CHEM 574 Polymer Chemistry C. J. Barrett Office 419 firstname.lastname@example.org Office hours anytime, but best to call/email for an appointment Evaluation:UndergradGraduate Mid Term Exam 35% 25% in class October 9 Final Exam 60% 40% ? in December Essay/Presentation ---- 30% due end of term Class Participation 5% 5% Grad students must follow the 2nd grading scheme, UG students have a choice. Research topics should be discussed, chosen, outlined by Oct 1.
CHEM 574 Polymer Chemistry Recommended Textbook: “Physical Polymer Science” by L. H. Sperling, 3rd Edition, 2001 ( http://www3.interscience.wiley.com/cgi-bin/bookhome/110527926 ) Other Suggested Reference Books: “Textbook of Polymer Science”, F.W. Billmeyer “Principles of Polymer Chemistry”, P.J. Flory “Physical Properties of Polymers”, J.E. Mark et al. “Polymer Chemistry: an Introduction”, Malcolm P. Stevens “Polymers: Chemistry and Physics” by J.M.G. Cowie, 2nd Edition
Outline of Material Covered Introduction and Overview (1 week) Chapters 1, 2 Molecular Weights, Sizes (~2 weeks) Chapter 3 Solid State Polymers, Solutions (~2 weeks) Chapters 4, 5, 6 Polymer Synthesis (~2 weeks) Chapter 11, + 8, 9, 10 bits Polymer Characterization (~1 week) various chapter bits Block Copolymers, Composites (~1 week) notes Polymers for NanoTech, Optics (~1 week) Chapter 14, plus notes. Biopolymers, Self Assembly (~2 weeks) notes Student Class Presentations (? days) questions on final exam ! We will only cover SOME of these chapters, and you will be responsible only for material covered in the class notes I will distribute.
Other 574 administration 1. Some important dates this term: Friday October 9th : Midterm exam (in class) Monday October 12th : Thanksgiving Holiday December 2,3 : No class– early study period for exam. (and some guest lectures still to be scheduled) 2. Background for CHEM 574: It is NOT assumed that students have had any introduction to Polymers. It IS assumed that students have some comfort with thermodynamics. Scientists and Engineers should feel equally (un)comfortable. It’s up to YOU to identify material that you’re not comfortable with.
Section 1. Introduction and Overview Important Question #1: What is a Polymer, and why is it not just a big molecule ? A1: The non-satisfying Latin answer: “polymer” = “many” + “units” A2: The more satisfying answer is found by examining the essence and limitations of Chemistry—the predictive science of Molecular Structure. H H H H H H Molecules: H – C – H or H – C – C – H or H – C – C – C – H H H H H H H CHEMISTRY the prediction of properties from molecular structure (molecular: size, shape, interaction with radiation (colour), smell, taste, reactivity, physiology) and group physical properties: (state, Mp, Bp, ‘feel’, surface energy, vapour pressure, ideal mechanical strength…)
Important Question #1: • CHEMISTRY the prediction of properties from molecular structure. • The idea is that: STRUCTURE ALL PROPERTIES. • BUT: There can be a few slight structural variations: • bond rotation, bond vibration ? This is accounted for, so negligible. • Structural isomers ? • vs. vs. • Still fast inter-conversion at most Temperatures, and in gas phase. • Therefore properties are usually observed as an AVERAGE, so negligible.
Important Question #1: • CHEMISTRY the prediction of properties from molecular structure. • The idea is that: STRUCTURE ALL PROPERTIES. • Stereochemistry however, is often NOT negligible: • 1 chiral centre: vs. • The 2 forms are distinct, but have identical properties except reactivity with other chiral systems (and therefore most physiological systems) • b) More than 1 chiral centre: other props NOT the same. (4 forms)
Important Question #1: • Stereochemistry however, is often NOT negligible: • 1 chiral centres: # of distinct forms = 2n • a large molecule can have many stereocentres n, with 2n distinct species. • Taxol n = 11 211 = 2048 different molecules • n = 20 ~ 1 million. • n = 40 ~1 x 1012 • Polytoxin n = 80 ~1 x 1012 • n = 320 ~1 x 1012 different molecules • Nomenclature then has to adopt the structure, + n different D, L labels. • (1 cellulose molecule can easily have 10,000 different stereocentres…)
Important Question #1: Polytoxin n = 80 ~1 x 1012 n = 320 ~1 x 1012 different molecules but we never have to worry, since any time we see these there’s 1 form. SO: STRUCTURE ALL PROPERTIES for all real molecules. Side question: Why not stop here? If molecules are so great, then why do we (or mother nature) need polymers? Answer: because individual molecules are structurally useless. If we look around, it’s hard to find any real materials that are individual molecules– they are metals, plastics, trees, paints, fabrics, fibres, papers, glass, ceramics, inorganic crystal but NOT individual molecules. If we look at the human body too, we are skin, bone, tendon, hair, teeth, muscle, DNA, proteins, cellular ‘gel’, etc, but NOT individual molecules.
