Step Reaction Polymerization C H E M I S T R Y AN INTRODUCTION Step Reaction Polymerization Malcolm P. Stevens

Distinguishing features of Chain- and Step Polymerizartion Mechanisms Step Polymerizations Chain Polymerizations Any two molecular species can react. Monomer disappears early. Polymer MW rises throughout. Growth of chains is usually slow (minutes to days). Long reaction times increase MW, but yield of polymer hardly changes. All molecular species are present throughout. Usually (but not always) polymer repeat unit has fewer atoms than had the monomer. Growth occurs only by addition of monomer to active chain end. Monomer is present throughout, but its concentration decreases. Polymer begins to form immediately. Chain growth is usually very rapid (second to microseconds). MW and yield depend on mechanism details. Only monomer and polymer are present during reaction. Usually (but not always) polymer repeat unit has the same atoms as had the monomer 2 Introduction to Polymer Chemistry

Condensation vs. Addition Carothers originally classified polymers based on a comparison of the atoms in the monomer to the atoms in the polymer repeat unit. Condensation polymers had fewer atoms in the repeat unit (i.e., some small molecule was emitted during polymerization). Addition polymers had the same atoms as their monomers. Step polymerization by addition of alcohols to diisocyanates to form polyurethanes: Chain polymerization (ring opening of heterocycle) with loss of CO2 to form polypeptide. 3 Introduction to Polymer Chemistry

A. Step-Reaction Polymerization - Kinetics 4 Introduction to Polymer Chemistry

A. Step-Reaction Polymerization - Kinetics 5 Introduction to Polymer Chemistry

A. Kinetics of Step-Growth Polymerization 6 Introduction to Polymer Chemistry

A. Kinetics of Step-Growth Polymerization 7 Introduction to Polymer Chemistry

A. Kinetics of Step-Growth Polymerization 8 Introduction to Polymer Chemistry

B. Stoichiometric Imbalance These are polyethers that are processed to an oligomer stage and are subsequently converted to network polymer by appropriate reactions of terminal epoxyide groups. With polyimides for fiber applications, molecular weight must often be limited because too high a viscosity is detrimental to extrusion of filaments through the fine holes of a spinneret. Three ways to limit M. W. in step polymerization 9 Introduction to Polymer Chemistry

B. Stoichiometric Imbalance 10 Introduction to Polymer Chemistry

B. Stoichiometric Imbalance 11 Introduction to Polymer Chemistry

B. Stoichiometric Imbalance 12 Introduction to Polymer Chemistry

C. Molecular Weight Distribution 13 Introduction to Polymer Chemistry

C. Molecular Weight Distribution Nx 14 Introduction to Polymer Chemistry

C. Molecular Weight Distribution 15 Introduction to Polymer Chemistry

C. Molecular Weight Distribution 16 Introduction to Polymer Chemistry

C. Molecular Weight Distribution 17 Introduction to Polymer Chemistry

D. Network Step-Polymerization : Theory of Gelation If monomers containing a functionality greater than two are used in step polymerization, chain branching results. If the reaction is carried to a high enough conversion, gelation occurs. The onset of gelation, or gel point, is accompanied by a sudden increase in viscosity such that the polymer undergoes an almost instantaneous change from a liquid to a gel. 18 Introduction to Polymer Chemistry

D. Network Step Polymerization 19 Introduction to Polymer Chemistry

D. Network Step Polymerization 20 Introduction to Polymer Chemistry

D. Network Step Polymerization 21 Introduction to Polymer Chemistry

D. Network Step Polymerization Branching point 22 Introduction to Polymer Chemistry

D. Network Step Polymerization 23 Introduction to Polymer Chemistry

D. Network Step Polymerization 24 Introduction to Polymer Chemistry

D. Network Step Polymerization 25 Introduction to Polymer Chemistry

D. Network Step Polymerization 26 Introduction to Polymer Chemistry

E. Step-Reaction Copolymerization 27 Introduction to Polymer Chemistry

E. Step-Reaction Copolymerization 28 Introduction to Polymer Chemistry

F. Step Polymerization Techniques 29 Introduction to Polymer Chemistry

F. Step Polymerization Techniques 30 Introduction to Polymer Chemistry

F. Step Polymerization Techniques 31 Introduction to Polymer Chemistry

F. Step Polymerization Techniques 32 Introduction to Polymer Chemistry

F. Step Polymerization Techniques 33 Introduction to Polymer Chemistry

G. Dendritic Polymers 34 Introduction to Polymer Chemistry

G. Dendritic Polymers 35 Introduction to Polymer Chemistry

G. Dendritic Polymers 36 Introduction to Polymer Chemistry

G. Dendritic Polymers 37 Introduction to Polymer Chemistry

G. Dendritic Polymers 38 Introduction to Polymer Chemistry

G. Dendritic Polymers 39 Introduction to Polymer Chemistry

G. Dendritic Polymers 40 Introduction to Polymer Chemistry

Commerically Important Polymers Prepared by Step-Reaction Polymerization Carbonyl addition-elimination Polyesters, polycarbonates, polyamides, polyimides... Aromatic addition-elimination Polysulfones, polysulfides, polyetherketones Carbonyl addition-condensation Phenol-formaldehyde and related polymers Polymeric heterocycles Addition to multiple bonds or epoxides Polyurethanes Epoxy polymers Miscellaneous Oxidative aromatic addition (polyphenylene oxide) Acyclic diene metathesis (ADMET) Aryl-aryl coupling Reductive coupling (polysilanes) Hydrolysis coupling (silicones) Diels-Alder cycloaddition Biradical coupling (polyxylylene) Friedel-Crafts chemistry SN2 reactions and a host of others... 41 Introduction to Polymer Chemistry

