MSE 457 Exam #1

What is the macromolecular hypothesis?

Polymers comprise covalently linked repeating units, not aggregates

What are the “Big 4” polymers?

PE: Polyethylene

PP: Polypropylene

PVC: Polyvinyl chloride

PS: Polystyrene

What does “monodispersed” system mean?

All polymers are of exactly equal chain lengths

How do you determine the molecular weight of a monodispersed polymeric system?

X * M₀, where X = degree of polymerization and M₀ = MW of the structural unit (if multiple SU per RU, divide to find average)

How do you determine the MW of a RU if there are multiple SUs?

Sum up the MW of all SUs that comprise 1 RU

How do you define the number-average MW of a polymer?

(∑n_iM_i)/∑n_i, where n_i = the # polymers with X = i, and M_i = MW of polymers with X = 1

How do you define the weight-average MW of a polymer?

(∑w_iM_i)/∑w_i, where w_i = the mass of polymers with X = i, and M_i = MW of polymers with X = 1

What is another way to define the weight-average MW of a polymer in terms of the 1st moment?

(∑n_iM_i²)/∑n_iM_i, where n_i = the # polymers with X = i, and M_i = MW of polymers with X = 1

How do you define the Z-average MW of a polymer?

(∑n_iM_i³)/∑n_iM_i², where n_i = the # polymers with X = i, and = MW of polymers with X = 1

What defines the “moment” of MW?

How many times the MW ratio has been multiplied by M_i/M_i, e.g., number-averaged is 1st moment, weight-averaged is 2nd moment, Z-averaged is 3rd moment

What is the importance of “moment” in the MW of polymers?

Higher moment is biased for heavier polymers, i.e, 3rd moment is much more heavily biased toward heavier polymers than 1st moment

What are general properties associated with each moment?

1st moment: solubility

2nd moment: viscosity

3rd moment: sedimentation rate

What is the relation between the second and first moments for monodispersed vs. polydispersed systems?

Monodispersed: Equal

Polydispersed: Second > first

Define the polydispersity index (PDI)

M_w/M_n

What are interpretations of values of PDI?

PDI = 1 (fullly monodispersed)

1 < PDI < 1.5 (polydispersed, “narrow” distribution)

PDI > 2 (“broad” distribution)

What are some analytical techniques to measure polymer MW?

SEC, MALDI, light scattering, etc.

What are some properties of polymers?

MW, tacticity/stereochemistry (crystalline vs. amorphous), architecture (chain, branch, star, etc.), conformation/configuration, other mechanical/chemical properties

Explain the curves for T_g (glass transition) and T_m (melting) on a molar/specific volume vs. temperature graph.

Glass transition: Second order, smooth curve

Melting: First order, vertical jump at T_m

What are macroproperties of polymers below T_g?

Glassy (rigid, frozen in the bead-spring model), amorphous regions stay amorphous

What temperature range is required for polymer crystallization?

Between T_g and T_m

Why do some polymers NOT have a T_m?

Fully amorphous: Over T_g, these become rubbery and flow, eventually burn after high heat

Thermoset: Too strongly crosslinked, become charred after high heat

What defines thermoplastic polymers, and what do their viscosity vs. temperature curves look like?

Polymers that begin to flow upon heating. The curve is flat until T_g, where a dip occurs followed by another plateau that eventually also dips once the polymer begins to flow more freely. The second plateau can be extended by increasing MW.

What type of polymers are generally thermoplastic?

Non-crosslinked

What defines thermoset polymers, and what do their modulus vs. temperature curves look like?

Polymers that keep a permanent structure upon heating. The curve is flat until T_g, where a small dip occurs followed by another plateau. This dip can be made smaller by increasing crosslinking density.

What type of polymers are generally thermoset?

Crosslinked

What defines addition polymers?

Polymers where the repeating units have the same atoms as the monomer, but different bonding. Example is PS

What defines condensation polymers?

Polymers where the repeating units have different atoms as the monomer. Example is 6,6-nylon

Why is 6-nylon not an example of either an addition or a condensation polymer?

It is formed via ring-operning (caprolactam)

What mechanism defines the chain-growth class of polymers?

Single monomer addition (one monomer at a time)

In general, what are the three steps of chain growth?

Initiation, propagation, termination

Is chain growth slow or fast, and why?

Fast, propagation quickly moves the highly reactive site to the ends for further elongation

How do the MW vs. % conversion (p) graphs differ for controlled vs. normal (but with termination) chain growth?

In normal chain growth, the graph resembles a highly fast-growing linear function that plateaus at termination. In controlled chain growth, the graph resembles a slower fast-growing linear function that eventually reaches termination.

What defines step growth polymers?

Polymerization involving the addition of i-mers of any length

What is the general architecture of the i-mers required for step growth polymerization?

A-o-B (where A and B are reactive groups and o is the unreactive rest of the monomer), or A-o-A + B-o-B

Why is ring opening considered its own class of polymer by mechanism?

These polymers can exhibit characteristics from both chain and step growth polymerization.

What defines a homopolymer vs. a copolymer?

Homopolymer: 1 RU

Copolymer: ≥2 RU

What are the three main types of copolymers, and what is their general structure?

Random: AABAABABAABABABAABBAAA

Alternating: ABABABABABABABABABA

Block: AAABBBAAABBBAAABBBAAABBB

What are the three main types of tacticity, and what properties do they lend?

Isotactic: All RUs exhibit the same streochemistry, crystalline, e.g. helix

Syndiotactic: RUs exhibit alternating stereochemistry, crystalline, e.g. lamellar crystal

Atactic: random stereochemistry, usually amorphous

What are five common step growth polymers, and how are they synthesized?

Polyamides: diacid + diamine

Polyester: diacid + diol + acid catalyst

Polycarbonate: diol + phosgene OR diol + chloroformate

Polyurethane: diisocyanate + diol

Polyurea: diisocyanate + diamine

In step growth polymerization, where/why are there deviations from the second-order regime?

At low p, water in the surrounding area is high in concentration, so equilibrium disfavors additional water ourput from the condensation reaction.

At high p, reactants have both degraded and there is more viscosity, making it more difficult for the condensation reaction to proceed.

For step growth polymers assuming equal stoichiometry, how do you determine X_n from the conversion rate p?

1/(1-p)

For step growth polymers assuming equal stoichiometry, how do you determine X_w from the conversion rate p?

(1+p)/(1-p)

For step growth polymers assuming equal stoichiometry, how can you quickly determine the polydispersity index from the conversion rate p?

1+p

For step growth polymers assuming equal stoichiometry, how do you determine the probability of X_n = i from the conversion rate p?

p^i-1

For step growth polymers assuming equal stoichiometry, how do you determine the number of polymers of length i, i.e., N_i from the conversion rate p and inital number of monomer units N₀?

N_i = N₀p^i-1(1-p)²

For step growth polymers assuming equal stoichiometry, how do you determine the weight fraction of polymers of length i, i.e., w_i from the conversion rate p and inital number of monomer units N₀?

w_i = ip^i-1(1-p)²

What are two insights about monomers for step growth polymers assuming equal stoichiometry that can be taken from N_i and w_i?

Monomers are the most abundant (from N_i) but they also contribute the least to the overall mass (from w_i)

What does the PDI of a transesterification polymerization approach?

2

How do you define r for non-stoichiometrically balanced polymer reactions?

r = N_A/N_B

How do you determine X_n for non-stoichiom