrangement of polymer (microstructures) there are essentially four main kinds of polymers- linear, branched, ladder- kinds and cross-linked network polymers. For each of the four kinds, there are sub-divisions or sub-types. All these types and sub-types of shapes are determined largely by the type of monomers involved, the type of catalysts used, the methods of polymerization or polymerization techniques, the conditions of polymerization, and the additive types present during polymerization.

2.1.1 Linear Arrangements

Linear arrangements are those in which the repeating units are linked together in one continuous chain or length to form a polymer molecule. In these linear arrangements for homopolymers and some block copolymers several structures called stereo isomers can also result depending on the type of catalysts involved and type of monomer used in terms of presence of assymmetricity of the main chain carbon atoms, the type of substituted groups along the sides of the main chain backbone and presence of specific rings along the chain backbone. While some monomers in the absence of stereo specific catalysts are self-stereo regulating, some do not. Some also become self-stereo regulating in the presence of specific types of catalysts.

Linear arrangements between two monomers or a single monomer with two polymerizable activation centers could also result to alternating placements with or without stereo specific placements of the monomers involved, depending on the symmetric or non-symmetric character of the monomer and the type of catalyst involved. In general, most linear stereo specific placement of the repeating units include-

(a) Isotactic d-and –l enantiomers arrangements/arrangements of substituted groups along same side of the main chain backbone-all on one side –dextro, that is, d or all on the other side -levo, (that is, l).

(b) Syndiotactic arrangements (arrangements of substituted groups alternately along the main chain backbone).

(c) Iso –Trans-tactic placements or arrangements (arrangement of internally located double bonds on opposite sides of main chain backbone with the substituted group internally or externally located, isotactically placed).

(d) Syndio-Trans-tactic placements or arrangements (arrangements of internally located double bonds on opposite sides of main chain backbone with the substituted group internally or externally located, syndiotactically placed).

(e) Iso-Cis-tactic placements or arrangements (arrangement of internally located double bonds on same side of main chain backbone with substituted group internally or externally located, isotactically placed).

(f) Cis-syndio- or disyndio-tactic placement or arrangement (cis-placement of rings in a syndiotactic arrangement).

(g) Cis-iso- or diiso-tactic placement or arrangement (cis-placement of rings in an isotactic manner-not common due to steric limitations).

(h) Trans-isotactic or diisotactic placement (Trans-placement of even membered rings in an isotactic arrangement).

(i) Trans-syndiotactic or disyndiotactic placement (Trans-placement of even membered rings in a syndiotactic manner-not common due to steric limitations).

(j) Erythro-cis-tactic arrangement (arrangement of at least two internally located double bonds on the same side of main chain backbone)

(k) Erythro- cis-isotactic arrangement (same placement as in (j), but with the substituted group isotactically placed).

(l) Erythro-trans-iso or syndio tactic arrangement (arrangement of at least two internally located double bonds alternatingly placed on both sides of the main chain backbone, with the substituted group iso- or syndio-tactically placed depending on the type of catalyst). There is also erythro-trans-tactic arrangement.

(m) Threo-cis-tactic arrangements (arrangement of one of the internal double bonds along the axis of one of the coordination catalyst centers, with the other double bonds on the same side of that axis). There is also the threo-cis-isotactic arrangement.

(n) Threo-trans-tactic arrangements (arrangement of one of the internal double bonds along the axis of one of the coordinated catalyst centers with the other double bonds alternatingly arranged on both sides of that axis). There are also the threo-trans- iso- or syndio-tactic arrangements depending on where the substituted group is located.

(a) and (b) arrangements above are limited to mono olefins, acetylenes, nitriles, isocyanates, ketenes, ring opening monomers, aldimines, ketimines and cumulenes, etc. (c), (d) and (e) are limited to 1, 3-di-olefins such as butadiene, 1, 3-pentadiene, isoprene, methyl sorbate etc. (f), (g), (h) and (i) are limited to cycloalkenes and similar types of monomers. The remaining placements or arrangements are limited to tri- and higher olefins. It is important to note that the list above is not indeed complete. Nevertheless, one has tried to cover most of the known monomers. Some of the structures cannot be fully understood until one has gone deep into the Volumes (in particular Volume V) dealing with propagation of species, the step in which the structures of the polymers are built. Therefore, only limited examples will be given in this chapter.

There are far more number of structures which should have been mentally possible, but cannot exist due to steric limitations, electrostatic/electrodynamic forces of repulsion and the type of monomers involved. Some of these structures are even presently thought to exist, when their existences are impossible unless special monomers such as diazoalkanes are used. With diazoalkanes it is possible for example to have the following structures, whereas with a

405661.png

2.1

Atactic (From methyl diazomethane)

monomer such as cis- or trans-2-butene, it is not readily possible to have such arrangements but the type shown below.