Important Question #1: (again) What is a Polymer, and why is it not just a big molecule ? SO, somehow Polymers can achieve a lot of things that molecules can’t. Perhaps we can get some insight by ‘growing’ a polymer from a molecule: H — CH2—n H n = 1 methane n = 2 ethane n = 3 propane n = 6 hexane n = 8 octane n = 10 decane n = 100 : n = 20 ~oil “polyethylene” n = 40 ~wax Nomenclature = poly (monomer) n = 80 some kind of solid.
Important Question #1: (again) What is a Polymer, and why is it not just a big molecule ? This is the challenge now: we have a ‘molecule’, but it doesn’t behave like our usual molecules do at all– properties are too slow to be averaged, it doesn’t adhere to solid/liquid/gas behaviour, and real properties are VERY hard to predict from the molecular structure. The molecular structure is also hard to define, since now there can be a lot of stereochemistry, many isomers, and especially ‘n’ is NOT a constant, but an average with a wide distribution. Some parts of the solid can be crystalline, and most important measurements depend on the timescale observed.
Important Question #1: (again) What is a Polymer, and why is it not just a big molecule ? Physical properties are still predictable from molecular structure, but one needs a LOT more information now, such as: -Molecular weight and MW distribution -fraction of the solid that’s crystalline -How the units attach to each other -Any stereochemistry of attachment -Lots of processing parameters -orientation of the chains if any -much clever statistical theory. BUT, control of these parameters synthetically and with processing, provides us with a great ability to tune and optimize properties of materials
Where do we start to address all of these considerations ? With Polymer preparation, to gain an appreciation of microstructure… There are TWO common different strategies to produce polymers from monomers, the choice depending on the chemistry of the final product, and on available starting materials. 1) Chain Polymerization starts at one end, and adds identical monofunctional mers one by one as the chain grows longer and longer. 2) Step Polymerization starts with two bifunctional mers, linking their reactive ends, doubling the molecular weight at every step.
1) Chain Polymerization starts at one end, and adds identical monofunctional mers one by one as the chain grows longer and longer. This mechanism usually gives better control over molecular weights, so is preferred if possible. If your mer can accommodate a double bond, and the system is free from generating H or water, this is preferred. This mechanism however is limited to polymers with a carbon backbone. We just need anhydrous solvent, a C=CR monomer, and an initiator I*. The initiator is usually a free radical R*, so this type of polymerization is most frequently known as Free Radical Polymerization. INITIATION: usually a peroxide –O-O-, which breaks down into 2 O* This O* radical readily attacks C=C double bonds
I* + C=C-R I-C-RC* + C=CR chain-C* + C=CR chain-C* … PROPAGATION: In this fashion, as long as there’s still C=CR monomer present, then the reaction keeps going, and going, and going… until: TERMINATION: 3 things can stop the chain from growing– running out of monomer, and the growing chain finding an H or another radical. The statistics of these terminations lead to a wide range of final molecular weights, = a “broad molecular weight distribution”. TYPICAL ‘R’ groups commonly used: H = polyethylene the most common commodity plastic. methyl = polypropylene rope, flexible plastic, sportswear Phenyl = polystyrene brittle plastic, wrapping, styrofoam Cl = poly(vinyl chloride) pipes, corrosion resistant (F = teflon) acrylates, methacrylates Plexiglass, plastics that look better.
Step Polymerization starts with two bifunctional mers, linking their reactive ends, doubling the molecular weight at every step. • This allows MUCH more flexibility for the composition of the backbone (softer materials, less brittle, more flexible), but the constraints are in the molecular weight obtainable • A – R1 – A + B - R2 – B A – R1 – R2 – B • As long as EACH species are still present, the Rx propagates. • BUT, not a growing chain, but molecules that double in size each step. • esters, amides (nylons), silanes, saccharides, • For example, nylon 48 is a 4 carbon amide + an 8 carbon carboxylic acid
Step Polymerization starts with two bifunctional mers, linking their reactive ends, doubling the molecular weight at every step. • BUT, for a lot of fibres, fabrics, and other soft materials, a high molecular weight is not crucial. Esters and amides most common. • TYPICAL ‘R’ groups commonly used: • ethyl and terepthalate poly(ethylene terepthalate) PETE • Urethanes lycra and spandex • dimethyl siloxanes silicone rubber • 2 phenyl groups Kevlar • NATURAL polymers are usually stepwise, such as as cellulose, wool, silk, • But natural rubber is a material that follows a chain-like production.