Carbonyl Addition-Elimination Step Polymerization : I. Polyester Mechanism : Structure-property relationships: I. Polyester Synthesis : 42 Introduction to Polymer Chemistry

Carbonyl Addition-Elimination Step Polymerization : I. Polyester PBT Other commercially important polyester: PEN PET 43 Introduction to Polymer Chemistry

Carbonyl Addition-Elimination Step Polymerization II. Polycarbonates III. Polyamides 44 Introduction to Polymer Chemistry

Carbonyl Addition-Elimination Step Polymerization IV. Polyimide 45 Introduction to Polymer Chemistry

Aromatic Addition-Elimination Polymerization Mechanism : This reaction is analogous to carbonyl addition-elimination, in that it is a two step process where the negative charge is accomodated by an electron withdrawing group. To emphasize the simularity, this example uses a ketone: Monomers : Bisphenols are most often used as the nucleophillic components. The chemistry begins when a base like NaOH or K2CO3 deprotonatea the bisphenol, as in this example for Bisphenol A: Krishnamurthy, S. J. Chem. Ed. 1982, 59, 543. 46 Introduction to Polymer Chemistry

Aromatic Addition-Elimination Polymerization I. Poly(etheretherketone), ‘’PEEK’’ The most common form of PEEK is the one shown, derived from Bisphenol A. This polymer is a remarkable material, highly crystalline, thermally stable, resistant to many chemicals, very tough. It can be melt-processed at very high temperatures (>300 °C), and is useful for special applications like pipes in oil refineries and chemical plants, and parts for aerospace, where high price is not a limitation. 47 Introduction to Polymer Chemistry

Aromatic Addition-Elimination Polymerization II. Polysulfone, ‘’PSF’’ Like polycarbonate, many other polysulfones could be synthesized, but the particular one shown here is by far the most common commercially, so that the general term "polysulfone" usually refers to this particular one. Worse, it is seldom called "poly(etherethersulfone)," despite its close structural similarity to PEEK Unlike PEEK, poly(etherethersulfone) is completely amorphous, probably a result of the relatively large size of the sulfonyl group, and the kink in the polymer backbone caused by the narrow C-S-C bond angle (close to 100°). Therefore, it can be processed at lower temperature than PEEK, but the material is not as resistant to heat and chemicals. 48 Introduction to Polymer Chemistry

Carbonyl Addition-Condensation Polymerization III. Phenol-Formaldehyde Polymers IV. Polymeric Heterocycles 49 Introduction to Polymer Chemistry

Carbonyl Addition-Condensation Polymerization The phenol-formaldehyde polymers are the oldest commercial synthetic polymers, first introduced around 100 years ago. Their inventor, Leo Bakeland, had no idea what was happening in his reaction kettles, but he was able to work out conditions to produce a tough, light, rigid, chemically resistant solid from two inexpensive ingredients. He soon became a rich man, in the same class as the famous industrialists of the time like Alfred Nobel, Henry Ford, Andrew Carnegie, George Eastman, etc. The actual chemistry is complicated, and still not competely understood. The polymers are usually thermosetting (i.e., crosslinked), and their insolubility limits the analytical techniques that can be brought to bear. The main reaction is the production of methylene bridges between aromatic rings, as shown below. Many side reactions also occur, and some of these give phenol-formaldehyde polymer its dark color. Of course, these crosslinked polymers cannot be melted or dissolved, so their synthesis must be conducted in molds for the actual product. In practice, the polymerization is usually carried out to somewhere below the gel point in a separate reactor, and then the "pre-polymer" is transferred to the mold, where the reaction is completed. Urea or melamine can be substituted for phenol. Methylene bridges can also be formed between the nitrogen atoms, giving rise to chemical relatives of the phenol-formaldehyde polymers. The urea and melamine based materials have much less color, and so are useful for decorative applications such as dinner plates and countertop materials (FormicaTM). 50 Introduction to Polymer Chemistry

Addition to Multiple Bonds or Epoxides Mechanism : The urethane linkage (often called carbamate) is usually made by adding an OH across the C=N of an isocyanate. The reaction is catalyzed by bases such as tertiary amines or by certain tin salts. I. Polyurethanes Polyurethanes are synthesized by the reaction of diols with diisocyanates: Many different polyurethanes have been synthesized, giving rise to materials with widely varying properties. For example, rubbery polyurethanes are used for Spandex fiber and for seat cushions in furniture and cars, while hard polyurethanes are used for wheels on roller skates, for bowling balls, and for paints and varnishes. The hydrogen bonds between the NH and CO groups provide toughness to the polymers. 51 Introduction to Polymer Chemistry

Addition to Multiple Bonds or Epoxides II. Epoxy Polymers These polymers are best known as two component thermosetting adhesives, although linear polymers can be prepared. The term "epoxy" polymers is something of a misnomer, because the epoxy groups are in the monomer, not in the polymer. To form the actual polymer, one reacts a multifunctional epoxide with a multifunctional nucleophile. Epoxy monomers based on Bisphenol A are by far the most common substrates, although others can be used. The nucleophiles are most often amines or phenoxides. The number of reactive functional groups on the components governs whether the polymer is linear or crosslinked. Epoxy Adhesive Chemistry The resulting network will not dissolve in any solvents, and resists all but the strongest chemical reagents. 52 Introduction to Polymer Chemistry

Addition to Multiple Bonds or Epoxides Other Epoxy Polymer The plurality of OH groups provides hydrogen bonding, useful for adhesion to polar surfaces like glass, wood, etc. Epoxy polymers are often used to form composite structures filled with glass or carbon fiber. 53 Introduction to Polymer Chemistry

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