405646.png

2.2

Di-syndiotactic cis-2-butene

405634.png

2.3

Di-isotactic trans-2-butene

It is almost impossible with the existing catalyst to have di-isotactic cis-2-butene or di-syndiotactic trans-2-butene particularly as the alkyl group increase in size due to steric limitations, when mono-olefins are involved. However, when very strong nucleophilic monomers such as diazoalkanes are involved (far more nucleophilic than olefins), any structure as shown in Equation 2.1 is favored, due to the type of monomer and catalyst involved.

When stereo specific structures, that is, regular structures cannot be obtained, for all the monomers, then atactic placements or structures are obtained. Atactic arrangement is one in which substituted groups are randomly placed along the main chain backbone. They do not only apply when the monomer is asymmetric but also when the monomer is symmetric for some cases where rings are present along the chain or where internal double bonds exist.

It has also been a very strong misconception over the years that head-to-head or tail-to- tail arrangement exists with polymeric structures. Chargedly, their existences are impossible. Radically, they only exist during termination by combination, a sub step under termination step. Nevertheless, chargedly and radically, one can also have mirror images of two long or short chains on both sides of a dead polymer depending on the step or sub step. But their presence is not due to head-to-head or tail-to-tail addition, but only via head-to-tail or tail-to-head addition. Only specific monomers radically favor termination by combination. One will begin with giving classical examples of all the structural cases above, both known and unknown.

Beginning with propylene for the first two cases (a) and (b), the followings are obtained-

405680.png

(i) Isotactic polypropylene d-form

2.4

405700.png

(ii) Isotactic polypropylene l-form

405529.png

2.5

Syndiotactic polypropylene

405521.png

2.6

Atactic polypropylene

405512.png

Atactic or isotactic or syndiotactic almost mirror images

2.7

Though propylene is presently known not to favor free-radical polymerization, the first three structures above are the structures favored on a linear chain radically and chargedly, while the fourth structure can be favored radically for propylene. There is no head-to-head or tail-to-tail arrangement as shown above. In Equation 2.7, there seems to be some resemblance of head-to-head addition. The (a) and (b) are images of two chains, which are involved in one of the sub steps during polymerization radically. Radically, largely atactic arrangement is favored for non-polar monomers such as propylene or α-olefins, styrene, etc. For a monomer such as styrene, the following additional arrangement is also possible.

405545.png

2.8

Atactic polystyrene

It is only during termination by combination radically during certain conditions that tail-to-tail or head-to-head addition is favored. Apart from that point of addition between the two growing polymer chains (a) and (b), all the other additions for the repeating units are head-to-tail additions. (a) and (b) are almost mirror images. For symmetric monomer such as ethylene or 1, 3-butadiene, these phenomena are not visibly present. The arrangements above are largely atactic, since the monomer is non-polar.

For di-olefins such as butadiene or pentadiene the followings are the possible structures linearly.

405572.png

2.9

Cis-tactic polybutadiene 2.9

405589.png

2.10

Trans-tactic polybutadiene

405337.png

2.11

Atactic-trans-cis-polybutadiene

405360.png

2.12

Iso-cis-tactic poly (1,3-pentadiene)

405384.png

2.13

Syndio-trans-tactic poly (1,3-pentadiene)

It should be noted that, these are new representations of the structures, slightly different from what has been known to be the case in the past. These new structures are based on current developments to be observed in the volumes and subsequent chapters.

For special monomers such as cyclohexenes and cyclobutene, the followings are some of the favored linear structures.

50359.png

2.14

Trans-isotactic polycylohexene

81461.png

2.15

Trans-di-isotactic polycylomethyl hexene

50316.png

2.16

Cis-di-syndiotactic polycylomethyl hexene

50308.png

2.17

Trans-syndiotactic polycyclobutene

50297.png

2.18

Trans-isotactic polycyclobutene

It is important to note that where trans-placement of the ring is favored, the ring is folded into two equal parts. Of the two monomers used above, only cyclobutene can also favor the opening of the ring, under certain conditions. There is no need to ask questions now about how these structures were obtained or under what conditions they are favored until later in the series. When the ring is not folded as shown in Equation 2.16, it is cis-placed. In this case the ring is placed on opposite sides of the main chain backbone. In the presence of a methyl group which is also similarly placed, cis-disyndio-tactic-placement is obtained, particularly in the presence of proper coordination.

Finally, for the last group of monomers which favor independent and different arrangements, the followings are obtained for hexatrienes.

