CHEM 7010 Macromolecular Synthesis 2 nd Introduction to

CHEM 7010 Macromolecular Synthesis 2 nd Introduction to Polymer Synthesis: General Aspects of Polymer Structures on Polymer Properties 2011

Multidimensional Order of Structure-Property Relationships in Polymers Microscopic Molecular Microscopic *Crystallinity *Phases *Orientation Molecular and Processing Synthesis *Chemical Composition Macroscopic *MW/MWD *Branching and/or Cross-linking *Strength Macroscopic *Modulus *Impact Resistance Permeability Clarity

IMPACT OF MOLECULAR WEIGHT ON MATERIAL PROPERTIES Increasing Degree of Polymerization, DP

Vinyl Monomers, CH 2=CH-X • • X H CH 3 Cl Phenyl CN COOCH 3 O-COCH 3 Polymer Abbreviation Polyethylene PE Polypropylene PP Poly(vinyl chloride) PVC Polystyrene PSt Polyacrylonitrile PAN Poly(methyl acrylate) PMA Poly(vinyl acetate) PVAc

Vinylidene Monomers, CH 2=C(X)Y • X Y Polymer • • • Abbreviation CH 3 Polyisobutylene PIB Cl Poly(vinylidene chloride) PVDC F F Poly(vinylidene fluoride) PVDF Phenyl CH 3 Poly( -methyl styrene) CH 3 COOCH 3 Poly(methyl methacrylate) PMMA • CN COOR Poly(alkyl -cyanoacrylate

Structural Complexity of Polymers • Homopolymers • Head to Tail vs. Head to Head Adducts e. g. a-olefins • 1, 2 - vs 1, 4 Adducts; e. g. butadiene • Tacticity of Enchainments • Branching

Tacticity • Isotactic • • All asymmetric carbons have same configuration Methylene hydrogens are meso Polymer forms helix to minimize substituent interaction • • • Asymmetric carbons have alternate configuration Methylene hydrogens are racemic Polymer stays in planar zig-zag conformation Heterotactic (Atactic) Asymmetric carbons have statistical variation of configuration Syndiotactic

Structural Complexity of Polymers • Copolymers • Identity and Number of Comonomers • Ratio and Distribution of Comonomers • Statistical • Blocks Alternating Gradient Grafts

Structural Complexity of Polymers • Molecular Weight – Molecular Weight Distribution, MWD – Polydispersity Index, PDI – Mn, Mw, Mz, Mv Averages • Crosslinking Density – Length of Crosslinks

Structural Complexity of Polymers Time Dependent Changes • Chemical Reactions • Hydrolysis • Dehydrohalogenation • Photodegradation • Oxidation

Structural Complexity of Polymers • Thermal Degradation • Processing • Aging • Crystallization • Changes in Polymorphism • Weathering-- Combination of Above • Plasticizer Loss -- Imbrittlement

Microscopic Properties (Intermolecular Interactions) • Morphology • Chain entanglement –amorphous • Chain ordering--liquid crystalline • Crystallinity • Phase separations (microdomains)

Types of Intermolecular Forces • Type of Force Relative Strength Low Molecular Analog Polymer • Dispersion or • Van der Waals Weak • Dipole-Dipole Medium Hydrogen bonding Strong Electrostatic Very Strong Methane Hexane Polyethylene Polypropylene CH 3 Cl CH 3 CO 2 CH 3 PVC PMMA H 2 O Cellulose CH 3 CONH 2 Proteins CH 3 CO 2 -Na+ Ionomers

GLASS TRANSITION, Tg • Definition: The onset of seqmental motion of seqments with 40 -50 carbons atoms • Physical Change Expansion of volume • Free volume required to allow segmental motion • Tg is an approximation • Depends upon measurement technique • Depends upon molecular weight • Polystyrene MW = 4000 Tg = 40 C • = 300, 000 = 100

GLASS TRANSITION, Tg • Properties Affected • Specific Volume / Density • Specific Heat, Cp • Refractive Index • Modulus • Dielectric Constant • Permeability

FACTORS INFLUENCING Tg • Tg is proportional to Rotational Freedom • For symmetrical polymers Tg, / Tm in K 1/2 • unsymmetical polymers 2/3 • 1. Chain flexibility • Silicone Ether Hydrocarbon Cyclic HC Aromatics

FACTORS INFLUENCING Tg 2. Steric Bulk of Substituents Tg = -120 C 5 C -24 C -50 C • Long side chains may act as plasticizers (C 6) – Tg = • -55 C 88 C

FACTORS INFLUENCING Tg • • 3. Molecular Symmetry Asymmetry increases chain stiffness. 4. Polar Interactions increase Tg Hydrogen bonding 5. Molecular Weight up to Critical Limit 6. Crosslinking Reduces Segment Mobility

FACTORS INFLUENCING Tm • 1. Chain flexibility • Silicone Ether Hydrocarbon Cyclic HC Aromatics • 2. Substituents Producing Lateral Dipoles • Hydrogen bonding • 3. Molecular Symmetry • Symmetry allows close packing

FACTORS INFLUENCING Tm • 4. No Bulky Substituents to Disrupt Lattice if placement is Random • 5. Structural Regularity • monomer placement • head to tail • 1, 2 - vs 1, 4 - • 1, 2 - vs 1, 3 - vs 1, 4 - aromatic substitution • geometric isomers of enchainments • cis or trans -C=C-; cyclic ring • tacticity

FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION • Thermodynamic • 1. Symmetrical chains which allow regular close packing in crystallite • 2. Functional groups which encourage strong intermolecular attraction to stabilize ordered alignment.

FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION • Kinetic • 1. Sufficient mobility to allow chain disentanglement and ultimate alignment • Optimum range for mobility • Tm -10 Tg + 30 • at Tm segmental motion too high • at Tg viscosity too high • 2. Concentration of nuclei • concentration of nucleating agents • thermal history of sample

Macroscopic Properties (Physical Behavior) • Tensile and/or Compressive Strength • Elasticity • Toughness • Thermal Stability • Flammability and Flame Resistance • Degradability • Solvent Resistance • Permeability • Ductility (Melt Flow)

Step Polymerization (Polycondensation) After many repetitions: Polyethyleneterephthalate (PET)

Step Polymerization Two Routes: A-A + B-B; or A-B 1 on 39 1 on 40 2 on 39 2 on 40 Either way, need: • high conversion • stoichiometric amount

Step vs. Chain Polymerization Step Polymerization • 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 hardly changes. • All molecular species are present throughout. • Usually (but not always) polymer repeat unit has fewer atoms than had the monomer.

Step vs. Chain, cont. Chain Polymerization • Growth occurs only by addition of monomer to active chain end. • Monomer is present throughout, but its concentration decreases. • High polymer forms immediately. • MW and yield depend on mechanism details. • Chain growth is usually very rapid (seconds to microseconds). • Only monomer and polymer are present during reaction. • Usually (but not always) polymer repeat unit has the same atoms as had the monomer.

Step Polymerization Concepts Comparison of Step and Chain Polymerization 1. Step: any 2 molecules in the system can react with each other Chain: chain growth occurs on end of growing polymer 2. Step: loss of monomer at early stage (dimers, tetramers, etc. ) Chain: monomer concentration decreases steadily 3. Step: broad molecular weight distribution in late stages Chain: narrower distribution; just polymer and monomer

Step Polymerization Basis for Kinetics (2 -1 a): 3 on 40 4 on 40 5 on 40 6 on 40

Step Polymerization Basis for Kinetics: 1 on 41 2 on 41

Step Polymerization Basis for Kinetics (2 -1 b): NOTE: Assume equal reactivity of functional groups table 2. 1 on 42 Thus, assumption looks ok

Step Polymerization Kinetics (2. 2) Example: diacid + diol Mechanism (acid catalyzed): 2 on 44 1 on 45 2 on 45

Chain Polymerization Concepts General Mechanism Initiation Propagation

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 a. Comparison of Chain and Step Polymerizations Chain: a. Rapid high mw b. Snapshot: monomer, high poly, growing chains c. MW does not change with time Step: a. Monomer gone fast, get dimer, trimer, etc. b. MW increases with time c. No high mw until end

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 b-1. General Considerations of Polymerizability Thermodynamics (Chain t-fer)

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 b-2. Effects of Substituents on C=C monomers for Ionic Polymerization A. Initiation opportunities: 2 types of bond cleavage/resonance B. Radical, Anionic, Cationic: Depends on: i. Inductance ii. Resonance

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 b-2. Effects of Substituents on C=C monomers for Ionic Polymerization C. Donating Groups i. Inductance ii. Resonance e. g. Polym of vinyl ether Good for cationic Example of resonance stabilization of cation by Delocalization of of positive charge. If oxygen not there, no stabilization

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 b-2. Effects of Substituents on C=C monomers for Ionic Polymerization C. Donating Groups ii. Resonance e. g. Polym of Styrene

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 b-2. Effects of Substituents on C=C monomers for Ionic Polymerization D. Withdrawing Groups i. inductance Good for anionic ii. resonance

Chain Polymerization Concepts: Nature of Chain Polymerization; 3 -1 b-2. Effects of Substituents on C=C monomers for radical polymerization A. Radicals not affected by charge, but can have resonance stabilization B. Also stabilized by primary<secondary<tertiary Draw your own pic

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3 -2 3 -1 a. Possible Modes of Propagation A. Two possible points of attachment i. On carbon 1: ii. On carbon 2:

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3 -2 3 -1 a. Possible Modes of Propagation B. If attachment is regular, get head-to-tail (H-T) C. If attachment is irregular, get head-to-head (H-H) [and T-T]

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3 -2 3 -1 a. Possible Modes of Propagation Usually only 1 -2% H-H. How do we know? A. Chemical Methods B. NMR Methods (we’ll check this out in the Spectroscopy section)