Diversity of Polymer Chains (two types):A) Low molar mass (small) moleculesB) PolymerCourse GoalsGoal 1: Structural and architectural controlGoal 2: Apply knowledge to processes in industrial and commercial settingsGoal 3: Awareness of new tools and approaches to materials designDescription of Molecular Weight in PolymersTypes of PolymerizationA) Chain growthB) Step growth10.569, Synthesis of Polymers Prof. Paula Hammond Lecture 1: Introduction Molecular structureProcessingFunctionStructureMorphologyCrystallinityPhase behaviorTg, TmFinal PropertiesMolecular structureProcessingFunctionStructureMorphologyCrystallinityPhase behaviorTg, TmFinal Properties Figure 1: Processing and molecular structure of a polymer determines its function, structure, and morphology, which in turn determines its final properties Diversity of Polymer Chains (two types): A) Low molar mass (small) molecules Example: OHHHCH3 Synthesis determines molecular structure One goal of synthesis is to avoid side reactions and achieve a pure product B) Polymer • Control molecular structure • Control regularity of backbone o Ex: stereochemistry RRRHHH Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 course materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.o Ex: sequencing in copolymers These three polymers are different even though they have the same number of monomers: abababab regular copolymer abbaaaba random copolymer aaaabbbb block copolymer • Control molecular weight o Impacts polydiversity: Polydisperse vs. Monodisperse o Overall molecular weight (MW) or mass If a polymer has low MW, it acts like a fluid above Tg If a polymer has high MW, it acts like a rubber above Tg MW also determines mechanical properties, viscosity, rheology • Control architecture linear chain polymer lightly branched polymer “combed” polymer “star polymer” 10.569, Synthesis of Polymers Lecture 1 Prof. Paula Hammond Page 2 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 course materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.Course Goals Goal 1: Structural and architectural control • To gain a sense of rational design and synthesis • To develop an intuition about the impact of a structure on property • The following two examples demonstrate how structure determines the polymer’s physical and chemical properties: o Ex 1: polyamides (Kevlar® by DuPont) HHOONNn • Kevlar®’s very low flexibility makes it a rigid structure • The hydrogen bonding enhances rigidity and makes it solvent-resistent • The long backbone gives it high mechanical strength • In fact, Kevlar® has a liquid crystalline structure o Ex 2: polydimethylsiloxane (PDMS) Si O CH3CH3n • The longer Si—O bond makes PDMS very flexible • CH3 makes the polymer hydrophobic • Tg ≈ -100°C Goal 2: Apply knowledge to processes in industrial and commercial settings • Determine which process is best for certain applications (Ex: there are ways to synthesize PDMS) • There are variables in polymer approach, synthetic route, starting materials and/or catalysts, and solvent conditions Goal 3: Awareness of new tools and approaches to materials design • Less traditional approaches • Functionalization of polymers • Self-assembly approaches 10.569, Synthesis of Polymers Lecture 1 Prof. Paula Hammond Page 3 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 course materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.Description of Molecular Weight in Polymers Each MW can be represented as Mi Ni = number of molecules of MW=Mi wi = weight fraction of given system of chains with MW=Mi iiiiiNMwNM=∑ nM= number average MW = sample in molecules # totalweight total = iiiNMN∑∑ wM= weight average MW = ()()()2ii i i iii iiNM M N MNM NM=∑∑∑∑ The following graph shows the relationship between wi and mi: wiMiMnMw Polydispersity can be measured by PDI (polydispersity index): 1.0wnMzM=≥. z = 1.03 or 1.05 is considered close to monodisperse 10.569, Synthesis of Polymers Lecture 1 Prof. Paula Hammond Page 4 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 course materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.Types of Polymerization A) Chain growth • In chain growth, a monomer is activated and polymerization propagates by activating neighboring monomers. The process is very rapid and high MW polymers are achieved quickly. • The following describes the chain growth reaction in which * represents the activated monomer M. This can be a free radical, negative charge, or positive charge: 1. R* + M Æ RM* 2. RM* + M Æ RMM* … RMn* + M Æ RMn+1* 3. Event that terminates B) Step growth • In chain growth, bifunctional monomers are added systematically to form covalent bonds. It generally involves 2 (or more) functional groups: “a” and “b.” Molecular weight increases “slowly” as dimers become trimers, which in turn become tetramers. • Examples of polymers formed by chain growth: nylons, polyesters, polypeptides (proteins) • [Handout] These are typical a and b groups: a + b Æ c + d where c = covalent link d = byproduct 1. a—a + b—b Æ a—c—b + d OCOHRCOOH + OHR1OH Æ (dialcohol) OCOHRCOOR1OH + OH2 ester link 2. a—c—b + a—a Æ a—c—c—a 3. a—c—c—a + b—c—c—c—a Æ a(c)6a + d 10.569, Synthesis of Polymers Lecture 1 Prof. Paula Hammond Page 5 of 5 Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 course materials, MIT OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology,
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