We can now appreciate the MW distribution, and the other microstructural variations between chains are clear too: a) -Molecular weight and MW distribution (from mech.) b) -How the units attach to each other (head to head, or head to tail) c) -Any stereochemistry of attachment (chirality of ‘attack’) d) -fraction of the solid that’s crystalline e) -orientation of the chains if any (processing) (later) (we just finished chapter 1 in the text).
b) -Molecular weight and MW distribution (from the mechanism) • Some nomenclature, and implications: • In Free Radical Polymerization (and in step polymerization with asymmetric mers), there are TWO ways to add a new mer: • Head to Tail (where each unit adds in the SAME orientation) • Head to Head (a unit adds in the opposite orientation to previous) • Additions are usually head-to-tail, so one can express any deviations simply as % of head-to-head, from 0 (assumed), to a few %. • Properties can scale with H-t-H ratio, for example Poly(isobutylene) can change its melting point from 5°C to 187°C from 0% to 100% HtH • We won’t worry too much more about the head/tail configurations.
c) -Stereochemistry of attachment • Some nomenclature, and implications: • In Free Radical Polymerization (and in step polymerization with chiral carbons at the reaction site), there are TWO ways to add a new mer: • i) ‘Left handed’ or ii) ‘Right handed’ • This time, it doesn’t really matter which– there is practically no difference, but what matters is the ALTERNATION of ‘R’ or ‘L’: • This relationship between adjacent stereochemistries is TACTICITY. • if the stereochemistry is all the same, it’s ISOTACTIC. • if the stereochemistry alternates, it’s SYNDIOTACTIC. • if it’s indifferent (statistical), then it’s ATACTIC. • Stereoregular Nomenclature: it-polystyrene, or st-polyisobutylene
c) -Stereochemistry of attachment Stereoregular Nomenclature: it-polystyrene, or st-polyisobutylene Stereoregular polymers crystallize more easily, while atactic polymers are usually completely amorphous. REAL Polymer properties are usually described well with ratios of diads and triads: meso ‘m’ retains the stereochemistry for 2 units racemic ‘r’ inverts the stereochemistry for 2 units. ‘mm’ triads keep the chirality over 3 consecutive units (ISOTACTIC) ‘rr’ triads invert the chirality over 3 units (SYNDIOTACTIC) ‘mr’ keeps, then inverts, chirality (HETEROTACTIC) There are 6 tetrads (mmm, mmr, rmr, mrm, rrm, rrr)… 10 pentads…
More nomenclature c) -Stereochemistry of attachment There is one other consideration if the mer has a double bond, that of cis- and trans- isomers (for example polybutadiene) Even more nomenclature b) mode of attachment, for 2 or more mers: These are COPOLYMERS (polymers with 2 flavours of mer) i) poly(A-co-B) no specification poly(styrene-co-MMA) ii) poly(A-stat-B) statistical with unequal reactivity ratios iii) poly(A-ran-B) random with equal reactivity ratios iv) poly(A-alt-B) alternating with very different react. ratios v) poly(A-per-B) periodic with very different react. ratios. vi) poly(A-block-B) block one mer first, then the other. (more on blocks and characterization later, so this finishes Chapter 2).
REACTIVITY RATIOS are not in the book, but are important (and easy) If there are 2 mers A and B in a solution with 2 growing chains: +A adds with rate Kaa A* or +B adds with rate Kab +A adds with rate Kba B * or +B adds with rate Kbb Reactivity Ratios: Ra = Kaa/Kab = preference to add to A Rb = Kbb/Kba = preference to add to B If Ra is high then A likes to add more A. If low, then A prefers more B
iii) poly(A-ran-B) random Ra = Rb = 1 (no preference) • iv) poly(A-alt-B) alternating Ra and Rb both << 1 • v) poly(A-per-B) periodic Ra and Rb both >> 1 • Note 1) “periodic” is usually called “blocky”. • 2) Real copolymers are always a bit blocky or alternating, never random. • This is all for EQUAL A and B available… and one needs to consider how much A and B remain in solution when calculating full structure. • This means that for real chains, our lives are even more complicated since one end of the polymer is usually different than the other.
So, we can now grow long chains, and we are now clever enough to be able to define the associated MICROSTRUCTURE : Firstly, the chemical structure of the monomer, then: a) -Molecular weight and MW distribution (g/mol or DP) b) -How the units attach to each other (head to head fraction) c) -the stereochemistry of attachment (tacticity) d) -fraction of the solid that’s crystalline NOW: we need to make a prediction about the MACROSTRUCTURE.