405249.png

2.19

Threo-cis-tactic polyhexatriene

405228.png

2.20

Threo-trans-tactic polyhexatriene

405170.png

2.21

Erythro-cis-tactic poly hexatriene

405189.png

2.22

Erythro-trans-tactic poly hexatriene

Provided the size of the monomer when activated does not provide steric limitations, when stereo specific catalysts are involved, the structural arrangements indicated above can readily be favored. In the Threo-configuration of the monomer, the internally located double bonds are trans-placed, while in the Erythro-configuration, they are cis-placed. The arrangements favored, will largely depend on the strength and type of catalyst involved for their polymerizations.

While some of the placements or arrangements for cycloalkenes and hexatriene have been observed, others have not. For those that have been observed, there have been wrong misrepresentations of their structures for the following reasons.

(i) The phenomena of activation of monomers have never been properly identified. What this implies will be fully explained down the series, particularly when it is realized that a monomer has never been defined in the true sense.

(ii) The types of catalyst involved in polymerization systems have never been fully identified and properly classified.

(iii) The many several other phenomena that take place with monomers-substituted mono-alkenes, di-alkenes, acetylene, ring-opening monomers, ring-forming monomers, etc., have never been known. In fact, how ring-opening monomers are opened have never been known. The reasons are too numerous to mention.

In some alternating placements, there are cases of stereo regular placements of the repeating units, such as alternating cis-trans arrangements of for example 1, 4-addition of 1, 3-butadiene. Randomness or lack of order of arrangement of repeating units in a polymer chain (atactic arrangements) prevents the orderly packing that is essential for crystalline structures. Provided substituted groups are not bulky, the degree of regularity is also reflected in the density, melting point and stiffness of polymer chains. Thus, complete stereo specific arrangement or regularity does not in general guarantee presence of complete crystallinity.

For some years now, there still seems to exist confusions about the conformations, configurations and arrangements of monomers or polymer chains in their use or definition. Sometimes, conformations and arrangements are used synonymously. While arrangements of monomers along a polymer chain backbone are fixed once formed, conformations are not. In general, changes in structure caused by only rotations about single bonds with no bond bending, breaking or stretching are called conformations. In a dilute solution, the conformation of a monomer or polymer chain is infinite and ever changing with time. In order words, it is the dynamic state of the structures or arrangements for polymer chain in solution. Of the infinite numbers of conformations however, some have been identified with different levels of stability. Some of the well-known conformations include staggered helical, sheet-like and eclipsed or spiraling conformations. Unlike conformations for small organic molecules, the conformations of polymer molecules are very important, since they can affect the macrostructures and the physical state of the polymer. That is, the types of substituted group(s) carried by the repeating units in a polymer chain is very important in determining the conformation which will be most favored all the time, in solutions with different concentrations. In trying to geometrically show or represent some of these conformations, several projections have been suggested, one of the most important of which is the Newman projections. While it should be known, that representation of conformations using projections in order to understand polymerization system kinetics, that is chain growth, is of no use. It is only the conformations and arrangements that can be represented using projections-the planar or Fischer projection. Irrespective the conformation of polymer chain, the Fischer projection remains the same and the only one required during polymerizations.

Arrangements are limited to polymer chains, as it indicates how the monomers are placed along the chain. Just as arrangement is fixed, configuration of a monomer or polymer chain is also fixed. However, configuration is the spatial arrangement of the substituted groups on a monomer or along the main chain of a polymer. Configuration can only be changed by breaking of bonds and replacing with other group(s) or interchanging the group(s) (isomers). Thus, it can be observed that though conformations, configurations and arrangements deal with structures, they are all uniquely different. In polymer reactor design, conformational analysis is of little or no significance, since conformer, unlike isomers are too readily convertible each into the other to be isolated as separate entities.

For linear copolymers, there are random, alternating and block types of copolymer structures as shown in figure 2.1 for two different monomers or repeating unit A and B or a single monomer with two activation centers.

407212.png

Fig2.1. Structural arrangement of linear copolymers

Random copolymers are for more common with monomers with activation centers than functional groups. Alternating copolymers are more common with monomers with Functional groups than those with Activation centers. Indeed, Step polymerization systems wherein two or more monomers are involved produce alternating placements. In (iii), the C and T refer to Cis- and trans- placements. An example of such stereo-alternating copolymer is 1, 4 polymerization of butadiene using special catalysts as will be shown later in the series.

50207.png

2.23

Alternating block copolymers can largely be found with monomers with functional groups. Regular block copolymers can be obtained from both types of monomers, those with activation centers and those with functional groups, using specific methods. Coupling block copolymers can largely be obtained using special coupling agents and catalyst, with monomers with special activation centers, such as some of those ring-opening monomers. A typical example is shown below.

50198.png

2.24

(Coupling block copolymer from an ethylene diamine and propylene oxide)

Stereo-block copolymers like stereo alternating copolymer are limited to monomers with activation centers using stereo-specific catalysts. An example of such case is shown for polypropylene, where the R in (iv)d of Figure 2.1 represents substituted group which for propylene is CH3.

50189.png

Stereo-block propylene polymer

2.25

Same monomer is involved, except that the pendant or substituted groups are arranged on opposite sides of the blocks. The blocks are both isotactic except that one is d, while the other is l as follows.

dddddddddddddddddddddlllllllllllllllllllllllllllllll

2.26

It is important to note that, all the structural placements and configurations have been represented using Fischer’s projection and this is the only projection that can and will be used, noting that the polymeric molecules, whether linear or branched are mostly coiled in solution.

2.1.2 Branched Arrangements

Branched polymer molecules are those in which there are side branches of linked monomer molecules protruding from different central branch points along the main polymer backbone or even side chains. They may be comb-like, T-shaped, cross-like or dendritic in structures, with long and short branches. Their presence is more largely favored when non-coordination or free-media catalysts such as radical and non-free-ionic catalysts are involved. Branches can be obtained either during homopoly-merization of some monomers or during copolymerizations. Unlike a metallic comb or cross, branches are flexible.

Graft copolymers like block copolymers have long sequences of the monomer types involved in the copolymer chain. However, the graft copolymer is a branched copolymer with a backbone of one monomer type to which are attached long branches of the second monomer. Unlike typical copoly-merization reactions where the two or more monomers are involved simultaneously in the reaction, in regular block, stereo block and graft copolymerizations, a dead polymer or a living polymer only for regular block is first obtained before homopolymerizing the second or more monomers on it. Presence of branches can also be favored for other non-graft copolymers depending on the type of monomers involved. They are similar to those of homopolymers. Several types of graft copolymers exist. Some of these include single graft copolymers favored by only Emulsion technique of polymerization, multiple graft copolymers and alternating graft copolymers. Single graft copolymers are those that have only one branch point upon which a different monomer is grafted. These are obtained from dead polymers with a single internally located activation center. There are specific ways by which such polymers can be exclusively produced. Multiple graft copolymers are those in which there are more than one branch point upon which a different monomer is grafted. These are the commonest type of graft copolymers. A notable example is high impact polystyrene (HIPS) in which styrene is grafted on a portion of rubber-polybutadiene. Alternating graft copolymers are those in which a monomer is grafted on sites generated on repeating units made from two monomers.

Figure 2.2 displays the structural arrangements of these classes of ideal branched polymers. These are not the full range of all types of branched polymers existing in polymeric systems, but represent a very broad range of their existence. Branch formations are more common with activation (π-bond) types of monomers and monomers with loose ionic or radical centers, than functional groups types of monomers. Branching on functional group types of polymers such as shown below can only be done via an activation center or via an ionic center of the type found in functional groups of Step monomers.

50180.png50174.png50164.png404998.png

Figure 2.2 Structural arrangements of Branched polymers.

50152.png

2.27

50144.png

2.28

Polymers such as poly (vinyl alcohol) from an activation type monomer can also be grafted on radically or ionically. Thus, if branch formations on these types of polymers, through activation centers or ionic centers are to occur with these polymers, the monomers from which they are obtained, must

50138.png

2.29

have more than two functional groups. An example of a polyfunctional monomer is glycerol or urea/formaldehyde.

451089.png

2.30

In glycerol, branching takes place via the No 3 Oxygen center. No 1 and 2 Oxygen centers are used for linear polymerization. In urea/formaldehyde, branching takes place via the internally located nitrogen centers.

It is interesting to note that even polyethylene (made from a monomer with an activation center), as simple as the repeating unit is, can be linear or branched depending on the method of polymerization. Thus, there is low density polyethylene (LDPE) which is branched and high density polyethylene (HDPE) which is linear. The branches are as a result of the abnormal operating conditions of such systems as will become clear in the Series. When branching is present in the polymer, the density is decreased, because the volume is larger. An important consequence of branching is that it interferes with the ordering of molecules, so that crystals are more difficult to form. Also, melt flow of branched molecules is more complicated by elastic effects. In branched polymers, the influence of secondary forces on physical properties of the polymers is far stronger than with linear polymers. Thermoplastic polymers can be identified with linear and branched structures. The term thermoplastic as has already been defined (see Chapter 1), is applied to polymers which soften (softening temperature) and flow without chemical change when heat or pressure is applied and harden (transition temperature) when